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    THE INTERNATIONAL SCIENTIFIC SERIES.

    VOLUME XLIX.




    THE INTERNATIONAL SCIENTIFIC SERIES.

    JELLY-FISH, STAR-FISH

    AND SEA-URCHINS

    BEING

    _A RESEARCH ON PRIMITIVE NERVOUS SYSTEMS_

    BY

    G. J. ROMANES, M.A., LL.D., F.R.S.

    ZOÖLOGICAL SECRETARY OF THE LINNEAN SOCIETY

    NEW YORK

    D. APPLETON AND COMPANY

    72 FIFTH AVENUE

    1898




PREFACE.


When I first accepted the invitation of the editors of the
International Scientific Series to supply a book upon Primitive
Nervous Systems, I intended to have supplemented the description of
my own work on the physiology of the _Medusæ_ and _Echinodermata_
with a tolerably full exposition of the results which have been
obtained by other inquirers concerning the morphology and development
of these animals. But it soon became apparent that it would be
impossible, within the limits assigned to me, to do justice to the
more important investigations upon these matters; and therefore I
eventually decided upon restricting this essay to an account of my
own researches.

With the exception of a few woodcuts in the last chapter (for the
loan of which I am indebted to the kindness of Messrs. Cassell),
all the illustrations are either original or copies of those in my
Royal Society papers. In the letter-press also I have not scrupled
to draw upon these papers, wherever it seemed to me that the
passages would be sufficiently intelligible to a general reader. I
may observe, however, that although I have throughout kept in view
the requirements of a general reader, I have also sought to render
the book of service to the working physiologist, by bringing together
in one consecutive account all the more important observations and
results which have been yielded by this research.

    G. J. R.

   LONDON, 1884.




CONTENTS.


    CHAPTER                                                PAGE

    INTRODUCTION                                              1

       I. STRUCTURE OF THE MEDUSÆ                            10

      II. FUNDAMENTAL EXPERIMENTS                            26

     III. EXPERIMENTS IN STIMULATION                         37

      IV. EXPERIMENTS IN SECTION OF COVERED-EYED MEDUSÆ      65

       V. EXPERIMENTS IN SECTION OF NAKED-EYED MEDUSÆ       104

      VI. CO-ORDINATION                                     130

     VII. NATURAL RHYTHM                                    146

    VIII. ARTIFICIAL RHYTHM                                 175

      IX. POISONS                                           213

       X. STAR-FISH AND SEA-URCHINS                         254




JELLY-FISH, STAR-FISH, AND SEA-URCHINS.

INTRODUCTION.


Among the most beautiful, as well as the most common, of the
marine animals which are to be met with upon our coasts are the
jelly-fish and the star-fish. Scarcely any one is so devoid of the
instincts either of the artist or of the naturalist as not to have
watched these animals with blended emotions of the æsthetic and the
scientific--feeling the beauty while wondering at the organization.
How many of us who live for most of the year in the fog and dust
of large towns enjoy with the greater zest our summer's holiday
at the seaside? And in the memories of most of us is there not
associated with the picture of breaking waves and sea-birds floating
indifferently in the blue sky or on the water still more blue, the
thoughts of many a ramble among the weedy rocks and living pools,
where for the time being we all become naturalists, and where those
who least know what they are likely to find in their search are
most likely to approach the keen happiness of childhood? If so, the
image of the red sea-stars bespangling a mile of shining sand, or
decorating the darkness of a thousand grottoes, must be joined with
the image, no less vivid, of those crystal globes pulsating with life
and gleaming with all the colours of the rainbow, which are perhaps
the most strange, and certainly in my estimation the most delicately
lovely creatures in the world.

It is with these two kinds of creatures that the present work is
concerned, and if it seems almost impious to lay the "forced fingers
rude" of science upon living things of such exquisite beauty, let
it be remembered that our human nature is not so much out of joint
that the rational desire to know is incompatible with the emotional
impulse to admire. Speaking for myself, I can testify that my
admiration of the extreme beauty of these animals has been greatly
enhanced--or rather I should say that this extreme beauty has been,
so to speak, revealed--by the continuous and close observation which
many of my experiments required: both with the unassisted eye and
with the microscope numberless points of detail, unnoticed before,
became familiar to the mind; the forms as a whole were impressed upon
the memory; and, by constantly watching their movements and changes
of appearance, I have grown, like an artist studying a face or a
landscape, to appreciate a fulness of beauty, the _esse_ of which
is only rendered possible by the _per cipi_ of such attention as is
demanded by scientific research. Moreover, association, if not the
sole creator, is at least a most important factor of the beautiful;
and therefore the sight of one of these animals is now much more to
me, in the respects which we are considering, than it can be to any
one in whose memory it is not connected with many days of that purest
form of enjoyment which can only be experienced in the pursuit of
science.

And here I may observe that the worker in marine zoology has one
great advantage over his other scientific brethren. Apart from the
intrinsic beauty of most of the creatures with which he has to deal,
all the accompaniments of his work are æsthetic, and removed from
those more or less offensive features which are so often necessarily
incidental to the study of anatomy and physiology in the higher
animals. When, for instance, I contrast my own work in a town
laboratory on vertebrated animals with that which I am now about
to describe upon the invertebrated in a laboratory set up upon the
sea-beach, it is impossible not to feel that the contrast in point
of enjoyment is considerable. In the latter case, a summer's work
resembles the pleasure-making of a picnic prolonged for months, with
the sense of feeling all the while that no time is being profitlessly
spent. Whether one is sailing about upon the sunny sea, fishing with
muslin nets for the surface fauna, or steaming away far from shore to
dredge for other material, or, again, carrying on observations in the
cool sea-water tanks and bell-jars of a neat little wooden workshop
thrown open to the sea-breezes, it alike requires some effort to
persuade one's self that the occupation is really something more than
that of finding amusement.

It is now twelve years since I first took to this kind of summer
recreation, and during that time most of my attention while at the
seaside has been devoted to the two classes of animals already
mentioned--viz. the jelly-fish and star-fish, or, as naturalists have
named them, the Medusæ and Echinodermata. The present volume contains
a tolerably full account of the results which during six of these
summers I have succeeded in obtaining. If any of my readers should
think that the harvest appears to be a small one in relation to the
time and labour spent in gathering it, I shall feel pretty confident
that those readers are not themselves working physiologists, and,
therefore, that they are really ignorant of the time and labour
required to devise and execute even apparently simple experiments,
to hunt down a physiological question to its only possible answer,
and to verify each step in the process of an experimental proof.
Moreover, the difficulties in all these respects are increased
tenfold in a seaside laboratory without adequate equipments or
attendance, and where, in consequence, more time is usually lost in
devising makeshifts for apparatus, and teaching unskilled hands how
to help, than is consumed in all other parts of a research. From the
picnic point of view, however, there is no real loss in this; such
incidental difficulties add to the enjoyment (else why choose to
make an extemporized grate and boil a kettle in the wood, when a
much more efficient grate, full of lighted coals, is already boiling
some other kettle at home?); and if they somewhat unduly prolong a
research, the full meaning of life is, after all, not exhausted by
the experiences of a mill-horse, and it is well to remember that so
soon as we cease to take pleasure in our work, we are most likely
sacrificing one part of our humanity to the altar of some other, and
probably less worthy, constituent.

I may now say a few words on the scope of the investigations which
are to be described in the present treatise. To some extent this is
conveyed by the title; but I may observe that, as the "primitive
nervous systems" whose physiology I have sought to advance are mainly
subservient to the office of locomotion, in my Royal Society papers
upon these researches I have adopted the title of "Observations on
the Locomotor System" of each of the classes of animals in question.
It is of interest to notice in this connection that the plan or
mechanism of locomotion is completely different in the two classes,
and that in the case of each class the plan or mechanism is unique,
_i.e._ is not to be met with elsewhere in the animal kingdom. It is
curious, however, that, in the case of one family of star-fish (the
_Comatulæ_), owing to an extreme modification of form and function
presented by the constituent parts of the locomotor organs, the
method of progression has come closely to resemble that which is
characteristic of jelly-fish.

There is still one preliminary topic on which I feel that it is
desirable to touch before proceeding to give an account of my
experiments, and this has reference to the vivisection which many
of these experiments have entailed. But in saying what I have to
say in this connection I can afford to be brief, inasmuch as it is
not needful to discuss the so-called vivisection question. I have
merely to make it plain that, so far as the experiments which I am
about to describe are concerned, there is not any reasonable ground
for supposing that pain can have been suffered by the animals. And
this it is easy to show; for the animals in question are so low
in the scale of life, that to suppose them capable of conscious
suffering would be in the highest degree unreasonable. Thus, for
instance, they are considerably lower in the scale of organization
than an oyster, and in none of the experiments which I have performed
upon them has so much laceration of living tissue been entailed
as that which is caused by opening an oyster and eating it alive,
after due application of pepper and vinegar. Therefore, if any one
should be foolish enough to object to my experiments on the score of
vivisection, _a fortiori_ they are bound to object to the culinary
use of oysters. Of course, it may be answered to this that two
blacks do not make a white, and that I have not by this illustration
succeeded in proving my negative. To this, however, I may in turn
reply that, for the purpose of morally justifying my experiments
on the ground which I have adopted, it is not incumbent on me to
prove any negative; it is rather for my critics to prove a positive.
That is to say, before convincing me of sin, it must be shown that
there is some reasonable ground for supposing that a jelly-fish or a
star-fish is capable of feeling pain. I submit that there is no such
ground. The mere fact that the animals are alive constitutes no such
ground; for the insectivorous plants are also alive, and exhibit even
more physiological "sensitiveness" and capability of rapid response
to stimulation than is the case with the animals which we are about
to consider. And if anyone should go so far as to object to Mr.
Darwin's experiments on these plants on account of its not being
demonstrable that the tissues did not suffer under his operations,
such a person is logically bound to go still further, and to object
on similar grounds to the horrible cruelty of skinning potatoes and
boiling them alive.

Thus, before any rational scruples can arise with regard to the
vivisection of a living organism, some reasonable ground must be
shown for supposing that the organism, besides being living, is also
capable of suffering. But no such reasonable ground can be shown in
the case of these low animals. We only know of such capability in
any case through the analogy based upon our own experience, and,
if we trust to this analogy, we must conclude that the capability
in question vanishes long before we come to animals so low in the
scale as the jelly-fish or star-fish. For within the limits of
our own organism we have direct evidence that nervous mechanisms,
much more highly elaborated than any of those which we are about
to consider, are incapable of suffering. Thus, for instance, when
the nervous continuity of the spinal cord is interrupted, so that a
stimulus applied to the lower extremities is unable to pass upwards
to the brain, the feet will be actively drawn away from a source of
irritation without the man being conscious of any pain; the lower
nervous centres in the spinal cord respond to the stimulation, but
they do so without _feeling_ the stimulus. In order to feel there
must be consciousness, and, so far as our evidence goes, it appears
that consciousness only arises when a nerve-centre attains to some
such degree of complexity and elaboration as are to be met with in
the brain. Whether or not there is a dawning consciousness in any
nerve-centres considerably lower in the scale of nervous evolution,
is a question which we cannot answer; but we may be quite certain
that, if such is the case, the consciousness which is present must
be of a commensurately dim and unsuffering kind. Consequently, even
on this positive aspect of the question, we may be quite sure that
by the time we come to the jelly-fish--where the object of the
experiments in the first instance was to obtain evidence of the very
existence of nerve-tissue--all question of pain must have vanished.
Whatever opinions, therefore, we may severally entertain on the
vexed question of vivisection as a whole, and with whatever feelings
we may regard the "blind Fury" who, in the person of the modern
physiologist, "comes with the abhorred shears and slits the thin-spun
life," we should be all agreed that in the case of these animals the
life is so very thin-spun that any suggestion of abhorrence is on the
face of it absurd.[1]

  [1] The relation of consciousness to the elaboration of
  nerve-centres throughout the animal kingdom is more fully
  considered in my work on "Mental Evolution in Animals" (Kegan
  Paul, Trench & Co.: 1883).




CHAPTER I.

STRUCTURE OF THE MEDUSÆ.


To give a full account of the morphology, development, and
classification of the Medusæ would be both unnecessary for our
present purposes and impracticable within the space which is allotted
to the present work.[2] But, for the sake of clearness in what
follows, I shall begin by briefly describing such features in the
anatomy of the jelly-fish as will afterwards be found especially to
concern us.

  [2] Those who may desire to read an excellent epitome of our
  most recent knowledge on these subjects, may refer to Professor
  E. Ray Lankester's article in the "Encyclopædia Britannica" on
  "Hydrozoa," together with Professor Haeckel's Report on the
  Medusæ of the _Challenger_ Expedition.

  [Illustration: Fig. 1.--Sarsia (natural size).]

In size, the different species of Medusæ vary from that of a small
pea to that of a large umbrella having streamers a hundred feet
long. The general form of these animals varies in different species
from that of a thimble (Fig. 1) to that of a bowl, a parasol, or a
saucer (see figures in subsequent chapters). Or we may say that the
form of the animals always resembles that of a mushroom, and that
the resemblance extends to a tolerably close imitation by different
species of the various forms which are characteristic of different
species of mushrooms, from the thimble-like kinds to the saucer-like
kinds. Moreover, this accidental resemblance to a mushroom is
increased by the presence of a central organ, occupying the position
of, and more or leas resembling in form, the stalk of a mushroom.
This organ is called the "manubrium," on account of its looking like
the "handle" of an umbrella, and the term "umbrella" is applied to
the other portion of the animal. The manubrium, like the umbrella,
varies much in size and shape in different species, as a glance
at any figures of these animals will show. Both the manubrium and
umbrella are almost entirely composed of a thick, transparent, and
non-contractile jelly; but the whole surface of the manubrium and the
whole _concave_ surface of the umbrella are overlayed by a thin layer
or sheet of contractile tissue. This tissue constitutes the earliest
appearance in the animal kingdom of true muscular fibres, and its
thickness, which is pretty uniform, is nowhere greater than that of
very thin paper.

The manubrium is the mouth and stomach of the animal, and at the
point where it is attached to or suspended from the umbrella its
central cavity opens into a tube-system, which radiates through the
lower or concave aspect of the umbrella. This tube-system, which
serves to convey digested material and may therefore be regarded as
intestinal in function, presents two different forms in the two
main groups into which the Medusæ are divided. In the "naked-eyed"
group, the tubes are unbranched and run in a straight course to the
margin of the umbrella, where they open into a common circular tube
which runs all the way round the margin (see Figs. 1 and 22). In
the "covered-eyed" group, on the other hand, the tubes are strongly
branched (see Fig. 8), although they likewise all eventually
terminate in a single circular tube. This circular or marginal tube
in both cases communicates by minute apertures with the external
medium.

The margin of the umbrella, both in the naked and covered eyed
Medusæ, supports a series of contractile tentacles, which vary
greatly in size and number in different species (see Figs. 1 and
8). The margin also supports another series of bodies which will
presently be found to be of much importance for us. These are
the so-called "marginal bodies," which vary in number, size, and
structure in different species. In all the covered-eyed species
these marginal bodies occur in the form of little bags of crystals
(therefore they are called "lithocysts"), which are protected by
curiously formed "hoods" or "covers" of gelatinous tissue; and it
is on this account that the group is called "covered-eyed," in
contradistinction to the "naked-eyed," where these little hoods or
coverings are invariably absent (compare Fig. 1 with Fig. 22), and
the crystals frequently so. In nearly all cases these marginal bodies
contain more or less brightly coloured pigments.

The question whether any nervous tissue is present in the Medusæ is
one which has long occupied the more or less arduous labours of many
naturalists. The question attracted so much investigation on account
of its being one of unusual interest in biology. Nerve-tissue had
been clearly shown to occur in all animals higher in the zoological
scale than the Medusæ, so that it was of much importance to ascertain
whether or not the first occurrence of this tissue was to be met
with in this class. But, notwithstanding the diligent application of
so much skilled labour, up to the time when my own researches began
there had been so little agreement in the results obtained by the
numerous investigators, that Professor Huxley--himself one of the
greatest authorities upon the group--thus defined the position of the
matter in his "Classification of Animals" (p. 22): "No nervous system
has yet been discovered in any of these animals."

The following is a list of the more important researches on this
topic up to the time which I have just named:--Ehrenberg, "Die
Acalephen des rothen Meeres und der Organismvs der Medusen der
Ostsee," Berlin, 1836; Kölliker, "Ueber die Randkörper der Quallen,
Polypen und Strahlthiere," Froriep's neue Notizen, bd. xxv., 1843;
Von Beneden, "Mémoire sur les Campanulaires de la côte d'Ostende,"
"Mémoires de l'Académie de Bruxelles," vol. xvii., 1843; Desor, "Sur
la Génération Medusipare des Polypes hydraires," "Annales d. Scienc.
Natur. Zool.," ser. iii. t. xii. p. 204; Krohn, "Ueber Podocoryna
carnea," "Archiv. f. Naturgeschichte," 1851, b. i.; McCrady,
"Descriptions of Oceania, etc.," "Proceedings of the Elliot Society
of Natural History," vol. i., 1859; L. Agassiz, "Contributions to the
Acaliphæ of North America," "Memoirs of the American Academy of Arts
and Sciences," vol. iii., 1860, vol. iv., 1862; Leuckart, "Archiv.
f. Naturgeschichte," Jahrg. 38, b. ii., 1872; Hensen, "Studien
über das Gehörorgan der Decapoden," "Zeitchr. f. wiss. Zool.," bd.
xiii., 1863; Semper, "Reisebericht," "Zeitschr. f. wiss. Zool.," bd.
xiii. vol. xiv.; Claus, "Bemerkungen über Clenophoren und Medusen,"
"Zeitschr. f. wiss. Zool.," bd. xiv., 1864; Allman, "Note on the
Structure of Certain Hydroid Medusæ," "Brit. Assoc. Rep.," 1867;
Fritz Müller, "Polypen und Quallen von S. Catherina," "Archiv. f.
Naturgesch.," Jahrg. 25, bd. i., 1859; also "Ueber die Randbläschen
der Hydroidquallen," "Archiv. f. Anatomie und Physiologie," 1852;
Haeckel, "Beiträge zur Naturgesch. der Hydromedusen," 1865; Eimer,
"Zoologische Untersuchungen," Würzburg, "Verhandlungen der Phys.-med.
Gesellschaft," N.F. vi. bd., 1874.

The most important of these memoirs for us to consider are the two
last. I shall subsequently consider the work of Dr. Eimer, which
up to this date was of a purely physiological character. Professor
Haeckel, who made his microscopical observations chiefly upon the
Geryonidæ, described the nervous elements as forming a continuous
circle all round the margin of the umbrella, following the course
of the radial or nutrient tubes throughout their entire length,
and proceeding also to the tentacles and marginal bodies. At the
base of each tentacle there is a ganglionic swelling, and it is from
these ganglionic swellings that the nerves just mentioned take their
origin. The most conspicuous of these nerves are those that proceed
to the radial canals and marginal bodies, while the least conspicuous
are those that proceed to the tentacles. Cells, as a rule, can
only be observed in the ganglionic swellings, where they appear as
fusiform and distinctly nucleated bodies of great transparency and
high refractive power. On the other hand, the nerves that emanate
from the ganglia are composed of a delicate and transparent tissue,
in which no cellular elements can be distinguished, but which is
longitudinally striated in a manner very suggestive of fibrillation.
Treatment with acetic acid, however, brings out distinct nuclei in
the case of the nerves that are situated in the marginal vesicles,
while in those that accompany the radial canals ganglion-cells are
sometimes met with.

A brief sketch of the contents of these and other memoirs on the
histology of the Medusæ is given by Drs. Hertwig in their more
recently published work on the nervous system and sense-organs of the
Medusæ, and these authors point to the important fact that before
the appearance of Haeckel's memoir, Leuckart was the only observer
who spoke for the fibrillar character of the so-called marginal
ring-nerve; so that in Haeckel's researches on Geryonia, whereby both
true ganglion-cells and true nerve-fibres were first demonstrated
as occurring in the Medusæ, we have a most important step in the
histology of these animals. Haeckel's results in these respects
have since been confirmed by Claus, "Grundzüge der Zoologie," 1872;
Allman, "A Monograph of the Gymnoblastic or Tubularian Hydroids,"
1871; Harting, "Notices Zoologiques," Niedlandisches "Archiv. f.
Zool.," bd. ii., Heft 3, 1873; F. E. Schulze, "Ueber den Bau von
Syncorzne Sarsii"; O. and R. Hertwig, "Das Nervensystem und die
Sinnesorgane der Medusen."

The last-named monograph is much the most important that has appeared
upon the histology of the Medusæ. I shall, therefore, give a
condensed epitome of the leading results which it has established.

There is so great a difference between the nervous system of the
naked and of the covered eyed Medusæ, that a simultaneous description
of the nervous system in both groups is not by these authors
considered practicable. Beginning, therefore, with the naked-eyed
division, they describe the nervous system as here consisting of two
parts, a central and a peripheral. The central part is localized in
the margin of the swimming-bell, and there forms a "nerve-ring,"
which is divided by the insertion of the "veil"[3] into an upper and
a lower nerve-ring. In many species the upper nerve-ring is spread
out in the form of a flattish layer, which is somewhat thickened
where it is in contact with the veil. In these species the nerve-ring
is only indistinctly marked off from the surrounding tissues. But
in other species the crowding together of the nerve-fibres at the
insertion of the veil gives rise to a considerable concentration
of nervous structures; while in others, again, this concentration
proceeds to the extent of causing a well-defined swelling of nervous
tissue against the epithelium of the veil and umbrella. In the
Geryonidæ this swelling is still further strengthened by a peculiar
modification of the other tissues in the neighbourhood, which had
been previously described by Professor Haeckel. In all species the
upper nerve-ring lies entirely in the ectoderm. Its principal mass
is composed of nerve-fibres of wonderful tenuity, among which are
to be found sparsely scattered ganglion-cells. The latter are for
the most part bi-polar, more seldom multi-polar. The fibres which
emanate from them are very delicate, and, becoming mixed with others,
do not admit of being further traced. Where the nervous tissue meets
the enveloping epithelium it is connected with the latter from
within, but differs widely from it; for the nerve-cells contain a
longitudinally striated cylindrical or thread-like nucleus which
carries on its peripheral end a delicate hair, while its central end
is prolonged into a fine nerve-fibre. There are, besides these, two
other kinds of cells which form a transition between the ganglion and
the epithelium cells. The first kind are of a long and cylindrical
form, the free ends of which reach as far as the upper surface of the
epithelium The second kind lie for the most part under the upper
surface. They are of a large size, and present, coursing towards
the upper surface, a long continuation, which at its free extremity
supports a hair. In some cases this continuation is smaller, and
stops short before reaching the outer surface. Drs. Hertwig observe
that in these peculiar cells we have tissue elements which become
more and more like the ordinary ganglion-cells of the nerve-ring the
more that their long continuation towards the surface epithelium
is shortened or lost, and these authors are thus led to conclude
that the upper nerve-ring was originally constituted only by such
prolongations of the epithelium-cells, and that afterwards these
prolongations gradually disappeared, leaving only their remnants to
develop into the ordinary ganglion-cells already described.

  [3] This is the name given to a small annular sheet of tissue
  which forms a kind of floor to the orifice of the swimming-bell,
  through the central opening of which floor the manubrium passes.
  The structure is shown in Fig. 1.

Beneath the upper nerve-ring lies the lower nerve-ring. It is
inserted between the muscle-tissue of the veil and umbrella, in
the midst of a broad strand wherein muscle-fibres are entirely
absent. It here constitutes a thin though broad layer which, like
the upper nerve-ring, belongs to the ectoderm. It also consists of
the same elements as the upper nerve-ring, viz. of nerve-fibres and
ganglion-cells. Yet there is so distinct a difference of character
between the elements composing the two nerve-rings, that even in
an isolated portion it is easy to tell from which ring the portion
has been taken. That is to say, in the lower nerve-ring there are
numerous nerve-fibres of considerable thickness, which contrast in
a striking manner with the almost immeasurably slender fibres of
the upper nerve-ring. A second point of difference consists in the
surprising wealth of ganglion-cells in the one ring as compared with
the other. Thus, on the whole, there is no doubt that the lower
nerve-ring presents a higher grade of structure than does the upper,
as shown not only by the greater multiplicity of nerve-cells and
fibres, but also by the relation in which these elements stand to the
epithelium. For in the case of the lower nerve-ring, the presumably
primitive connections of the nervous elements with the epithelium is
well-nigh dissolved--this nerve-ring having thus separated itself
from its parent structure, and formed for itself an independent
layer beneath the epithelium. The two nerve-rings are separated from
one another by a very thin membrane, which, in some species at all
events, is bored through by strands of nerve-fibres which serve to
connect the two nerve-rings with one another.

The peripheral nervous system is also situated in the ectoderm,
and springs from the central nervous system, not by any observable
nerve-trunks, but directly as a nervous plexus composed both of
cells and fibres. Such a nervous plexus admits of being detected
in the sub-umbrella of all Medusæ, and in some species may be
traced also into the tentacles. It invariably lies between the
layer of muscle-fibre and that of the epithelium. The processes of
neighbouring ganglion-cells in the plexus either coalesce or dwindle
in their course to small fibres: at the margin of the umbrella
these unite themselves with the elements of the nerve-rings. There
are also described several peculiar tissue elements, such as, in
the umbrella, nerve-fibres which probably stand in connection with
epithelium-cells; nerve-cells which pass into muscle-fibres, similar
to those which Kleinenberg has called neuro-muscular cells; and, in
the tentacles, neuro-muscular cells joined with cells of special
sensation (_Sinneszellen_).

No nervous elements could be detected in the convex surface of the
umbrella, and it is doubtful whether they occur in the veil.

In some species the nerve-fibres become aggregated in the region of
the generative organs, and in that of the radial canals, thus giving
rise in these localities to what may be called nerve-trunks. But in
other species no such aggregations are apparent, the nervous plexus
spreading out in the form of an even trellis-work.

In the covered-eyed Medusæ the central nervous system consists
of a series of separate centres which are not connected by any
commissures. These nerve-centres are situated in the margin of the
umbrella, and are generally eight in number, more rarely twelve,
and in some species sixteen. They are thickenings of the ectoderm,
which either enclose the bases of the sense-organs, or only cover
the ventral side of the same. Histologically they consist of cells
of special sensation, together with a thick layer of slender
nerve-fibres. Ganglion-cells, however, are absent, so that the
nerve-fibres are merely processes of epithelium-cells.

Drs. Hertwig made no observations on the peripheral nervous system
of the covered-eyed Medusæ; but they do not doubt that such a system
would admit of being demonstrated, and in this connection they cite
the observations of Claus, who describes numerous ganglion-cells as
occurring in the sub-umbrella of Chrysaora. Here I may appropriately
state that before Drs. Hertwig had published their results, Professor
Schäfer, F.R.S., conducted in my laboratory a careful research upon
the histology of the Medusæ, and succeeded in showing an intricate
plexus of cells and fibres overspreading the sub-umbrella tissue of
another covered-eyed Medusa (Aurelia aurita).[4] He also found that
the marginal bodies present a peculiar modification of epithelium
tissue, which is on its way, so to speak, towards becoming fully
differentiated into ganglionic cells.

  [4] See "Observations on the Nervous System of _Aurelia aurita_,"
  _Phil. Trans._, pt. ii., 1878.

Lastly, returning to the researches of Drs. Hertwig, these authors
compare the nervous system of the naked-eyed with that of the
covered-eyed Medusæ, with the view of indicating the points which
show the latter to be less developed than the former. These points
are, that in the nerve-centres of the covered-eyed Medusæ there
are no true ganglion-cells, or only very few; that the mass of the
central nervous system is very small; and that the centralization
of the nervous system is less complete in the one group than in the
other. In their memoir these authors further supply much interesting
information touching the structure of the sense-organs in various
species of Medusæ; but it seems scarcely necessary to extend the
present _résumé_ of their work by entering into this division of
their subject.

In a later publication, entitled "Der Organismus der Medusen und
seine Stellung zur Keimblättertheorie," Drs. Hertwig treat of
sundry features in the morphology of the Medusæ which are of great
theoretical importance; but here again it would unduly extend the
limits of the present treatise if I were to include all the ground
which has been so ably cultivated by these industrious workers.

It will presently be seen in how striking a manner all the
microscopical observations to which I have now briefly alluded are
confirmed by the physiological observations--or, more correctly, I
might say that the microscopical observations, in so far as they
were concerned with demonstrating the existence of nerve-tissue in
the Medusæ, were forestalled by these physiological experiments;
for, with the exception of Professor Haeckel's work on Geryonidæ,
they were all of later publication. But in matters of scientific
inquiry mere priority is not of so much importance as it is too often
supposed to be. Thus, in the present instance, no one of the workers
was in any way assisted by the publications of another. In each case
the work was independent and almost simultaneous.

The remark just made applies also to the only research which still
remains to be mentioned. This is the investigation undertaken and
published by Professor Eimer.[5] He began, like myself, by what in
the next chapter I call the "fundamental observation" on the effects
of excising the nerve-centres, and from this basis he worked both at
the physiology and the morphology of the neuro-muscular tissues. In
point of time, I was the first to make the fundamental observation,
and he was the first to publish it. The sundry features in which our
subsequent investigations agreed, and those in which they differed, I
shall mention throughout the course of the following pages.

  [5] "Die Medusen physiologisch und morphologisch auf ihr
  Nervensystem untersucht" (Tübingen, 1878).

       *       *       *       *       *

I shall now conclude this chapter by giving a brief account of those
general principles of the physiology of nerve and muscle with which
it is necessary to be fully acquainted, in order to understand the
course of the following experiments.

Nerve-tissue, then, universally consists of two elementary
structures, viz. very minute nerve-cells and very minute
nerve-fibres. The fibres proceed to and from the cells, so in some
cases serving to unite the cells with one another, and in other cases
with distant parts of the animal body. Nerve-cells are usually found
collected together in aggregates, which are called nerve-centres or
ganglia, to and from which large bundles of nerve-fibres come and go.

To explain the _function_ of nerve-tissue, it is necessary to begin
by explaining what physiologists mean by the term "excitability."
Suppose that a muscle has been cut from the body of a freshly killed
animal; so long as it is not interfered with in any way, so long
will it remain quite passive. But every time a stimulus is supplied
to it, either by means of a pinch, a burn, an electrical shock, or
a chemical irritant, the muscle will give a single contraction in
response to every stimulation. And it is this readiness of organic
tissues to respond to a suitable stimulus that physiologists
designate by the term "excitability."

Nerves, no less than muscles, present the property of being
excitable. If, together with the excised muscle, there had been
removed from the animal's body an attached nerve, every time any
part of this nerve is stimulated the attached muscle will contract
as before. But it must be carefully observed that there is this
great difference between these two cases of response on the part of
the muscle--that while in the former case the muscle responded to
a stimulus _applied directly to its own substance_, in the latter
case the muscle responded to a stimulus applied at a _distance from
its own substance_, which stimulus was then _conducted_ to the
muscle by the nerve. And in this we perceive the characteristic
function of _nerve-fibres_, viz. that of conducting stimuli to a
distance. The function of nerve-_cells_ is different, viz. that of
accumulating nervous energy, and, at fitting times, of discharging
this energy into the attached nerve-fibres. The nervous energy,
when thus discharged, acts as a stimulus to the nerve-fibre; so
that if a muscle is attached to the end of a fibre, it contracts
on receiving this stimulus. I may add that when nerve-cells are
collected into ganglia, they often appear to discharge their energy
spontaneously; so that in all but the very lowest animals, whenever
we see apparently _spontaneous_ action, we infer that ganglia are
probably present. Lastly, another important distinction must be borne
in mind--the distinction, namely, which is to be drawn between muscle
and nerve. A stimulus applied to a nerveless muscle can only course
through the muscle by giving rise to a visible wave of contraction,
which spreads in all directions from the seat of disturbance as from
a centre. A nerve, on the other hand, conducts the stimulus without
sensibly moving or undergoing any change of shape. Now, in order not
to forget this distinction, I shall always speak of muscle-fibres
as conveying a _visible_ wave of _contraction_, and of nerve-fibres
as conveying an _invisible_, or _molecular_, wave of _stimulation_.
Nerve-fibres, then, are functionally distinguished from
muscle-fibres--and also from protoplasm--by displaying the property
of conducting invisible, or molecular, waves of stimulation from
one part of an organism to another, so establishing physiological
continuity between such parts, _without the necessary passage of
waves of contraction_.




CHAPTER II.

FUNDAMENTAL EXPERIMENTS.


The naked-eyed Medusæ are very much smaller in size than the
covered-eyed, and as we shall find that the distribution of their
nervous elements is somewhat different, it will be convenient to use
different names for the large umbrella-shaped part of a covered-eyed
Medusæa and the much smaller though corresponding part of a
naked-eyed Medusa. The former, therefore, I shall call the umbrella,
and the latter the swimming-bell, or nectocalyx. In each case alike
this portion of the animal performs the office of locomotion,
and it does so in the same way. I have already said that this
mushroom-like organ, which constitutes the main bulk of the animal,
is itself mainly constituted of thick transparent and non-contractile
jelly, but that the whole of its concave surface is lined with a
thin sheet of muscular tissue. Such being the structure of the
organ, the mechanism whereby it effects locomotion is very simple,
consisting merely of an alternate contraction and relaxation of the
entire muscular sheet which lines the cavity of the bell. At each
contraction of this muscular sheet the gelatinous walls of the bell
are drawn together; the capacity of the bell being thus diminished,
water is ejected from the open mouth of the bell backwards, and the
consequent reaction propels the animal forwards. In these swimming
movements, systole and diastole follow one another with as perfect a
rhythm as they do in the beating of a heart.


_Effects of excising the entire Margins of Nectocalyces._

Confining our attention under this heading to the naked-eyed Medusæ,
I find that the following proposition applies to every species of the
group which I have as yet had the opportunity of examining: _Excision
of the extreme margin of a nectocalyx causes immediate, total, and
permanent paralysis of the entire organ_. Nothing can possibly be
more definite than in this highly remarkable effect. I have made
hundreds of observations upon various species of the naked-eyed
Medusæ, of all ages and conditions of freshness, vigour, etc.; and I
have constantly found that if the experiment be made with ordinary
care, so as to avoid certain sources of error presently to be named,
the result is as striking and decided as it is possible to desire.[6]
Indeed, I do not know of any case in the animal kingdom where the
removal of a centre of spontaneity causes so sudden and so complete
a paralysis of the muscular system, there being no subsequent
movements or twitchings of a reflex kind to disturb the absolute
quiescence of the mutilated organism. The experiment is particularly
beautiful if performed on Sarsia; for the members of this genus
being remarkably active, the death-like stillness which results from
the loss of so minute a portion of their substance is rendered by
contrast the more surprising.

  [6] I have only met with one individual exception. This occurred
  in a specimen of _Staurophora laciniata_, where, after removal
  of the entire margin, three centres of spontaneity were found to
  remain in the sheet of contractile tissue lining the nectocalyx.

From this experiment, therefore, I conclude that in the margin of
all the species of naked-eyed Medusæ which I have as yet had the
opportunity of examining, there is situated an intensely localized
system of centres of spontaneity, having at least for one of its
functions the origination of impulses, to which the contractions of
the nectocalyx, under ordinary circumstances, are exclusively due.
And this obvious deduction is confirmed (if it can be conceived to
require confirmation) by the behaviour of the severed margin. This
continues its rhythmical contractions with a vigour and a pertinacity
not in the least impaired by its severance from the main organism,
so that the contrast between the perfectly motionless swimming-bell
and the active contractions of the thread-like portion which has
just been removed from its margin is as striking a contrast as it is
possible to conceive. Hence it is not surprising that if the margin
be left _in situ_, while other portions of the swimming-bell are
mutilated to any extent, the spontaneity of the animal is not at
all interfered with. For instance, if the equator of any individual
belonging to the genus Sarsia (Fig. 1) be cut completely through,
so that the swimming-bell instead of being closed at the top is
converted into an open tube, this open tube continues its rhythmical
contractions for an indefinitely long time, notwithstanding that
the organism so mutilated is, of course, unable to progress. Thus
it is a matter of no consequence how small or how large a portion
of contractile tissue is left adhering to the severed margin of
the swimming-bell; for whether this portion be large or small, the
locomotor centres contained in the margin are alike sufficient to
supply the stimulus to contraction. Indeed, if only the tiniest piece
of contractile tissue be left adhering to a single marginal body
cut out of the bell of Sarsia, this tiny piece of tissue, in this
isolated state, will continue its contractions for hours, or even for
days.


_Effects of excising the entire Margins of Umbrellas._

Turning now to the covered-eyed division of the Medusæ, I find, in
all the species I have come across, that excision of the margins of
umbrellas produces an effect analogous to that which is produced by
excision of the margins of swimming-bells. There is an important
difference, however, between the two cases, in that the paralyzing
effect of the operation on umbrellas is neither so certain nor so
complete as it is on swimming-bells. That is to say, although in the
majority of experiments such mutilation of umbrellas is followed
by immediate paralysis, this is not invariably the case; so that
one cannot here, as with the naked-eyed Medusæ, predict with any
great confidence what will be the immediate result of any particular
experiment. Further, although such mutilation of an umbrella is
usually followed by a paralysis as sudden and marked as that which
follows such mutilation of a swimming-bell, the paralysis of the
former differs from the paralysis of the latter, in that it is very
seldom _permanent_. After periods varying from a few seconds to half
an hour or more, occasional weak and unrhythmical contractions begin
to manifest themselves, or the contractions may even be resumed with
but little apparent change in their character and frequency. The
condition of the animal before the operation, as to general vigour,
etc., appears to be one factor in determining the effect of the
operation; but this is very far from being the only factor.

Upon the whole, then, although in the species of covered-eyed Medusæ
which I have as yet had the opportunity of examining, the effects
which result from excising the margins of umbrellas are such as
to warrant me in saying that the main supply of locomotor centres
appears to be usually situated in that part of these organs, these
effects are nevertheless such as to compel me at the same time to
conclude that the locomotor centres of the covered-eyed Medusæ are
more diffused or segregated than are those of the naked-eyed Medusæ.
Lastly, it should be stated that all the species of covered-eyed
Medusæ resemble all the species of naked-eyed Medusæ, in that their
members will endure any amount of section it is possible to make
upon any of their parts other than their margins without their
spontaneity being in the smallest degree affected.


_Effects of excising Certain Portions of the Margins of Nectocalyces._

The next question which naturally presents itself is as to whether
the locomotor centres are equally distributed all round the margin
of a swimming organ, or situated only, or chiefly, in the so-called
marginal bodies. To take the case of the naked-eyed Medusæ first,
it is evident that in most of the genera, in consequence of the
intertentacular spaces being so small, it is impossible to cut
out the marginal bodies (which are situated at the bases of the
tentacles) without at the same time cutting out the intervening
portions of the margin. The genus Sarsia, however, is admirably
adapted (as a glance at Fig. 1 will show) for trying the effects of
removing the marginal bodies without injuring the rest of the margin,
and _vice versâ_. The results of such experiments upon members of
this genus are as follow.

Whatever be the condition of the individual operated upon as to
freshness, vigour, etc., it endures excision of three of its marginal
bodies without suffering any apparent detriment; but in most cases,
as soon as the last marginal body is cut out, the animal falls to
the bottom of the water quite motionless. If the subject of the
experiment happens to be a weakly specimen, it will, perhaps, never
move again: it has been killed by something very much resembling
nervous shock. On the other hand, if the specimen operated upon be
one which is in a fresh and vigorous state, its period of quiescence
will probably be but short; the nervous shock, if we may so term
it, although evidently considerable at the time, soon passes away,
and the animal resumes its motions as before. In the great majority
of cases, however, the activity of these motions is conspicuously
diminished.

The effect of excising all the marginal tissue from between the
marginal bodies and leaving the latter untouched, is not so definite
as is the effect of the converse experiment just described. Moreover,
allowance must here be made for the fact that in this experiment the
principal portion of the "veil"[7] is of necessity removed, so that
it becomes impossible to decide how much of the enfeebling effect
of the section is due to the removal of locomotor centres from the
swimming-bell, and how much to a change in the merely mechanical
conditions of the organ. From the fact, however, that excision of the
entire margin of Sarsia produces total paralysis, while excision of
the marginal bodies alone produces merely partial paralysis, there
can be no doubt that both causes are combined. Indeed, it has been
a matter of the greatest surprise to me how very minute a portion
of the intertentacular marginal tissue is sufficient, in case of
this genus, to animate the entire swimming-bell. Choosing vigorous
specimens of Sarsia, I have tried, by cutting out all the margin
besides, to ascertain how minute a portion of intertentacular tissue
is sufficient to perform this function, and I find that this portion
may be so small as to be quite invisible without the aid of a lens.

  [7] See Fig. 1.

From numerous observations, then, upon Sarsia, I conclude that in
this genus (and so, from analogy, probably in all the other genera of
the true Medusæ) locomotor centres are situated in every part of the
extreme margin of a nectocalyx, but that there is a greater supply of
such centres in the marginal bodies than elsewhere.


_Effects of excising Certain Portions of the Margin of Umbrellas._

Coming now to the covered-eyed Medusæ, I find that the concentration
of the locomotor centres of the margin into the marginal bodies, or
lithocysts, is still more decided than it is in the case of Sarsia.
Taking Aurelia aurita as a type of the group, I cannot say that,
either by excising the lithocysts alone or by leaving the lithocysts
_in situ_ and excising all the rest of the marginal tissue, I have
ever detected the slightest indications of locomotor centres being
present in any part of the margin of the umbrella other than the
eight lithocysts; so that all the remarks previously made upon this
species, while we were dealing with the effects of excising the
entire margin of umbrellas, are equally applicable to the experiment
we are now considering, viz. that of excising the lithocysts alone.
In other words, but for the sake of symmetry, I might as well have
stated at the first that in the case of the covered-eyed Medusæ all
the remarkable paralyzing effects which are obtained by excising
the entire margin of an umbrella are obtained in exactly the same
degree by excising the eight lithocysts alone; the intermediate
marginal tissue, in the case of these Medusæ, is totally destitute of
locomotor centres.


_Effects upon the Manubrium of excising the Margin of a Nectocalyx or
Umbrella._

Lastly, it must now be stated, and always borne in mind, that neither
in the case of naked nor covered-eyed Medusæ does excision of the
margin of a swimming organ produce the smallest effect upon the
manubrium. For hours and days after the former, in consequence of
this operation, has ceased to move, the latter continues to perform
whatever movements are characteristic of it in the unmutilated
organism--indeed, these movements are not at all interfered with even
by a complete severance of the manubrium from the rest of the animal.
In many of the experiments subsequently to be detailed, therefore, I
began by removing the manubrium, in order to afford better facilities
for manipulation.


_Summary of Chapter._

With a single exception to hundreds of observations upon six widely
divergent genera of naked-eyed Medusæ, I find it to be uniformly
true that removal of the extreme periphery of the animal causes
instantaneous, complete, and permanent paralysis of the locomotor
system. In the genus Sarsia, my observations point very decidedly to
the conclusion that the principal locomotor centres are the marginal
bodies, but that, nevertheless, every microscopical portion of the
intertentacular spaces of the margin is likewise endowed with the
property of originating locomotor impulses.

In the covered-eyed division of the Medusæ, I find that the
_principal_ seat of spontaneity is the margin, but that the latter is
not, as in the naked-eyed Medusæ, the exclusive seat of spontaneity.
Although in the vast majority of cases I have found that excision
of the margin impairs or destroys the spontaneity of the animal for
a time, I have also found that the paralysis so produced is very
seldom of a permanent nature. After a variable period occasional
contractions are usually given, or, in some cases, the contractions
may be resumed with but little apparent detriment. Considerable
differences, however, in these respects are manifested by different
species, and also by different individuals of the same species.
Hence, in comparing the covered-eyed group as a whole with the
naked-eyed group as a whole, so far as my observations extend,
I should say that the former resembles the latter in that its
representatives usually have their main supply of locomotor centres
situated in their margins, but that it differs from the latter in
that its representatives usually have a greater or less supply of
their locomotor centres scattered through the general contractile
tissue of their swimming organs. But although the locomotor centres
of a covered-eyed Medusa are thus, generally speaking, more diffused
than are those of a naked-eyed Medusæ, _if we consider the organism
as a whole_, the locomotor centres in the _margin_ of a covered-eyed
Medusa are _less_ diffused than are those in the _margin_ of a
naked-eyed Medusæ. In no case does the excision of the margin of
a swimming organ produce any effect upon the movements of the
manubrium.




CHAPTER III.

EXPERIMENTS IN STIMULATION.


_Mechanical, Chemical, and Thermal Stimulation._

So far as my observations extend, I find that all Medusæ, after
removal of their locomotor centres, invariably respond to every
kind of stimulation. To take the case of Sarsia as a type, nothing
can possibly be more definite than is the single sharp contraction
of the mutilated nectocalyx in response to every nip with the
forceps. The contraction is precisely similar to the ordinary ones
that are performed by the unmutilated animal; so that by repeating
the stimulus a number of times, the nectocalyx, with its centres
of spontaneity removed, may be made to progress by a succession of
contractions round and round the vessel in which it is contained,
just as a frog, with its cerebral hemispheres removed, may be made to
hop along the table in response to a succession of stimulations.[8]

  [8] In the case of the covered-eyed Medusæ, however, the
  paralyzed umbrella sometimes responds to a single stimulation
  with two, and more rarely with three contractions, which are
  separated from one another by an interval of the same duration as
  the normal diastole of the unmutilated animal.

Different species of Medusæ exhibit different degrees of irritability
in responding to stimuli; but in all the cases I have met with
the degree of irritability is remarkably high. Thus, I have seen
responsive contractions of the whole umbrella follow upon the
exceedingly slight stimulus caused by a single drop of sea-water
let fall upon the irritable surface from the height of one inch. As
regards chemical stimulation, dilute spirit or other irritant, when
dropped on the paralyzed swimming organ of Aurelia aurita, often
gives rise to a whole series of rhythmical pulsations, the systoles
and diastoles following one another at about the same rate as is
observable in the normal swimming motions of the unmutilated animal.

It is somewhat difficult, in the case of paralyzed swimming organs,
to prove the occurrence of a contraction in response to thermal
stimulation, from the fact that while these tissues are not nearly
so sensitive to this mode of excitation as might be anticipated,
they are, as just observed, extraordinarily sensitive to mechanical
excitation. It therefore becomes difficult to administer the
appropriate thermal stimulus without at the same time causing a
sufficient mechanical disturbance to render it doubtful to which
of the stimuli the response is due. This may be done, however, by
allowing a few drops of heated sea-water to run over the excitable
surface while it is exposed to the air. In this and in other ways I
have satisfied myself that the paralyzed tissues of swimming organs
respond to sudden elevations of temperature.


_Luminous Stimulation._

It is interesting to note that, in the case of some of the naked-eyed
Medusæ, the action of light as a stimulus is most marked and
unfailing. In the case of Sarsia, for instance, a flash of light let
fall upon a living specimen almost invariably causes it to respond
with one or more contractions. If the animal is vigorous and swimming
freely in water, the effect of a momentary flash thrown upon it
during one of the natural pauses is immediately to originate a bout
of swimming. But if the animal is non-vigorous, or if it be removed
from the water and spread flat upon an object-glass, it usually gives
only one contraction in response to every flash. There can thus be
no doubt that a sudden transition from darkness to light acts upon
Sarsia as a stimulus, and this even though the transition be but of
momentary duration. The question therefore arises as to whether the
stimulus consists in the presence of light, or in the occurrence
of the sudden transition from darkness to light and from light to
darkness. To answer this question, I tried the converse experiment
of placing a vigorous specimen in sunlight, waiting till the middle
of one of the quiescent stages in the swimming motions had come
on, and then suddenly darkening. In no case, however, under these
circumstances, did I obtain any response; so that I cannot doubt it
is the light per se, and not the sudden nature of the transition
from darkness to light, which in the former experiment acted as
the stimulus. Indeed, the effect of the converse experiment just
described is rather that of inhibiting contractions; for, if the
sunlight be suddenly shut off during the occurrence of a swimming
bout, it frequently happens that the quiescent stage immediately
sets in. Again, in a general way, it is observable that Sarsia are
more active in the light than they are in the dark, the comparative
duration of the quiescent stages being less in the former than in
the latter case. Light thus appears to act towards these animals as
a constant stimulus. Lastly, it may be stated that when the marginal
bodies of Sarsia are removed, the swimming-bell, although still able
to contract spontaneously, no longer responds to luminous stimulation
of any kind or degree. But if only one body be left _in situ_, or if
the severed margin alone be experimented upon, the same unfailing
response may be obtained to luminous stimulation as that which is
obtained from the entire animal.

The fact last mentioned indicates that the marginal bodies are organs
of special sense, adapted to respond to luminous stimulation; or,
in more simple words, that they perform the functions of sight. Now
it has long been thought more or less probable that these marginal
bodies are rudimentary or incipient "eyes," but hitherto the
supposition has not been tested by experiment, and was therefore of
no more value than a guess.[9] The guess in this instance, however,
happens to have been correct, as the results of the following
experiments will show.

  [9] As Professor Haeckel observes in his monograph already
  alluded to, "Die Deutung der Sinnesorgane niederer Thiere gehört
  ohne Zweifel zu den schwierigsten Objecten der vergleichenden
  Physiologie und ist der grössten Unsicherheit unterworfen. Wir
  sind gewohnt, die von den Wirbelthieren gewonnenen Anschauungen
  ohne Weiteres auch auf die wirbellosen Thiere der verschiedenen
  Kinese zu übertragen und bei diesen analoge Sinnesempfindungen
  anzunehmen als wir selbst besitzen ... Noch weniger freilich als
  die von den meisten Autoren angenommene Deutung der Randbläschen
  unserer Medusen als Gehörorgane kann die von Agassiz und Fritz
  Müller vertretene Ansicht befriedigen, dass dieselben Augen
  seien.... Alle diese Verhältnisse sind mit der Deutung der
  Concretion als 'Linse' und des sie umschliessenden Sinnesganglion
  als 'Sehnerv' durchaus unvereinbar."

  It may not be unnecessary to say that, although the simple
  experiment above described effectually proves that the marginal
  bodies have a visual function to subserve, we are not for this
  reason justified in concluding that these are so far specialized
  as organs of sight as to be precluded from ministering to any
  other sense.

Having put two or three hundred Sarsia into a large bell-jar, I
completely shut out the daylight from the room in which the jar
was placed. By means of a dark lantern and a concentrating lens,
I then cast a beam of light through the water in which the Sarsia
were swimming. The effect upon the latter was most decided. From all
parts of the bell-jar they crowded into the path of the beam, and
were most numerous at that side of the jar which was nearest to the
light. Indeed, close against the glass they formed an almost solid
mass, which followed the light wherever it was moved. The individuals
composing this mass dashed themselves against the glass nearest
the light with a vigour and determination closely resembling the
behaviour of moths under similar circumstances. There can thus be no
doubt about Sarsia possessing a visual sense.

The method of ascertaining whether this sense is lodged in the
marginal bodies was, of course, extremely simple. Choosing a dozen of
the most vigorous specimens, I removed all the marginal bodies from
nine, and placed these, together with the three unmutilated ones, in
another bell-jar. After a few minutes the mutilated animals recovered
from their nervous shock, and began to swim about with tolerable
vigour. I now darkened the room, and threw the concentrated beam of
light into the water as before. The difference in the behaviour of
the mutilated and of the unmutilated specimens was very marked. The
three individuals which still had their marginal bodies sought the
light as before, while the nine without their marginal bodies swam
hither and thither, without paying it any regard.

A further question, however, still remained to be determined. The
pigment spot of the marginal body in Medusæ is, as L. Agassiz
observed, placed _in front of_ the presumably nervous tissue, and
for this reason he naturally enough suggested that if the marginal
body has a visual function to perform, the probability is that the
rays by which the organ is affected are the heat-rays lying beyond
the range of the visible spectrum. Accordingly I brought a heated
iron, just ceasing to be red, close against the large bell-jar which
contained the numerous specimens of Sarsia; but not one of the latter
approached the heated metal.

From these observations, therefore, I conclude that in Sarsia
the faculty of appreciating luminous rays is present, and that
this faculty is lodged exclusively in the marginal bodies; while
from observations conducted on the covered-eyed Medusæ, I have
come to the same conclusion respecting them. But although I have
tested many species of naked-eyed Medusæ besides Sarsia, I have
obtained indications of response to luminous stimulation only
in the case of one other. This is a species which I have called
Tiaropsis polydiademata, and the response which it gives to luminous
stimulation is even more marked and decided than that which is given
by Sarsia; for a sudden exposure to sunlight causes this animal to
go into a kind of tonic spasm, the whole of the nectocalyx being
drawn together in a manner resembling cramp. Now, in one remarkable
particular this response to luminous stimulation on the part of
Tiaropsis polydiademata differs from that given by Sarsia tubulosa;
and the difference consists in the fact that, while with Sarsia the
period of latency (_i.e._ the time between the fall of the stimulus
and the occurrence of the response) is, so far as the eye can judge,
as instantaneous in the case of response to luminous stimulation as
it is in the case of response to any other kind of stimulation, such
is far from being true with Tiaropsis polydiademata. The period of
latency in the last-named species is, so far as the eye can judge,
quite as instantaneous as it is in the case of Sarsia, when the
stimulus employed is other than luminous; but in response to light,
the characteristic spasm does not take place till slightly more than
a second has elapsed after the first occurrence of the stimulus.
As this extraordinary difference in the latent period exhibited by
the same animal towards different kinds of stimuli appeared to me
a matter of considerable interest, I was led to reflect upon the
probable cause of the difference. It occurred to me that the only
respect in which luminous stimulation of the Medusæ differed from
all the other modes of stimulation I had employed consisted in
this--that, as proved by my previous experiments on Sarsia, which I
repeated on Tiaropsis, luminous stimulation directly affected the
ganglionic tissues. Now, as in Tiaropsis polydiademata luminous
stimulation differed from all the other modes of stimulation in
giving rise to an immensely longer period of latency, I seemed
here to have an index of the difference between the rapidity of
the response to stimuli by the contractile and by the ganglionic
tissues respectively. The next question, therefore, which presented
itself was as to whether the enormous length of time occupied by
the process of stimulation in the ganglia was due to any necessity
on the part of the latter to accumulate the stimulating influence
prior to originating a discharge, or to an immensely lengthened
period of latent stimulation manifested by the ganglia under the
influence of light.[10] This is an interesting question, because if
such a lengthened period of latent stimulation occurs in this case,
it would stand in curious antithesis to the very short period of
latent stimulation manifested by the contractile tissues of the same
animal under other modes of irritation. To test these alternative
hypotheses, I employed the very simple method of first allowing a
continuous flood of light to fall suddenly on the Medusa, and then
noting the time at which the responsive spasm first began. This
time, as already stated, was slightly more than one second. I next
allowed the animal to remain for a few minutes in the dark to recover
shock, and, lastly, proceeded to throw in single flashes of light
of measured duration. I found that unless the flash of light was of
slightly more than one second in its duration, no response was given;
that is to say, the minimal duration of a flash required to produce
a responsive spasm was just the same as the time during which a
continuous flood of light required to operate in order to produce a
similar spasm. From this, therefore, I conclude that the enormously
long period of latent excitation in response to luminous stimuli was
not, properly speaking, a period of latent excitation at all; but
that it represented the time during which a certain summation of
stimulating influence was taking place in the ganglia, which required
somewhat more than a second to accumulate, and which then caused
the ganglia to originate an abnormally powerful discharge. So that
in the action of light upon the ganglionic matter of this Medusa we
have some analogy to its action on certain chemical compounds in
this respect, that, just as in the case of those compounds which
light is able to split up, a more or less lengthened exposure to its
influence is necessary in order to admit of the summating influence
of its vibrations on the molecules, so in the ease of this ganglionic
material, the decomposition which is effected in it by light, and
which terminates in an explosion of nervous energy, can only be
effected by a prolonged exposure of the unstable material to the
summating influence of the luminous vibrations. Probably, therefore,
we have here the most rudimentary type of a visual organ that is
possible; for it is evident that if the ganglionic matter were a
very little more stable than it is, it would altogether fail to be
thrown down by the luminous vibrations, or would occupy so long a
time in the process that the visual sense would be of no use to its
possessor. How great is the contrast between the excitability of
such a sense-organ and that of a fully evolved eye, which is able
to effect the needful molecular changes in response to a flash as
instantaneous as that of lightning.

  [10] The period of latent stimulation merely means the time
  after the occurrence of an excitation during which a series of
  physiological processes are taking place, which terminate in
  a contraction; so that, whether the excitation is of a strong
  or of a weak intensity, the period of latent stimulation is
  not much affected. The above question, therefore, was simply
  this--Does the prolonged delay on the part of these ganglia, in
  responding to light, represent the time during which the series
  of physiological processes are taking place in response to an
  adequate stimulus, or does it represent the time during which
  light requires to act before it becomes an adequate stimulus?

With regard to luminous stimulation, it is only necessary further to
observe that responses were given equally well to direct sunlight,
diffused daylight, and to light reflected from a mirror inclined at
the polarizing angle. It must also be stated that responses are given
to any of the luminous rays of the spectrum when these are employed
separately; but that neither the non-luminous rays beyond the red,
nor those beyond the violet, appear to exert the smallest degree of
stimulating influence.


_Electrical Stimulation._

All the excitable parts of all the Medusæ which I have examined are
highly sensitive to electrical stimulation, both of the constant and
of the induced current.

Exploration with needle-point terminals and induction shocks of
graduated strength showed that certain parts or tracts of the
nectocalyx are more sensitive than others. The most sensitive
parts are those which correspond with the distribution of the main
nerve-trunks, _i.e._ round the margin of the nectocalyx and along
the course of the radial tubes. The external or convex surface of a
nectocalyx or umbrella is totally insensitive to stimulation, and
the same statement applies to the whole thickness of the gelatinous
substance to which the neuro-muscular sheet is attached.

In all other respects the excitable tissues of the Medusæ in their
behaviour towards electrical stimulation conform to the rules which
are followed by excitable tissues of other animals. Thus, closure
of the constant current acts as a stronger stimulus than does
opening of the same, while the reverse is true of the induction
shock; and exhaustion supervenes under the influence of prolonged
excitation. Moreover, I have obtained evidence of that polarization
of nerve-tissues under the influence of the constant current,
which is known to physiologists by the term "electrotonus;" but
it would be somewhat tedious to detail the evidence on this head
which I have already published elsewhere.[11] Tetanus produced by
faradaic electricity is not of the nature of an apparently single
and prolonged contraction, but that of a number of contractions
rapidly succeeding one another, as in the case of the heart under
similar excitation. This at least applies to Sarsia. In the case of
Aurelia, tolerably strong faradization does cause a more or less
well-pronounced tetanus. The continuity of the spasm is, indeed,
often interrupted by momentary and partial relaxations. These
interruptions are the more frequent the weaker the current; so that,
at a certain strength of the latter, the tetanus is of a wild and
tumultuous nature; but with strong currents the spasm is tolerably
uniform. That in all cases the tetanus is due to summation of
contractions may be very prettily shown by the following experiment.
An Aurelia is cut into a spiral strip, and all its lithocysts are
removed; single induction-shocks are then thrown in with a key at
one end of the strip--every shock, of course, giving rise to a
contraction wave. If these shocks are thrown in at a somewhat fast
rate, two contraction waves may be made at the same time to course
along the spiral strip, one behind the other; but if the shocks are
thrown in at a still faster rate, so as to diminish the distance
between any two successive waves, a point soon arrives at which
every wave mounts upon its predecessor; and if several waves be thus
made to coalesce, the whole strip becomes thrown into a state of
persistent contraction.

  [11] See "Croonian Lectures," 1875. _Philosophical Transactions_,
  vol. 166, part I. pp. 284-6.

In this way sustained tetanus, or single contraction waves, or any
intermediate phase, may be instantly produced at pleasure. In such
experiments, moreover, it is interesting to observe that, no matter
how long the strip be, whatever disturbances are set up at one end
are faithfully transmitted to the other. For instance, if an Aurelia
be cut into the longest possible strip with a remnant of the disk
left attached at one end, as represented in Fig. 11 (p. 70), then
all the peculiar time relations between successive contractions
which are intentionally caused by the experimenter at one end of the
strip, are afterwards accurately reproduced at the other end of the
strip by the remainder of the disk. Now, as this fact is observable
however complex these time relations may be, and however rapidly
the successive stimuli are thrown in, I think it is a point of some
interest that these complicated relations among rapidly succeeding
stimuli do not become blended during their passage along the thirty
or forty inches of contractile tissue. The fact, of course, shows
that the rate of transmission is so identical in the case of all
the stimuli originated, that the sum of the effects of any series
of stimuli is delivered at the distal end of the strip, with all
its constituent parts as distinct from one another as they were at
starting from the proximal end of the strip.


_Period of Latency, and Summation of Stimuli._

I shall now give an account of my experiments in the period of
latency and the summation of stimuli. To do this, I must first
describe the method which I adopted in order to obtain a graphic
record of the movements which were given in response to the stimuli
supplied. As Aurelia aurita is the only species on which I have
experimented in this connection, my remarks under this heading will
be confined to it alone.

The method by which I determined the latent period in the case of
this species was as follows. A basin containing the Medusa was
filled to its brim with sea-water, and placed close beside a smoked
cylinder, which, while it lay in a horizontal position, could be
rotated at a known rate. The Aurelia[12] was placed with its concave
aspect uppermost, and an inch or two below the surface of the water.
The animal was held firmly in this position by means of a pair of
compasses thrust through it and forced into a piece of wood, which
was fastened to the bottom of the basin. The legs of the compasses
were provided with india-rubber sliders, so that by placing these
under the Medusa, the latter might be kept at any elevation in the
water which might be desired. The manubrium and lithocysts were now
removed, and also a segment of the umbrella. A light straw was then
forced through the gelatinous substance of the umbrella in a radial
direction, and close to the gap caused by the missing segment. The
other, or free, end of this straw was firmly joined to a capillary
glass rod, which was suitably bent to avoid contact with the rim of
the basin, and also to write on the smoked cylinder. If the straw
was not itself sufficient to support the weight of the capillary
rod, a small cross-piece of cork might easily be tied to it, so as
to add to the flotation power. A part of the excitable tissue was
now raised above the surface of the water by means of a disk of cork
placed beneath it, and on the part of the tissue thus raised there
were placed a pair of platinum electrodes. These electrodes proceeded
from an electro-magnetic apparatus, which was arranged in such a
way that every time the current in it was opened or closed, it gave
an induction shock and moved a lever at the same instant of time.
This lever was therefore placed upon the cylinder immediately above
the capillary glass-writer which proceeded from the Medusa, care
being taken to place the two writers in the same line, parallel to
the axis of the cylinder. Such being the arrangement, the cylinder
was rotated, and thus two parallel lines were marked upon it by the
two writers. If the current was now closed, an induction shock was
thrown into the tissue at the same instant that the electro-magnet
writer recorded the fact, by altering its position on the cylinder.
Again, as soon as the paralyzed Medusa responded to the induction
shock, the radii of the vacant segment were drawn apart, and in
this way a curve was obtained by the other writer on the rotating
cylinder. Now, by afterwards dropping a perpendicular line from
the point at which the electro-magnet writer changed its position,
to the parallel line made by the other writer, and then measuring
the distance between the point of contact and the point on the
last-mentioned line on which the curve began, the period of latent
stimulation was determined. A glance at Figs. 3 and 4 (p. 55)
will render this description clear to any one who is not already
acquainted with the method, when it is stated that the upper line is
a record of the movements of the electro-magnet writer, and the lower
line that of the movements of the other writer. It will be observed
that the point _a_ in the upper line marks the point at which the
induction shock was thrown in; so that by first producing the
perpendicular till it meets the lower line at _b_, and then measuring
the distance between the point _b_ and the point _c_, at which the
curve in the lower line first begins, the latent period (_b c_) is
determined--the time occupied by the rotation of the cylinder from
_b_ to _c_ being known.

  [12] It may here be stated that in all the experiments on
  stimulation subsequently to be detailed, there is no difference
  to be observed between the behaviour of an entire swimming organ
  deprived of its ganglia, and that of a portion of any size which
  may be separated from it.

_Summation of Stimuli._--In this way I have been able to ascertain
the period of latent stimulation in Aurelia aurita with accuracy.
It must be stated at the outset, however, that the period is subject
to great variations under certain varying conditions, so that we can
only arrive at a just estimation of it by understanding the nature
of the modifying causes. To take the simplest cause first, suppose
that the paralyzed Aurelia has been left quiet for several minutes in
sea-water at forty-five degrees, and that it is then stimulated by
means of a single induction shock; the responsive contraction will
be comparatively feeble with a very long period of latency, viz.
five-eighths of a second. If another shock of the same intensity be
thrown in as soon as the tissue has relaxed, a somewhat stronger
contraction, with a somewhat shorter latent period, will be given.
If the process is again repeated, the response will be still more
powerful, with a still shorter period of latency; and so on, perhaps,
for eight or ten stages, when the maximum force of contraction of
which the tissue is capable will have been attained, while the period
of latency will have been reduced to its minimum. This period is
three-eighths of a second, or, in some cases, slightly less.

Now, we have here a remarkable series of phenomena, and as it is
a series which never fails to occur under the conditions named,
I append tracings to give a better idea of the very marked and
striking character of the results. The first tracing (Fig. 2) is a
record of the successive increments of the responses to successive
induction shocks of the same intensity, thrown in at three seconds'
intervals--the cylinder being stationary during each response, and
rotated a short distance with the hand during each interval of repose.

  [Illustration: Fig. 2.]

The second tracing (Figs. 3 and 4) is a record of the difference
between the lengths of the latent period, and also between the
strengths of the contraction, in the case (_a_) of the first of
such a series of responses (Fig. 3), and (_b_) of the last of such
a series (Fig. 4). From these tracings it will be manifest, without
further comment, how surprising is the effect of a series of stimuli;
first, in _arousing_ the tissue, as it were, to increased _activity_,
and, second, in developing a state of _expectancy_.

In accordance with the now customary terminology, I shall call such
a series of responses as are given in Fig. 2 a "staircase." Such a
staircase has a greater number of steps in it if caused by a weak
current (compare Figs. 2 and 5); and if the strength of the current
be suddenly increased after the maximum level of a staircase has
been reached by using a feeble current, this level admits of being
slightly raised (see Fig. 5). Moreover, I find that a stimulus,
which at the bottom of a staircase is of less than minimal intensity,
is able, at the top of a staircase, to give rise to a contraction
of very nearly maximum intensity. That is to say, by employing
an induction stimulus of slightly less than minimal intensity in
relation to the original irritability of the tissue, no response is
given to the first two or three shocks of a series; but at the third
or fourth shock a slight response is given, and from that point
onward the staircase is built up as usual. This was the case in the
experiment of which Fig. 2 is a record, no response having been given
to the first two shocks.

  [Illustration: Fig. 3.]

  [Illustration: Fig. 4.]

  [Illustration: Fig. 5.]

With regard to this interesting staircase action, two questions
naturally present themselves. In the first place, we are anxious
to know whether the arousing effect which is so conspicuous in
a staircase series is due to the occurrence of the previous
_stimulations_, or to that of previous _contractions_; and, in the
next place, we should like to know whether, during the _natural_
rhythm of the tissue, each contraction exerts a beneficial influence
on its successor, analogous to that which occurs in the case of
contractions which are due to _artificial_ stimuli. To answer the
first of these questions, therefore, I built up a staircase in the
ordinary way, and then suddenly transferred the electrodes to the
opposite side of the umbrella from that on which they rested while
constructing the staircase. On now throwing in another shock at this
part of the contractile tissue so remote from the part previously
stimulated, the response was a maximum response. Similarly, if the
electrodes were transferred in the way just described, not after the
maximum effect had been attained, but at any point during the process
of constructing a staircase, the response given to the next shock was
of an intensity to make it rank as the next step in the staircase.
Hence, shifting the position of the electrodes in no wise modifies
the peculiar effect we are considering; and this fact conclusively
proves that the effect is a general one, pervading the whole mass of
the contractile tissue, and not confined to the locality which is
the immediate seat of stimulation. Nevertheless, this fact does not
tend to prove that the staircase effect depends on the process of
contraction as distinguished from the process of stimulation, because
the wave of the former process must always precede that of the
latter. But, on the other hand, in this connection it is of the first
importance to remember the fact already stated, viz. that a current
which at the beginning of a series of stimulations is of slightly
less than minimal intensity presently becomes minimal, and eventually
of much more than minimal intensity--a staircase being thus built up
of which the first observable step (or contraction) only occurs in
response to the second, third, or even fourth shock of the series.
This fact conclusively proves that the staircase effect, at any
rate at its commencement, depends on the process of stimulation as
distinguished from that of contraction; for it is obvious that the
latter process cannot play any part in thus constructing what we may
term the invisible steps of a staircase.

To answer the second of the above questions, I placed an Aurelia with
its concave surface uppermost, and removed seven of its lithocysts;
I then observed the spontaneous discharge of the remaining one, and
found it to be conspicuous enough that, after the occurrence of one
of the natural pauses (if this were of sufficient duration), the
first contraction was feeble, the next stronger, the next still
stronger, and so on, till the maximum was attained. This natural
staircase action admits of being very prettily shown in another
way. If a tolerably large Aurelia is cut into a spiral strip of
small width and great length, and if all the lithocysts are removed
except one at one end of the strip, it may be observed that,
after the occurrence of a natural pause, the first discharge only
penetrates perhaps about a quarter of the length of the strip, the
next discharge penetrates a little further, the next further, and so
on, till finally the contraction waves pass from end to end. On now
removing the ganglion, waiting a few minutes, and then stimulating
with successive induction shocks, the same progressive penetration is
observable as that which previously took place with the ganglionic
stimulation. Lastly, the identity of natural and artificial
staircase action may be placed beyond all doubt by an experiment in
which the effects of induction shocks and of ganglionic discharges
are combined. To accomplish this, all the lithocysts save one are
removed, and a staircase is then built up in the ordinary way by
successive induction shocks. It will now occasionally happen that the
ganglion originates a discharge during the process of constructing
the staircase, which is being built up by the artificial stimuli;
when this happens the resulting contraction takes its proper rank
in the series, and this at whatever point the natural contraction
happens to come in.

Thus, then, to summarize and conclude these observations, we have
seen that if a single stimulation, whether of a natural or artificial
kind, is supplied to the excitable tissues of a jelly-fish, a short
period, called the period of latency, will elapse, and then the
jelly-fish will give a single weak contraction. If, as soon as the
tissue has relaxed, the stimulation is again repeated, the period of
latency will be somewhat shorter, and will be followed by a somewhat
stronger contraction. Similarly, if the stimulation is repeated a
third time, the period of latency will be still shorter, and the
ensuing contraction still stronger. And so on up to nine or ten
times, when the period of latency will be reduced to its _minimum_,
while the force of the contraction will be raised to its _maximum_;
so that in the jelly-fish, the effect of a series of excitations
supplied at short intervals from one another is that of both arousing
the tissue into a state of increased _activity_, and also of
producing in it a state of greater _expectancy_. We have, moreover,
seen that this effect depends upon the repetition of the process of
stimulation, and not upon that of the process of contraction.

Now, effects very similar to these have been found to occur in the
case of the excitable plants by Dr. Burdon-Sanderson; in the case of
the frog's heart by Dr. Bowditch; and in the case of reflex action
of the spinal cord by Dr. Stirling. Indeed, the only difference in
this respect between these four tissues, so widely separated from one
another in the biological scale, consists in the _time_ which may be
allowed to elapse between the occurrence of the successive stimuli,
in order to produce this so-called summating effect of one stimulus
upon its successor: the _memory_, so to speak, of the heart-tissue
for the occurrence of a former stimulus being longer than the memory
of the jelly-fish tissue; while the memory of the latter is longer
than that of the plant tissue. And I may here add that even in our
own organization we may often observe the action of this principle of
the summation of stimuli. For instance, we can tolerate for a time
the irritation caused by a crumb in the larynx, but very rapidly
the sense of irritation accumulates to a point at which it becomes
impossible to avoid coughing. And similarly with tickling generally,
the convulsive reflex movements to which it gives rise become more
and more incontrollable the longer the stimulation is continued,
until they reach a _maximum_ point, where, in persons susceptible
to this kind of stimulation, the muscular action passes completely
beyond the power of the will. Lastly, I may further observe, what I
do not think has ever been observed before, that even in the domain
of psychology the action of this principle admits of being clearly
traced. We find it, for instance, in the rhythmical waves of emotion
characteristic of grief, and at the other extreme we find it in the
case of the ludicrous. We can endure for a short time, without giving
any visible response, the psychological stimulation which is supplied
by a comical spectacle; but if the latter continues sufficiently
long in a sufficiently ludicrous manner, our appropriate emotion
rapidly runs up to a point at which it becomes uncontrollable, and
we burst into an explosion of ill-timed laughter. But in this case
of psychological tickling, as in the previous case of physiological
tickling, some persons are much more susceptible than others.
Nevertheless, there can be no doubt that from the excitable tissues
of a plant, through those of a jelly-fish and a frog, up even to the
most complex of our psychological processes, we have in this recently
discovered principle of the summation of stimuli a very remarkable
uniformity of occurrence.


_Effects of Temperature on Excitability._

I shall now conclude this chapter with a brief statement of the
effects of temperature on the excitability of the Medusæ; and before
stating my results, I may observe that in all my experiments in this
connection I changed the temperature of the Medusæ by drawing off
the water in which they floated with a siphon, while at the same
time I substituted water of a different temperature from that which
I thus abstracted. In this way, without modifying any of the other
conditions to which the animals were exposed, I was able to observe
the effects of changing the temperature alone.

With regard to the effect of temperature on the latent period of
stimulation, the following table, setting forth the results of one
among several experiments, explains itself.

Period of latent stimulation of the deganglionated tissues of Aurelia
aurita as affected by temperature:--

    -------------------------------+-------------------------------
     Temperature of water (Fahr.). | Period of latent stimulation.
    -------------------------------+-------------------------------
                  70°              |           1/5 second
                  50°              |           1/3 second
                  35°              |           2/5 second
                  20°              |           1/2 second
    -------------------------------+-------------------------------

In the case of each observation, several shocks were administered
before the latent period was taken, in order to decrease this period
to its _minimum_ by the staircase action. When this is not done, the
latent period at 20° may be as long as 1-1/5 seconds; but soon after
this irritability disappears.

The extraordinary sluggishness of the latent period at very low
temperatures is fully equalled by the no less extraordinary
sluggishness of the contraction.

In order to render apparent the degree in which both these effects
are produced, I here append a pair of tracings which were procured
from the same piece of tissue when exposed to the different
temperatures named. (N.B.--The seconds are wrongly marked in Fig. 7;
they ought to be the same as in Fig. 6.)

  [Illustration: Fig. 6.]

  [Illustration: Fig. 7.]

I may as well state here that in water at all temperatures, within
the limits where responses to stimuli are given at all, the staircase
action admits of being equally well produced; but in order to
procure the _maximum_ effect for any given temperature, the rate at
which the successive stimuli are thrown in must be quicker in warm
than in cold water.




CHAPTER IV.

EXPERIMENTS IN SECTION OF COVERED-EYED MEDUSÆ.

   _Amount of Section which the Neuro-muscular Tissues of the
   Medusæ will endure without suffering Loss of their Physiological
   Continuity._


The extent to which the neuro-muscular tissues of the Medusæ may
be mutilated without undergoing destruction of their physiological
continuity is in the highest degree astonishing. For instance, to
begin with the covered-eyed Medusæ, I shall briefly state three modes
of section, the results of which serve to show in a striking manner
the fact in question.

  [Illustration: Fig. 8.]

  [Illustration: Fig. 9.]

The annexed woodcuts represent the umbrella of Aurelia aurita, with
its manubrium cut off at the base, and the under or concave surface
of the umbrella exposed to view, shewing in the centre the ovaries,
and radiating from them the branched system of nutrient tubes. The
umbrella when fully expanded, as here represented, is about the size
of a soup plate, and, as previously stated, all the marginal ganglia
are aggregated in the eight marginal bodies or lithocysts. Therefore
if the reader will imagine the first of the diagrams (Fig. 8) to
be overspread with a disc of muslin, the fibres and mesh of which
are finer than those of the finest and closest cobweb, and if he
will imagine the mesh of these fibres to start from these marginal
ganglia, he will gain a tolerably correct idea of the lowest nervous
system in the animal kingdom. Now, suppose that seven of these eight
ganglia are cut out, the remaining one then continues to supply its
rhythmical discharges to the muscular sheet of the bell, the result
being, at each discharge, two contraction waves, which start at the
same instant, one on each side of the ganglion, and which then course
with equal rapidity in opposite directions, and so meet at the point
of the disc which is opposite to the ganglion. Suppose, now, a number
of radial cuts are made in the disc according to such a plan as this
(Fig. 9), wherein every radial cut deeply overlaps those on either
side of it. The contraction waves which now originate from the
ganglion must either become blocked and cease to pass round the disc,
or they must zigzag round and round the tops of these overlapping
cuts. Now, remembering that the passage of these contraction waves
is presumably dependent on the nervous network progressively
distributing the ganglionic impulse to the muscular fibres, surely
we should expect that two or three overlapping cuts, by completely
severing all the nerve-fibres lying between them, ought to destroy
the functional continuity of these fibres, and so to block the
passage of the contraction wave. Yet this is not the case; for even
in a specimen of Aurelia so severely cut as the one here represented,
the contraction waves, starting from the ganglion, continued to
zigzag round and round the entire series of sections.

  [Illustration: Fig. 10.]

The second mode of section to which I have alluded is as follows
(Fig. 10). The central circle (_x_) stands for an open space cut out
of the umbrella; the outer circle indicates the margin of the animal,
with all lithocysts save one (_l_) removed; and the median circular
line represents a cut. It will be seen that the effect of this cut is
almost completely to sever the mass of tissue at _z_ from the rest of
the umbrella, the only connection between them being the narrow neck
of tissue at _z_. Yet, in the case to which I refer, the contraction
waves emanating from _l_ passed in the directions represented by
the arrows without undergoing any appreciable loss of vigour. Upon
completing the circular cut at _z_, the ring of tissue (_y z_) became
totally paralyzed, while the outer circle, of course, continued
its contractions as before. Now, the neck of tissue at _z_ measured
only one-eighth of an inch across, while the ring of tissue (_y z_),
when cut through and straightened out upon the table, measured one
inch across and sixteen inches in length; that is to say, sixteen
square inches of tissue derived its impulse to vigorous contractions
through a channel one-eighth of an inch wide, notwithstanding that
the latter was situated at the furthest point of the circle from the
discharging lithocyst which the form of the section rendered possible.

  [Illustration: Fig. 11.]

Lastly, the third mode of section is represented in the next cut.
Here seven of the marginal ganglia having been removed as before,
the eighth one was made the point of origin of a circumferential
section, which was then carried round and round the bell in the
form of a continuous spiral--the result, of course, being this long
ribbon-shaped strip of tissue with the ganglion at one end and the
remainder of the swimming-bell at the other. Well, as before, the
contraction-waves always originated at the ganglion; but now they
had to course all the way along the strip until they arrived at its
other extremity; and, as each wave arrived at that extremity, it
delivered its influence into the remainder of the swimming-bell,
which thereupon contracted. Now, in this experiment, when the spiral
strip is only made about half an inch broad, it may be made more than
a yard long before all the bell is used up in making the strip; and
as nothing can well be imagined as more destructive of the continuity
of a nerve-plexus than this spiral mode of section must be, we cannot
but regard it as a very remarkable fact that the nerve-plexus should
still continue to discharge its function. Indeed, so remarkable
does this fact appear, that to avoid accepting it we may well
feel inclined to resort to another hypothesis, namely, that these
contraction-waves do not depend for their passage on the nervous
network at all, but that they are of the nature of the muscle-waves,
or of the waves which we see in undifferentiated protoplasm,
where all parts of the mass being equally excitable and equally
contractile, however severely we cut the mass, as long as we do not
actually divide it, contraction-waves will pass throughout the whole
mass. But this very reasonable hypothesis of the contraction-waves in
the Medusæ being possibly nothing more than muscle-waves is negatived
by other facts, which I shall now proceed to state.

In the first place, if a number of experiments be tried in any of
the three modes of section above described, it will be found that
extreme variations are manifested as regards the degree of tolerance.
In the spiral mode of section, for instance, it will sometimes happen
that the contraction-wave will become blocked when the contractile
strip is only an inch long, while in other cases (as in the one
represented) the wave will continue to pass through a strip more
than thirty times that length; and between these two extremes there
are all possible grades of tolerance. Now it seems to me that if the
tissue through which these contraction-waves pass is supposed (as far
as they are concerned) to be of a functionally homogeneous nature,
no reason can be assigned why there should be such great differences
in the endurance of the tissue in different individual cases; while,
if we suppose that the passage of the contraction-waves is more or
less dependent on the functional activity of the nervous plexus which
we know from microscopical examination to be present, we encounter
no such difficulty; for it is almost to be expected that in some
cases it would happen that important nerves would soon be encountered
by the section, while in other cases it would happen that such
nerves would escape the section for a longer distance. It is indeed
incredible that any one nerve should happen to pursue a spiral course
twice or thrice round the umbrella, and at the same time happen to be
concentric with the course pursued by the section; but, as we shall
presently see, such an hypothesis as this is not necessary to account
for the facts.

Again, in the second place, strong evidence that the passage of
the contraction-waves is dependent on the functional activity
of the nervous plexus, and therefore that they are not merely
muscle-waves, is furnished by the fact that at whatever point in
a spiral strip which is being progressively elongated by section
the contraction-wave becomes blocked, the blocking is sure to
take place _completely_ and _exclusively_ at that point. Now, as
I have tried this experiment a great number of times, and always
tried it by carefully feeling the way round (_i.e._ only making a
very short continuation of the cut after the occurrence of each
contraction-wave, and so very precisely localizing the spot at
which the contraction-wave ceased to pass), I can scarcely doubt
that in every case the blocking is caused by the cutting through of
nerves.[13]

  [13] In a highly interesting paper recently published by Dr. W.
  H. Gaskell, F.R.S., on "The Innervation of the Heart" (_Journ.
  of Physiol._, vol. iv. p. 43, _et seq._), it is shown that the
  experiments in section thus far described yield strikingly
  similar results when performed upon the heart of the tortoise and
  the heart of the skate. Dr. Gaskell inclines to the belief that
  in these cases the contraction-waves are merely muscle-waves.
  There is one important fact, however, which even here seems to
  me to indicate that the propagation of the wave is at least in
  some measure dependent on nervous conduction. This fact is, that
  after a contraction-wave has been blocked by the activity of a
  spiral or other form of section, it may again be made to force a
  passage under the influence of vagus stimulation. Moreover, in
  a paper still more recently published by Drs. Brunton and Cash
  on "Electrical Stimulation of the Frog's Heart" (_Proc. Roy.
  Soc._, vol. xxxv., No. 227, p. 455, _et seq._) it is remarked,
  "Another interesting consideration is, whether the stimulus
  which each cavity of the heart transmits to the succeeding one
  consists in the propagation of an actual muscular wave, or in
  the propagation of an impulse along the nerves. The observations
  of Gaskell have given very great importance to the muscular wave
  occurring in each cavity of the heart of cold-blooded animals
  as a stimulus to the contraction of the next succeeding cavity.
  Our observations appear to us to show that, while this in an
  important factor, it is not the only one in the transmission of
  stimuli.... We consider that stimuli are also propagated from one
  chamber of the heart to another through nervous channels: thus we
  find that irritation of the venus sinus will sometimes produce
  simultaneous contractions of the auricle and ventricle, instead
  of the ventricular beat succeeding the auricular in the ordinary
  way. This we think is hardly consistent with the hypothesis, that
  a stimulus consists of the propagation of a muscular wave only
  from the auricle to the ventricle."

  [Illustration: Fig. 12.]

But, lastly, the strongest evidence in favour of this view as
afforded by the following observations. At the beginning of this
treatise I stated that the distinguishing function of nerve consists
in its power of conducting stimuli to a distance, irrespective of
the passage of a contraction-wave; and I may here add that when
a stimulus so conducted reaches a ganglion, or nerve-centre, it
causes the ganglion to discharge by so-called "reflex action."
Now, this distinguishing function of nerve can plainly be proved
to be present in the Medusæ. For instance, take such a section of
Aurelia as this one (Fig. 12), wherein the bell has been cut into
the form of a continuous parallelogram of tissue with the ovaries
and a single remaining ganglion at one end. (The cuts interposed in
the parallelogram may, for the present, be disregarded.) Now, if
the end marked _a_ of the neuro-muscular sheet most remote from
the ganglion be gently brushed with a camel's hair brush--_i.e._
too gently to start a responsive contraction-wave--the ganglion at
the other end will shortly afterwards discharge, as shown by its
starting a contraction-wave at its own end of the parallelogram,
_b_; thus proving that the stimulus caused by brushing the tissue at
the other end, _a_, must have been conducted all the way along the
parallelogram to the terminal ganglion, _b_, so causing the terminal
ganglion to discharge by reflex action. Indeed, in many cases, the
passage of this nervous wave of stimulation admits of being _seen_.
For the numberless tentacles which fringe the margin of Aurelia are
more highly excitable than is the general contractile tissue of the
bell; so that on brushing the end _a_ of the parallelogram remote
from the ganglion, the tentacles at this end respond to the stimulus
by a contraction, then those next in the series do the same, and so
on--a wave of contraction being thus set up in the tentacular fringe,
the passage of which is determined by the passage of the nervous wave
of stimulation in the superjacent nervous network. This tentacular
wave is in the illustration represented as having traversed nearly
half the whole distance to the terminal ganglion, and when it reaches
that ganglion it will cause it to discharge by reflex action, so
giving rise to a visible wave of muscular contraction passing in the
direction _b a_, opposite to that which the nervous or tentacular
wave had previously pursued. Now this tentacular wave, being an
optical expression of a passage of a wave of stimulation, is a sight
as beautiful as it is unique; and it affords a first-rate opportunity
of settling this all-important question, namely, Will this conductile
or nervous function prove itself as tolerant towards a section of
the tissue as the contractile or muscular function has already
proved itself to be? For, if so, we shall gain nothing on the side
of simplicity by assuming that the _contraction_-waves are merely
muscle-waves, so long as the _conduction_ or _undoubtedly nervous_
waves are equally able to pass round sections interposed in their
path. Briefly, then, I find that the nervous waves of stimulation
are quite as able to pass round these interposed sections as are the
waves of contraction. Thus, for instance, in this specimen (Fig. 12),
the tentacular wave of stimulation continued to pass as before, even
after I had submitted the parallelogram of tissue to the tremendously
severe form of section which is represented in the illustration;
and this fact, in my opinion, is one of the most important that has
been brought to light in the whole range of invertebrate physiology.
For what does it prove? It proves that the distinguishing function
of nerve, where it first appears upon the scene of life, admits of
being performed vicariously to almost any extent by all parts of
the same tissue-mass. If we revert to our old illustration of the
muslin as representing the nerve-plexus, it is clear that, however
much we choose to cut the sheet of muslin with such radial or spiral
sections as are represented in the illustrations, one could always
trace the threads of the muslin with a needle round and round the
disc, without once interrupting the continuity of the tracing; for
on coming to the end of a divided thread, one could always double
back on it and choose another thread which might be running in the
required direction. And this is what we are now compelled to believe
takes place in the fibres of this nervous network, if we assume that
these visible fibres are the only conductile elements which are
present. Whenever a stimulus wave reaches a cut, we must conclude
that it doubles back and passes into the neighbouring fibres, and so
on, time after time, till it succeeds in passing round and round any
number of overlapping cuts.

This is, no doubt, as I have already observed, a very remarkable
fact; but it becomes still more so when we have regard to the
histological researches of Professor Schäfer on the structural
character of this nerve-plexus. For these researches have shown that
the nerve-fibres which so thickly overspread the muscular sheet of
Aurelia do not constitute a true plexus, but that each fibre is
comparatively short and nowhere joins with any of the other fibres;
that is to say, although the constituent fibres of the network
cross and recross one another in all directions--sometimes, indeed,
twisting round one another like the strands of a rope--they can
never be actually seen to join, but remain anatomically insulated
throughout their length. So that the simile by which I have
represented this nervous network--the simile, namely, of a sheet
of muslin overspreading the whole of the muscular sheet--is, as a
simile, even more accurate than has hitherto appeared; for just as
in a piece of muslin the constituent threads, although frequently
meeting one another, never actually coalesce, so in the nervous
network of Aurelia, the constituent fibres, although frequently in
contact, never actually unite.

Now, if it is a remarkable fact that in a fully differentiated
nervous network the constituent fibres are not improbably capable
of vicarious action to almost any extent, much more remarkable does
this fact become when we find that no two of these constituent
nerve-fibres are histologically continuous with one another. Indeed,
it seems to me we have here a fact as startling as it is novel. There
can scarcely be any doubt that _some_ influence is communicated from
a stimulated fibre _a_ to the adjacent fibre _b_ at the point where
these fibres come into close apposition. But what the nature of the
process may be whereby a disturbance in the excitable protoplasm of
_a_ sets up a sympathetic disturbance in the anatomically separate
protoplasm of _b_, supposing it to be really such--this is a question
concerning which it would as yet be premature to speculate. But I
think it may be well for physiologists to keep awake to the fact that
a process of this kind probably takes place in the case of these
nerve-fibres. For it thus becomes a possibility which ought not to
be overlooked, that in the fibres of the spinal cord, and in ganglia
generally, where histologists have hitherto been unable to trace any
anatomical or structural continuity between cells and fibres, which
must nevertheless be supposed to possess physiological or functional
continuity--it thus becomes a possibility that in these cases no such
anatomical continuity exists, but that the physiological continuity
is maintained by some such process of physiological induction as
probably takes place among the nerve-fibres of Aurelia.[14]

  [14] That it can scarcely be _electrical induction_ would seem
  to be shown by the fact that such effects can only be produced
  on nerves by strong currents, and also by the fact that the
  saline tissues of the swimming-bell must short-circuit any feeble
  electrical currents as soon as they are generated.

I have now to detail another fact of a very puzzling nature, but
one which is certainly of importance. When the spiral section
is performed on Aurelia aurita, and when, as a consequence, the
contraction-waves which traverse the elongating strip become at some
point suddenly blocked, if the section be stopped at this point it
not unfrequently happens that after a time the blocking suddenly
ceases, the contraction-waves again passing from the strip into
the umbrella as freely as they did before the section reached the
point at which the blocking occurred. The time required for this
restoration of physiological continuity is very variable, the limits
being from a few seconds to an hour or more; usually, however, it
is from two to four minutes. This process of re-establishing the
physiological connections, although rapid, is not so instantaneous
as is that of their destruction by section. In general it requires
the passage of several contraction-waves before the barrier to the
passage of succeeding waves is completely thrown down. The first
wave which effects a passage appears to have nearly all its force
expended in overcoming the barrier, the residue being only sufficient
to cause a very feeble, and sometimes almost imperceptible,
contraction of the umbrella. The next wave, however, passes across
the barrier with more facility, so that the resulting contraction
of the umbrella is more decided. The third wave, again, causes a
still more pronounced contraction of the umbrella; and so on with
all succeeding waves, until every trace of the previous blocking has
disappeared. When this is the case, it generally happens that the
strip will again admit of being elongated for a short distance before
a blocking of the contraction-waves again supervenes. Sometimes it
will be found that this second blockage will also be overcome, and
that the strip will then admit of being still further elongated
without the passage of the waves being obstructed; and so on
occasionally for three or four stages.

The same series of phenomena may be shown in another way. If
a contractile strip of tolerable length be obtained, with the
waves passing freely from one end to the other, and if a series
of parallel and equidistant cuts be made along one side of the
strip, in a direction at right angles to the length, and each cut
extending two-thirds of the breadth of the strip, the chances are
in favour of the contraction-waves being wholly unaffected by the
sections, however numerous these may be. But now, if another series
of parallel and equidistant cuts of the same length as the first
ones, and alternating with them, be made along the other side
of the contractile strip, the result is, of course, a number of
interdigitating cuts; and it is easy to see that by beginning with a
few such cuts and progressively increasing their number, a point must
somewhere be reached at which one portion will become physiologically
separated from the rest. The amount of such section, however, which
contractile strips will sometimes endure is truly surprising. I
have seen such a strip twenty inches long by one and a half inches
wide with ten such cuts along each side, and the contraction-waves
passing without impediment from end to end. But what I wish more
especially to observe just now is, that by progressively increasing
the number of such interdigitating cuts up to the point at which the
contraction-wave is blocked, and then leaving the tissue to recover
itself, in many cases it will be observed that the blocking is sooner
or later overcome; that on then adding more interdigitating cuts the
blocking again supervenes; but that in time it may again be overcome,
and so on. It is, however, comparatively rare to find cases in which
blocking is overcome twice or thrice in succession.

Section is not the only way in which blocking of waves may be caused
in contractile strips. I find that pressure, even though very gentle,
exerted on any part of a strip causes a blocking of the waves at that
part, even after the pressure has been removed. If the pressure has
been long continued, after its removal the blocking will probably
be permanent; but if the pressure has been only of short duration,
the blocking will most likely be transitory. Even the slight strains
caused by handling a contractile strip in the air are generally
followed by a decrease in the rate of the waves, and sometimes by
their being completely blocked. Other methods by which the passage of
waves in contractile strips admits of being blocked will be alluded
to farther on.

Now, in all these cases of temporary blocking we must conclude that
when the contraction-waves succeed in at last forcing a passage,
some structural change has taken place in the tissue at the
region of injury, corresponding with the functional change of the
re-establishment of physiological continuity. The waves previously
stopped at a certain point of section or otherwise, after beating for
a time on the physiological barrier, are at last able to throw down
the barrier, and thenceforward to proceed on their way unhindered.
What, then, is the nature of the structural change which has taken
place?

In the early days of this research, before the presence of a
nerve-plexus had been proved histologically, I argued in favour of
such a plexus on the grounds furnished by many of the foregoing
experiments; and at a lecture given in the Royal Institution I
ventured to say that if a careful investigation of the histology of
these tissues should fail to show the plexus which the result of
those experiments required me to assume, we should still be compelled
to suppose that the plexus was present, although not sufficiently
differentiated to admit of being seen. I further ventured to
suggest that in this event the facts just stated might be taken to
substantiate the theory of Mr. Herbert Spencer on the genesis of
nerve-tissue in general. This theory is that which supposes incipient
conductile tissues, or rudimentary nerve-fibres, to be differentiated
from the surrounding contractile tissues, or homogeneous protoplasm,
by a process of integration which is due simply to use; so that just
as water continually widens and deepens the channel through which
it flows, so molecular or nervous waves of stimulation, by always
flowing through the same tissue-tracts, tend ever more and more to
excavate for themselves functionally differentiated lines of passage.

Such being Mr. Spencer's theory, I applied it hypothetically to the
above facts in the words which I may here quote.

"As the successive waves beat rhythmically on the area of
obstruction, more or less of the molecular disturbances must every
time be equalized through these lines of discharge, which from the
first have been almost sufficient to maintain the physiological
continuity of the tissue. Therefore, according to the hypothesis,
every wave that is blocked imposes upon these particular lines of
discharge a much higher degree of functional activity than they were
ever before required to exercise; and this greater activity causing
in its turn greater permeability, a point will sooner or later arrive
at which these lines of discharge, from having been _almost_, become
_quite_ able to draft off sufficient molecular motion, or stimulating
influence, to carry on the contraction-waves beyond the areas of
previous blocking. In such instances, of course, we should expect
to find what I always observed to be the case, viz. that the first
contraction-wave which passes the barrier is only very feeble, the
next stronger, the next still stronger, and so on, according as the
new passage becomes more and more permeable by use, until at last
the contraction-waves pour over the original barrier without any
perceptible diminution of their force. In some cases, by exploring
with graduated stimuli and needle-point terminals, I was able to
ascertain the precise line through which this eruption of stimulating
influence had taken place."

I have now to state the effect upon this hypothesis which in my
opinion has been produced by the histological proof that the plexus
in question is composed of fully differentiated nerves. Briefly,
then, I think that the hypothesis still holds to the extent of
being the only one available whereby to explain the facts; but
there is this great difference, viz. that the hypothesis need not
now be applied to the genesis of nerve-tissue out of comparatively
undifferentiated contractile tissue, but rather to the increasing of
the functional activity of already well-differentiated nerve-tissue.
In other words, we have not now to suppose that nerve-tissue is
formed _de novo_ in the region of blocking; but, in my opinion,
we still have to suppose that the nerve-fibres which were already
there have their functional capabilities so far improved by the
greater demand imposed upon them, that whereas at first they were
not able, eventually they became able to draft off enough molecular
disturbance to carry on a stimulus adequate to cause a muscular
contraction. It will be observed that the difference thus expressed
is one of considerable importance; for now the facts cease to lend
any countenance to Mr. Spencer's theory touching the formation
of nerves out of protoplasm, or other contractile material. They
continue, however, to countenance his views touching the improvement
of functional capacity which nerve-fibres, when already formed,
undergo by use; and this, which is in itself an important matter, is
the point with which I was mainly concerned in the lecture of the
Royal Institution just alluded to. For, as I then observed, in this
theory of nerve-fibres becoming more and more functionally developed
by use, we probably have a physical explanation, which is as full and
complete as such an explanation can ever be, of the genesis of mind.
"For from the time that intelligence first dawned upon the scene of
life, whenever a new relation had to be established in the region of
mind, it could only be so established in virtue of some new line of
discharge being excavated through the substance of the brain. The
more often this relation had to be repeated in the mind, the more
often would this discharge require to take place in the brain, and
so the more easy would every repetition of the process become....
Thus it is, according to the theory, that there is always a precise
proportion between the constancy with which any relations have
been joined together during the history of intelligence, and the
difficulty which intelligence now experiences in trying to conceive
of such relations as disjoined. Thus it is that, even during the
history of an individual intelligence, 'practice makes perfect,' by
frequently repeating the needful stimulations along the same lines
of cerebral discharge, so rendering the latter ever more and more
permeable by use. Thus it is that a child learns its lessons by
frequently repeating them; and thus it is that all our knowledge is
accumulated."[15]

  [15] I have associated the above theory of nerve-genesis with the
  name of Mr. Spencer, because it occupies so prominent a place in
  his "Principles of Psychology." But from what I have said in the
  text, I think it is clear that the theory, as presented by Mr.
  Spencer, consists of two essentially distinct hypotheses--the
  one relating to the formation of nerve-tissue out of protoplasm,
  and the other to the increase of functional capacity in a
  nerve-fibre by use (a third hypothesis of Mr. Spencer relating to
  the formation of ganglion-tissue does not here concern us). The
  latter hypothesis, however, ought not to be associated with Mr.
  Spencer's name without explaining that it has likewise occurred
  to other writers, the first of which, so far as I can ascertain,
  was Lamarck, who says, "Dans toute action, le fluide des nerfs
  qui la provoque, subit un mouvement de déplacement qui y donne
  lieu. Or, lorsque cette action a été plusieurs fois répétée, il
  n'est pas douteux que le fluide qui l'a exécutée, ne se soit
  frayé une route, qui lui devient alors d'autant plus facile à
  parcourir, qu'il l'a effectivement plus souvent franchie, et
  qu'il n'ait lui-même une aptitude plus grand à suivre cette route
  frayée que celles qui le sont moins." ("Phil. Zool.," tom ii. pp.
  318-19.)


_Rate of Transmission of Stimuli._

The rate at which contraction-waves traverse spiral strips of Aurelia
is variable. It is largely determined by the length and width of
the strip; so that the best form of strip to use for the purpose
of ascertaining the _maximum_ rate is one which I shall call the
circular strip. A circular strip is obtained by first cutting out
the central bodies (_i.e._ manubrium and ovaries), and then, with a
single radial cut, converting the animal from the form of an open
ring to that of a continuous band. I distinguish this by the name
"circular" band or strip, because the two ends tend to preserve their
original relative positions, so giving the strip more or less of a
circular form. Such a strip has the advantage of presenting all the
contractile tissue of the swimming-bell in one continuous band of
the greatest possible width, and is therefore the form of strip that
yields the _maximum_ rate at which contraction-waves are able to
pass. The reason why the _maximum_ rate should be the one sought for
is because this is the rate which must most nearly approximate the
natural rate of contraction-waves in the unmutilated animal. This
rate, at the temperature of the sea and with vigorous specimens, I
find to be eighteen inches per second.

In a circular strip the rate of the waves is uniform over the whole
extent of the strip; so that the time of their transit from one
point to another varies directly as the length of the strip. But
on now narrowing such a strip, although the rate is thus slowed,
the relation between the narrowing and the slowing is not nearly so
precise as to admit of our saying that the rate varies inversely as
the width. The following figure will serve to show the proportional
extent to which the passage of contraction-waves is retarded by
narrowing the area through which they pass:--

  [Illustration: Fig. 13.

  Time from end to end of a circular strip ...

  Time after width has been reduced to one-half ...

  Time after width has been reduced to one-quarter ...

  Time after width has been reduced to one-eighth ...

  Fig. 14.

  In such experiments it generally happens, as here represented,
  that reducing the width of a circular strip by one-half produces
  no effect, or only a slight effect, on the rate, while further
  narrowing to the degree mentioned produces a conspicuous
  effect. I may also state that if, as occasionally happens, the
  immediate effect of narrowing a circular strip to one-half is to
  temporarily block the contraction-waves, when the latter again
  force their passage, their rate is slower than it was before. It
  seems as if the more pervious tissue tracts having been destroyed
  by the section, the less pervious ones, though still able to
  convey the contraction-wave, are not able to convey it so rapidly
  as were the more pervious tracts.]

  [Illustration: Fig. 14.]

In order to ascertain whether certain zones of the circular
contractile sheet in all individuals habitually convey more of
the contractile influence than do other zones, I tried a number
of experiments in the following form of section. Having made a
circular strip, I removed all the lithocysts save one, and then cut
the strip as represented in Fig. 14. On now stimulating the end
_a_, or on watching the lithocyst there discharge, the resulting
contraction-wave would be observed to bifurcate at _b_, and then pass
on as two separate waves through the zones, _b_ _c_, _b_ _d_. Now,
as these two waves were started at the same instant of time, they
ran, as it were, a race in the two zones, and in this way the eye
could judge with perfect ease which wave occupied the shortest time
in reaching its destination. This experiment could be varied by again
bisecting each of these two zones, thus making four zones in all, and
four waves to run in each race. A number of experiments of this kind
showed me that there is no constancy in the relative conductivity
of the same zones in different individuals. In some instances, the
waves occupy less time in passing through the zone _b_ _c_ than in
passing through the zone _b_ _d_; in other instances, the time in the
two zones is equal; and, lastly, the converse of the first-mentioned
case is of equally frequent occurrence. Very often the waves become
blocked in _b_ _c_, while they continue to pass in _b_ _d_, and
_vice versâ_. Now, all these various cases are what we might expect
to occur, in view of the variable points at which contraction-waves
become blocked in spiral strips, etc.; for if the contractile tissues
are not functionally homogeneous, and if the relatively pervious
conductile tracts are not constant as to their position in different
individuals, the results I have just described are the only ones
that could be yielded by the experiments in question. Considering,
however, that in these experiments the central zones are not so long
as the peripheral zones, I think it may fairly be said that the
conductile power of the latter is greater than that of the former;
for, otherwise, the above experiments ought to yield a large majority
of races won by the waves that course through the central zones,
and this is not the case. Indeed, it is surprising how often the
race is, as it were, neck and neck, thus showing that the relative
conductivity of all the zones is precisely adjusted to their relative
lengths; and forasmuch as in the unmutilated animal this adjustment
must clearly serve the purpose of securing to the contraction-wave
a passage of uniform rate over the whole radius of the umbrella,
I doubt not that, if it were possible to perform the race-course
section without interrupting any of the lines of conduction-tissue,
neck and neck races would be of invariable occurrence.

Interdigitating cuts, as might be expected, prolong the time of
contraction-waves in their passage through the tissue in which
the cuts are interposed. For example, in a spiral strip measuring
twenty-six inches in length, the time required for the passage of
a contraction-wave from one end to the other is represented by the
line _a_ _b_ in the annexed woodcut. But after twenty interdigitating
cuts had been interposed, ten on each side of the strip, the time
increased to _c_ _d_, the line _e_ _f_ representing one second. And
more severe forms of section are, of course, attended with a still
more retarding influence.

  [Illustration: Fig. 15.]

The effects of temperature on the rate of contraction-waves are
very striking. For instance, in a rather narrow strip measuring
twenty-eight inches long and one and a half inches wide, the
following variations in rate occurred:--

    -------------------------+-------------------------------
      Temperature of water.  |  Time occupied in passage of
                             |    contractile waves.
    -------------------------+-------------------------------
              26°            |          4 seconds.
              32°            |          3 seconds.
              42°            |          2-2/5 seconds.
              65°            |          2 seconds.
              75°            |          1-3/5 seconds.
              85°            |          Blocked.


Or, adopting again the graphic method of statement, these variations
may be represented as follows:--

  [Illustration: Fig. 16.

  26°......

  32°......

  42°......

  65°......

  75°......

  85°......

  One second ...]

Submitting a contractile strip to slight strains has also the
effect of retarding the rate of the waves while they pass through
the portions of the strip which have been submitted to strain. The
method of straining which I adopted was to pass my finger below
the strip, and then, by raising my hand, to bring a portion of
the strip slightly above the level of the water. The irritable or
contractile surface was kept uppermost, and therefore suffered a
gentle strain; for the weight of the tissue on either side of the
finger made the upper surface somewhat convex. By passing the finger
all the way along the strip in this way, the latter might be gently
strained throughout its entire length, the degree of straining
being determined by the height out of the water to which the tissue
was raised. Of course, if the strip is too greatly strained, the
contraction-waves become blocked altogether, as described above;
but shortly before this degree of straining was reached, I could
generally observe that the rate of the waves was diminished. To give
one instance, a contractile strip measuring twenty-two inches had
the rate of its waves taken before and after straining of the kind
described. The result was as follows:--

  [Illustration: Fig. 17. Before straining ...

  After straining ...

  One second ... ]

Immediately after severe handling of this kind, the retardation
of contraction-waves, is sometimes even more marked than here
represented; but I think this may be partly due to shock, for on
giving the tissue a little while to recover, the rate of the waves
becomes slightly increased.

Anæsthetics likewise have the effect of slowing the rate of
contraction-waves before blocking them. Taking, for instance, the
case of chloroform, a narrow spiral strip between one and two feet
long was immersed in sea-water containing a large dose of the
anæsthetic; the observations being taken at six seconds' intervals,
the following were the results:--

  [Illustration: Fig 18.

  Normal water ...

  Six seconds after transference to chloroform

  Six seconds later ...

  Six seconds later ...

  Six seconds later ...

  Six seconds later ...

  One second ... ]

In such experiments, the recovery of the normal rate in unpoisoned
water is gradual. Taking, for instance, the case of a spiral strip
in morphia (Fig. 19), it will be seen that the original rate did not
fully return. Some substances, however, exert a more marked permanent
effect of this kind than do weak solutions of morphia. Here, for
instance, is an experiment with alcohol (see Fig. 20).

  [Illustration: Fig. 19

  In normal water ...

  Quarter of an hour after exposed to morphia ...

  One minute after strengthening dose ...

  Four minutes later, and just before blocking of wave ...

  Fifteen seconds later, wave continuing blocked ...

  Immediately after passage of wave on restoration to normal
  sea-water ...

  Four minutes later ...

  Quarter of an hour later ...

  An hour later ...

  One second ... ]

  [Illustration: Fig. 20

  In normal water ...

  Quarter of an hour after exposure to weak dose ...

  Two minutes after strengthening of dose ...

  Five minutes later, and just before blocking of wave ...

  Fifteen seconds later, wave continuing blocked ...

  Immediately after passage of wave on restoration to normal
  sea-water ...

  An hour later ...

  One second ... ]

From these experiments, however, it must not be definitely concluded
that it is the anæsthesiating property of such substances which
exerts this slowing and blocking influence on contraction-waves,
for I find that almost any foreign substance, whether or not an
anæsthetic, will do the same. That nitrite of amyl, caffein, etc.,
should do so, one would not be very surprised to hear; but it might
not so readily be expected that strychnine, for instance, should
block contraction-waves; yet it does so, even in doses so small as
only just to taste bitter. Nay, even fresh water completely blocks
contraction-waves after the strip has been exposed to its influence
for about half an hour, and exerts a permanently slowing effect after
the tissue is restored to sea-water. These facts show the extreme
sensitiveness of the neuro-muscular tissues of the Medusæ to any
change in the character of their surrounding medium, a sensitiveness
which we shall again have occasion to comment upon when treating of
the effects of poisons.

In conclusion, I may mention an interesting fact which is probably
connected with the summation of stimuli before explained. When a
contractile strip is allowed to rest for a minute or more, and
when a wave is then made to traverse it, careful observation will
show that the passage of the first wave is slower than that of its
successor, provided the latter follows the former after not too great
an interval of time. The difference, however, is exceedingly slight,
so that to render it apparent at all the longest possible strips
must be used, and even then the experimenter may fail to detect the
difference, unless he has been accustomed to signalling, by which
method all these observations on rate have to be made.


_Stimulus-waves._

The rate of transmission of tentacular waves is only one-half that of
contraction-waves, viz. nine inches a second. This fact appeared to
me very remarkable in view of the consideration that the tentacular
wave is the optical expression of a stimulus-wave, and that there can
be no conceivable use in a stimulus-wave being able to pass through
contractile tissue independently of a contraction-wave, unless the
former is able to travel more rapidly than the latter; for the only
conceivable use of the stimulus-wave is to establish physiological
harmony between different parts of the organism, and if this wave
cannot travel more rapidly than a contraction-wave which starts from
the same point, it would clearly fail to perform this function.

In view of this anomaly, I was led to think that if the rate of the
stimulus-wave is dependent in a large degree on the strength of the
stimulus that starts it, the slow rate of nine inches a second might
be more than doubled, if, instead of using a stimulus so gentle as
not to start a contraction-wave, I used a stimulus sufficiently
strong to do this. Accordingly I chose a specimen of Aurelia wherein
the occurrence of tentacular waves was very conspicuous, and found,
as I had hoped, that every time I stimulated too gently to start a
contraction-wave, the tentacular wave travelled only at the rate
of nine inches a second; whereas, if I stimulated with greater
intensity, I could always observe the tentacular wave coursing an
inch or two in front of the contraction-wave.

It is remarkable, however, that in this, as in all the other
specimens of Aurelia which I experimented upon, the reflex response
of the manubrium was equally long, whatever strength of stimulus
I applied to the umbrella; or, at any rate, the time was only
slightly less when a contraction-wave had passed than when only a
tentacular wave had done so. The loss of time, however, appears to
take place in the manubrium itself, where the rate of response is
astonishingly slow. Thus, if one lobe be irritated, it is usually
from four to eight seconds before the other lobes respond. But the
time required for such sympathetic response may be even more variable
than this--the limits I have observed being as great as from three
to ten seconds. In all cases, however, the response, when it does
occur, is sudden, as if the distant lobe had then for the first
time received the stimulus. Moreover, one lobe--usually one of those
adjacent to the lobe directly irritated--responds before the other
two, and then a variable time afterwards the latter also respond.
This time is, in most cases, comparatively short, the usual limits
being from a quarter of a second to two seconds. How much of these
enormous intervals is occupied by the period of ganglionic latency,
and how much by that of transmission, it is impossible to say; but I
have determined that the rate of transmission from the end of a lobe
of the manubrium to a lithocyst (deducting a second for the double
period of latent stimulation) is the same as the rate of a tentacular
wave, viz. nine inches a second. The presumption, therefore, is that
the immense lapse of time required for reflex response on the part of
the manubrium is required by the lobular ganglia, or whatever element
it is that here performs the ganglionic function.


_Exhaustion._

In various modes of section of Aurelia I have several times observed
a fact that is worth recording. It sometimes happens that when the
connecting isthmus between two almost severed areas of excitable
tissue is very narrow, the passage of contraction-waves across the
isthmus depends upon the freshness, or freedom from exhaustion,
of the tissue which constitutes the isthmus. That is to say, on
faradizing one of the two tissue-areas which the isthmus serves
to connect, the resulting contraction-waves will at first pass
freely across the isthmus; but after a time it may happen in some
preparations that every now and then a contraction-wave fails to
pass across the isthmus. When this is the case, if the stimulation
is still continued, a greater and greater proportion of waves fail
to pass across the isthmus, until perhaps only one in every five or
six becomes propagated from the one area to the other. If single
induction-shocks be then substituted for the faradaic stimulation,
it may be found that by leaving an interval of four or five seconds
between the successive shocks, every wave that is started in the
one area will be propagated across the isthmus to the other area.
But if the interval between the successive shocks be reduced to
two or three seconds, every now and then a wave will fail to pass
across the isthmus; and if the interval be still further reduced to
one second, or half a second, comparatively few of the waves will
pass across. Now, however, if the tissue be allowed five minutes'
rest from stimulation, and the single shocks be thrown in at one
second's intervals, all the first six or ten waves will pass across
the isthmus, after which they begin to become blocked as before. It
may be observed also that when the waves are thus blocked, owing to
exhaustion of the connecting isthmus, they may again be made to force
a passage by increasing the intensity of the stimulation, and so
giving rise to stronger waves having a greater power of penetration.
Thus, on re-enforcing the electrical stimulus with the simultaneous
application of a drop of spirit, the resulting waves of contraction
are almost sure to pass across the isthmus, even though this has been
exhausted in the manner just described.


_Ganglia appearing to assert their Influence at a Distance from their
own Seat._

Another fact, which I have several times noticed during my sections
of Aurelia, also deserves to be recorded. I have observed it under
several modes of section, but it will be only necessary to describe
one particular case.

In the Aurelia of a portion of which the accompanying woodcut (p.
102) is a representation, seven of the lithocysts were removed,
while the remaining one was almost entirely isolated from the
general contractile tissue by the incisions _aa_, _bb_, _cc_. The
lithocyst continued to animate the tissue-area _xxxx_, and through
the connecting passage _y_ the contraction-waves spread over the
remainder of the sub-umbrella tissue _zzzz_. So far, of course, the
facts were normal; but very frequently it was observed that the
contraction-waves did not start from the lithocyst, or from the area
_xxxx_, but from the point o in the area _zz_. After this origination
of the contraction-waves from the point _o_ had been observed a great
number of times, I removed the lithocyst. The effect was not only
to prevent the further origination of contraction-waves in the area
_xxxx_, but also to prevent their further origination from the point
_o_, the entire umbrella thus becoming paralyzed. Hence, before the
removal of the lithocyst, the contraction-waves which originated at
the point _o_, no less than those which originated at the lithocyst
itself, must in some way or other have been due to the ganglionic
influence emanating from the lithocyst and asserting itself at the
distant point _o_.

  [Illustration: Fig. 21.]

This property, which lithocysts sometimes present, of asserting their
ganglionic influence at a distance from their own locality, can only,
I think, be explained by supposing that at the point where under
these circumstances the contractions originate, there are situated
some scattered ganglionic cells of considerable functional power,
but yet not of power enough to originate contraction-waves unless
re-enforced by some stimulating influence, which reaches them from
the lithocyst through the nervous plexus.


_Regeneration of Tissues._

The only facts which remain to be stated in the present chapter
have reference to the astonishing rapidity with which the excitable
tissues of the Medusæ regenerate themselves after injury. In this
connection I have mainly experimented on Aurelia aurita, and shall,
therefore, confine my remarks to this one species.

If with a sharp scalpel an incision be made through the tenuous
contractile sheet of the sub-umbrella of Aurelia, in a marvellously
short time the injury is repaired. Thus, for instance, if such an
incision be carried across the whole diameter of the sub-umbrella,
so as entirely to divide the excitable tissues into two parts while
the gelatinous tissues are left intact, the result of course is that
physiological continuity is destroyed between the one half of the
animal and the other, while the form of the whole animal remains
unchanged--the much greater thickness of the uninjured gelatinous
tissues serving to preserve the shape of the umbrella. But although
the contractile sheet which lines the umbrella is thus completely
severed throughout its whole diameter, it again reunites, or heals
up, in from four to eight hours after the operation.




CHAPTER V.

EXPERIMENTS IN SECTION OF NAKED-EYED MEDUSÆ.


_Distribution of Nerves in Sarsia._

My experiments have shown that the nervous system in the naked-eyed
Medusæ is more highly organized, or integrated, than it is in the
covered-eyed Medusæ; for whereas in the latter I obtained no evidence
of the gathering together of nerve-fibres into definite bundles or
trunks (the plexus being evenly distributed over the entire surface
of the neuro-muscular sheet lining the umbrella), in the former I
found abundant evidence of this advance in organization. And as the
experiments in this connection serve to substantiate the histological
researches of Professors Haeckel, Schultz, Eimer, and Hertwig, in
as far as the distribution of the main nerve-trunks is concerned, I
shall here detail at some length the character and results of these
experiments in the case of Sarsia.

The occurrence of reflex action in Sarsia is of a very marked and
unmistakable character. I may begin by stating that when any part
of the internal surface of the bell is irritated, the manubrium
responds; but as there is no evidence of ganglia occurring in the
manubrium, this cannot properly be regarded as a case of reflex
action. But now the converse of the above statement is likewise true,
viz. that when any part of the manubrium is irritated, the bell
responds; and it is in this that the unequivocal evidence of reflex
action consists, for while the sympathy of the manubrium with the
bell is not in the least impaired by removing the marginal ganglia of
the latter, the sympathy of the bell with the manubrium is by this
operation entirely destroyed.

We have thus very excellent demonstration of the occurrence of reflex
action in the Medusæ. Further experiments show that the reflex action
occurs, not between the marginal ganglia and every part of the
manubrium, but only between the marginal ganglia and the point of the
bell from which the manubrium is suspended--it being only the pull
which is exerted upon this point when the manubrium contracts that
acts as a stimulus to the marginal ganglia. But the high degree of
sensitiveness shown by the marginal ganglia to the smallest amount
of traction of this kind is quite as remarkable as their lack of
sensitiveness to disturbances going on in the manubrium.

Turning now to the physiological evidence of the distribution of
nerves in Sarsia, when one of the four tentacles is very gently
irritated, it alone contracts. If the irritation be slightly
stronger, all the four tentacles, and likewise the manubrium,
contract. If one of the four tentacles be irritated still more
strongly, the bell responds with one or more locomotor contractions.
If in the latter case the stimulus be not too strong, or, better
still, if the specimen operated on be in a non-vigorous or in a
partly anæsthesiated state, it may be observed that a short interval
elapses between the response of the tentacles and that of the bell.
Lastly, the manubrium is much more sensitive to a stimulus applied
to a tentacle, or to one of the marginal bodies, than it is to a
stimulus applied at any other part of the nectocalyx.

These facts clearly point to the inference that nervous connections
unite the tentacles with one another and also with the manubrium;
or, perhaps more precisely, that each marginal body acts as a
co-ordinating centre between nerves proceeding from it in four
directions, viz. to the attached tentacle, to the margin on either
side, and to the manubrium. This, it will be observed, is the
distribution which Haeckel describes as occurring in Geryonia, and
Schultz as occurring in Sarsia. It is, further, the distribution to
which my explorations by stimulus would certainly point. But, in
order to test the matter still more thoroughly, I tried the effects
of section in destroying the physiological relations which I have
just described. These effects, in the case of the tentacles, were
sufficiently precise. A minute radial cut (only just long enough to
sever the tissues of the extreme margin) introduced between each pair
of adjacent marginal bodies completely destroyed the physiological
connection between the tentacles. If only three marginal cuts were
introduced, the sympathy between those two adjacent tentacles
between which no cut was made continued unimpaired, while the
sympathy between them and the other tentacles was destroyed.

The nervous connections between the tentacles and the manubrium
are of a more general character than those described between the
tentacles themselves; that is to say, severing the main radial
nerve-trunks produces no appreciable effect upon the sympathy between
the tentacles and the manubrium.

The nervous connections between the whole excitable surface of the
nectocalyx and the manubrium are likewise of this general character,
so that, whether or not the radial nerve-trunks are divided, the
manubrium will respond to irritation applied anywhere over the
internal surface of the nectocalyx. The manubrium, however, shows
itself more sensitive to stimuli applied at some parts of this
surface than it is to stimuli applied at other parts, although in
different specimens there is no constancy as to the position occupied
by these excitable tracts.


_Distribution of Nerves in Tiaropsis Indicans._

  [Illustration: Fig. 22.]

We have seen that in Sarsia reflex action obtains between the
manubrium and the nectocalyx; we shall now see that in Tiaropsis
indicans something resembling reflex action obtains between the
nectocalyx and the manubrium. The last-named species is a new one,
which I have described elsewhere, and I have called it "indicans"
from a highly interesting and important peculiarity of function
which is manifested by its manubrium. The Medusa in question measures
about one and a half inches in diameter, and is provided with a
manubrium of unusual proportional size, its length being about
five-eighths of an inch, and its thickness being also considerable.
Now, if any part of the nectocalyx be irritated, the following
series of phenomena takes place. Shortly after the application of
the stimulus, the large manubrium suddenly contracts--the appearance
presented being that of an exceedingly rapid crouching movement.
The crouching attitude in which this movement terminates continues
for one or two seconds, after which the organ begins gradually to
resume its former dimensions. Concurrently with these movements on
the part of the manubrium, the portion of the nectocalyx which has
been stimulated bends inwards as far as it is able. The manubrium
now begins to deflect itself towards the bent-in portion of the
nectocalyx; and this deflection continuing with a somewhat rapid
motion, the extremity of the manubrium is eventually brought, with
unerring precision, to meet the in-bent portion of the nectocalyx.
I here introduce a drawing of more than life-size to render a
better idea of this _pointing_ action by the manubrium to a seat
of irritation located in the bell. It must further be stated that
in the unmutilated animal such action is quite invariable, the
tapered extremity of the manubrium never failing to be placed on the
exact spot in the nectocalyx where the stimulation is being, or had
previously been, applied. Moreover, if the experimenter irritates
one point of the nectocalyx, with a needle or a fine pair of forceps
for instance, and while the manubrium is applied to that point he
irritates another point, then the manubrium will leave the first
point and move over to the second. In this way the manubrium may be
made to indicate successively any number of points of irritation;
and it is interesting to observe that when, after such a series
of irritations, the animal is left to itself, the manubrium will
subsequently continue for a considerable time to visit first one and
then another of the points which have been irritated. In such cases
it usually dwells longest and most frequently on those points which
have been irritated most severely.

I think the object of these movements is probably that of stinging
the offending body by means of the urticating cells with which the
extremity of the manubrium is armed. But, be the object what it
may, the fact of these movements occurring is a highly important
one in connection with our study of the distribution of nerves in
Medusæ, and the first point to be made out with regard to these
movements is clearly as to whether or not they are truly of a reflex
character. Accordingly, I first tried cutting off the margin, and
then irritating the muscular tissue of the bell; the movements in
question were performed exactly as before. I was thus led to think
it probable that the reflex centres of which I was in search might
be seated in the manubrium. Accordingly, I cut off the manubrium,
and tried stimulating its own substance directly. I found, however,
that no matter how small a portion of this organ I used, and no
matter from what part of the organ I cut it, this portion would do
its best to bend over to the side which I irritated. Similarly,
no matter how short a stump of the manubrium I left in connection
with the nectocalyx, on irritating any part of the latter, the
stump of the manubrium would deflect itself towards that part of
the bell, although, of course, from its short length it was unable
to reach it. Hence there can be no doubt that every portion of the
manubrium--down, at least, to the size which is compatible with
conducting these experiments--is independently endowed with the
capacity of very precisely localizing a point of irritation which is
seated either in its own substance or in that of the bell.

We have here, then, a curious fact, and one which it will be well to
bear in mind during our subsequent endeavours to frame some sort of a
conception regarding the nature of these primitive nervous tissues.
The localizing function, which is so very efficiently performed
by the manubrium of this Medusa, and which if anything resembling
it occurred in the higher animals would certainly have definite
ganglionic centres for its structural co-relative, is here shared
equally by every part of the exceedingly tenuous contractile tissue
that forms the outer surface of the organ. I am not aware that such
a diffusion of ganglionic function has as yet been actually proved
to occur in the animal kingdom, but I can scarcely doubt that future
investigation will show such a state of things to be of common
occurrence among the lower members of that kingdom.[16]

  [16] The only case I know which rests on direct observation,
  and which is at all parallel to the one above described, is the
  case of the tentacles of Drosera. Mr. Darwin found, when he cut
  off the apical gland of one of these tentacles, together with
  a small portion of the apex, that the tentacle thus mutilated
  would no longer respond to stimuli applied directly to itself.
  Thus far the case differs from that of the manubrium of Tiaropsis
  indicans, and, in respect of localization of co-ordinating
  function, resembles that of ganglionic action. But Mr. Darwin
  also found that such a "headless tentacle" continued to be
  influenced by stimuli applied to the glands of neighbouring
  tentacles--the headless one in that case bending over in whatever
  direction it was needful for it to bend, in order to approach the
  seat of stimulation. This shows that the analogue of ganglionic
  function must here be situated in at least more than one part
  of a tentacle; and I think it is not improbable that, if trials
  were expressly made, this function would be found to be diffused
  throughout the whole tentacle.

I shall now proceed to consider the nature of the nervous connections
between the nectocalyx and manubrium of this Medusa.

Bearing in mind that in an unmutilated Tiaropsis indicans the
manubrium invariably localizes with the utmost precision any minute
point of irritation situated in the bell, the significance of the
following facts is unmistakable, viz. that when a cut is introduced
between the base of the manubrium and the point of irritation in the
bell, the localizing power of the former, as regards that point in
the latter, is wholly destroyed. For instance, if such a cut as that
represented at _a_ (see Fig. 22) be made in the nectocalyx of this
Medusa, the manubrium will no longer be able to localize the seat of
a stimulus applied below that cut, as, for instance, at _b_. Now,
having tried this experiment a number of times, and having always
obtained the same result, I conclude that the nervous connections
between the nectocalyx and the manubrium, which render possible the
localizing action of the latter, are connections the functions of
which are intensely specialized, and the distribution of which is
radial.

So far, then, we have highly satisfactory evidence of tissue-tracts
performing the function of afferent nerves. But another point
of interest here arises. Although, in the experiment just
described, the manubrium is no longer able to _localize_ the seat
of stimulation in the bell, it nevertheless continues able to
perceive, so to speak, that stimulation is being applied in the bell
_somewhere_; for every time any portion of tissue below the cut a
is irritated, the manubrium actively dodges about from one part of
the bell to another, applying its extremity now to this place and
now to that one, as if seeking in vain for the offending body. If
the stimulation is persistent, the manubrium will every now and
then pause for a few seconds, as if trying to decide from which
direction the stimulation is proceeding, and will then suddenly move
over and apply its extremity, perhaps to the point that is opposite
to the one which it is endeavouring to find. It will then suddenly
leave this point and try another, and then another, and another,
and so on, as long as the stimulation is continued. Moreover, it is
important to observe that there are _gradations_ between the ability
of the manubrium to localize correctly and its inability to localize
at all, these gradations being determined by the circumferential
distance from the end of the cut and the point of stimulation. For
instance, in Fig. 22, suppose a cut A B, quarter of an inch long,
to be made pretty close to the margin and concentric with it, then
a stimulus applied at the point _c_, just below the middle point of
A B, would have the effect of making the manubrium move about to
various parts of the bell, without being able in the least degree
to localize the seat of irritation. But if the stimulus be applied
at _d_, the manubrium will probably be so far able to localize the
seat of irritation as to confine its movements, in its search for the
offending body, to perhaps the _quadrant_ of the bell in which the
stimulation is being applied. If the stimulation be now supplied at
_e_, the localization on the part of the manubrium will be still more
accurate; and if applied at _f_ (that is, _almost_ beneath one end of
the cut A B), the manubrium may succeed in localizing quite correctly.

These facts may also be well brought out by another mode of section,
viz. by cutting round a greater or less extent of the marginal
tissue, leaving one end of the resulting slip free, and the other
end attached _in situ_. If this form of section be practised on
Tiaropsis indicans, as represented at _g_ _k_ in the figure, it may
also be observed that irritation of a distant point in the marginal
strip, such as _g_ or _h_, causes the manubrium to move in various
directions, without any special reference to that part of the bell
which the irritated point of the marginal strip would occupy if _in
situ_. But if the stimulation be applied only one or two millims.
from the point of attachment of the marginal strip, as at _i_, the
manubrium will confine its localizing motions to perhaps the proper
quadrant of the bell; and if the stimulus be applied still nearer
to the attachment of the severed strip, as at _j_, the localizing
motions of the manubrium may become quite accurate.

Again, with regard to _radial_ distance, if the cut A B in the figure
were situated higher up in the bell, as at A´ B´, and the arc, _c_,
_d_, _e_, _f_, of the margin irritated as before, the manubrium
would be able to localize better than if, as before, the radial
distance between A B and _c_, _d_, _e_, _f_ were less. The greater
this radial distance, the better would be the localizing power of
the manubrium; so that, for instance, if the cut A´ B´ were situated
nearly at the base of the manubrium, the latter organ might be able
to localize correctly a stimulus applied, not only as before at _f_,
but also at _e_ or _d_. In such comparative experiments, however, it
is to be understood that the higher up in the bell a cut is placed,
the shorter it must be; for a fair comparison requires that the
two ends of the cut shall always touch the same two radii of the
nectocalyx. Still, if the cut is only a very short one (say one or
two millims. long), this consideration need not practically be taken
into account; for such a cut, if situated just above the margin, as
represented at _a_, will have the effect of destroying the localizing
power of the manubrium as regards the corresponding arc of the
margin; but if situated high up in the bell, even though its length
be still the same, it will not have this effect.

From all this, then, we have seen that the connections which render
possible the _accurate_ localizing functions of the manubrium are
almost, though not quite, exclusively radial. We have also seen that
between accurate localization and mere random movements on the part
of the manubrium there are numerous gradations, the degree of decline
from one to the other depending on the topographical relations
between the point of stimulation and the end of the section (the
section being of the form represented by A B in the figure). These
relations, as we have seen, are the more favourable to correct
localization: (_a_) the greater the radial distance between the point
of stimulation and the end of the section; and (_b_) the less the
circumferential distance between the point of the stimulation and
the radius let fall from the end of the section. But we have seen
that the limits as regards severity of section within which these
gradations of localizing ability occur, are exceedingly restricted--a
cut of only a few millims. in length, even though situated at the
greatest radial distance possible, being sufficient to destroy all
localizing power of the manubrium as regards the middle point of the
corresponding arc of the margin, and a stimulus applied only a few
millims. from the attached end of a severed marginal strip entirely
failing to cause localizing action of the manubrium. Lastly, we have
seen that even after all localizing action of the manubrium has been
completely destroyed by section of the kinds described, this organ
nevertheless continues actively, though ineffectually, to search for
the seat of irritation.

  [Illustration: Fig. 23.]

The last-mentioned fact shows that after excitational continuity
of a higher order has been destroyed, excitational continuity of a
lower order nevertheless persists; or, to state the case in other
words, the fact in question shows that after severance of the almost
exclusively radial connections between the bell and the manubrium, by
which the perfect or unimpaired localizing function of the latter
is rendered possible, other connections between these organs remain
which are not in any wise radial. I therefore next tested the degree
in which these non-radial connections might be cut without causing
destruction of that excitational continuity of a lower order which
it is their function to maintain. It will here suffice to record one
mode of section which has yielded definite results. A glance at the
accompanying illustration (Fig. 23) will show the manner in which
the Medusa is prepared. The margin having been removed (in order to
prevent possible conduction by the marginal nerve-fibres), a single
deep radial cut (_a_ _a_) is first made, and then a circumferential
cut (_a_, _b_, _c_) is carried nearly all the way round the base of
the manubrium. In this way the nectocalyx, deprived of its margin,
is converted into a continuous band of tissue, one of the ends of
which supports the manubrium. Now it is obvious that this mode of
section must be very trying to nervous connections of any kind
subsisting between the bell and the manubrium. Nevertheless, in many
cases, irritating any part of the band _a_ _l_ has the effect of
causing the manubrium to perform the active random motions previously
described. In such cases, however, it is observable that the further
away from the manubrium the stimulus is applied, the less active
is the response of this organ. In very many instances, indeed, the
manubrium altogether fails to respond to stimuli applied at more than
a certain distance from itself. For example, referring to Fig. 23,
the manubrium might actively respond to irritation of any point in
the division _d_, _e_, _f_, _g_, while to irritation of any point
in the division _f_, _g_, _h_, _i_ its responses would be weaker,
and to irritation of any point in _h_, _i_, _j_, _k_, they would be
very uncertain or altogether absent. Hence in this form of section
we have reached about the limit of tolerance of which the non-radial
connections between the bell and manubrium are capable.

Another interesting fact brought out by this form of section is, that
the radial tubes are tracts of comparatively high irritability as
regards the manubrium; for the certainty and vigour with which the
manubrium responds to a stimulus applied at one of the severed radial
tubes, _f_, _g_, or _h_, _i_, or _j_, _k_, contrast strongly with the
uncertainty and feebleness with which it often responds to stimuli
applied between any of these tubes. Indeed, it frequently happens
that a specimen which will not respond at all to a stimulus applied
between two radial tubes, will respond at once to a stimulus applied
much further from the manubrium, but in the course of the radial tube
_f_ _k_.

And this leads us to another point of interest. In such a form of
section, when any part of the mutilated nectocalyx is irritated, the
manubrium shows a very marked tendency to touch some point in the
tissue-mass _a_ _a_ _d_ _e_ (Fig. 23) by which it still remains in
connection with the bell, and through which, therefore, the stimulus
must pass in order to reach the manubrium. And it is observable that
this tendency is particularly well marked if the section has been
planned as represented in Fig. 23, _i.e._ in such a way as to leave
the tissue-tract _a_ _a_ _d_ _e_ pervaded by a nutrient-tube _d_ _e_,
this tube being thus left intact. When this is done, the manubrium
most usually points to the uninjured nutrient-tube _d_ _e_ every time
any part of the tissue-band _a_ _l_ is irritated.

Let us now very briefly consider the inferences to which these
results would seem to point. The fact that the localizing power of
the manubrium is completely destroyed as regards all parts of the
bell lying beyond an incision in the latter, conclusively proves,
as already stated, that all parts of the bell are pervaded by
radial lines of differentiated tissue, which have at least for one
of their functions the conveying of impressions to the manubrium.
The fact in question also proves that the particular effect which
is produced on the manubrium by stimulating any one of these lines
cannot be so produced by stimulating any of the other lines. But
although these tracts of differentiated tissue thus far resemble
afferent nerves in their function, we soon see that in one important
particular they differ widely from such nerves; for we have seen
that, after they have been divided, stimulation of their peripheral
parts still continues to be transmitted to their central parts,
as shown by the non-localizing movements of the manubrium. Of
course this transmission cannot take place through the divided
tissue-tracts themselves; and hence the only hypothesis we can
frame to account for the fact of its occurrence is that which would
suppose these tissue-tracts, or afferent lines, to be capable of
vicarious action. Such vicarious action would probably be effected
by means of intercommunicating fibres, the directions of which would
probably be various. In this way we arrive at the hypothesis of the
whole contractile sheet being pervaded by an intimate plexus of
functionally differentiated tissue, the constituent elements of which
are capable of a vicarious action in a high degree.

Now we know from histological observation that there is a plexus of
nerve-fibres pervading the whole expanse of the contractile sheet,
and therefore we may conclude that this is the tissue through which
the effects are produced. But, if so, we must further conclude that
the fibres of this nerve-plexus are capable of vicarious action in
the high degree which I have explained.

And this hypothesis, besides being recommended by the consideration
that it is the only one available, is confirmed by the fact that
the stimuli which it supposes to escape from a severed phalanx of
nerve-fibres, and then to reach the manubrium after being diffused
through many or all of the other radial lines (such stimuli thus
converging from many directions), are responded to when they reach
the manubrium, not by any decided localizing action on the part of
the latter, but, as the hypothesis would lead us to expect, by the
tentative and apparently random motions which are actually observed.
Moreover, we must not neglect to notice that these tentative or
random movements resemble in every way the localizing movements,
save only in their want of precision. Again, this hypothesis is
rendered more probable by the occurrence of those _gradations_ in
the localizing power of the manubrium which we have seen to be
so well marked under certain conditions. The occurrence of such
gradations under the conditions I have named is what the theory
would lead us to expect, because the closer beneath a section that
a stimulus is applied, the greater must be the immediate lateral
spread of the stimulus through the plexus before it reaches the
manubrium. Similarly, the further the circumferential distance from
the nearest end of such a section that the stimulus is applied, the
greater will be its lateral spread before reaching the manubrium.
Lastly, the present hypothesis would further lead us to anticipate
the fact that when Tiaropsis indicans is prepared as represented in
Fig. 23, the manubrium refers a stimulus applied anywhere in the
mutilated nectocalyx to the band of tissue by which it is still left
in connection with that organ; for it is evident that, according to
the hypothesis, the radial fibres occupying such a band are the only
ones whose irritation the manubrium is able to perceive, and hence it
is to be expected that it should tend to refer to these particular
fibres a source of irritation occurring anywhere in the mutilated
bell.

It is not quite so easy to understand why, in the last-mentioned
experiment, the manubrium should tend to refer a seat of irritation
to the unsevered nutrient tube, or nerve-trunk, rather than to the
unsevered nerves in the general nerve-plexus on either side of
that nerve-trunk; for if this nerve-trunk at all resembles in its
functions the nerve-trunks of higher animals, the afferent elements
collected in it ought to communicate to the manubrium the impression
of having had their _distal_ terminations irritated, and therefore
the fact of a number of such elements being collected into a single
trunk ought not to cause the manubrium to refer a distant seat of
irritation to that trunk rather than to any of the parts from which
the plexus-elements may emanate. Concerning this difficulty, however,
I may observe that we seem to have in it one of those cases in which
it would be very unsafe to argue, with any confidence, from the
highly integrated nervous systems with which we are best acquainted,
to the primitive nervous systems with which we are now concerned. And
although it would occupy too much space to enter into a discussion
of this subject, I may further observe that I think it is not at
all improbable that the manubrium of Tiaropsis indicans should, in
the absence of more definite information, refer a distant seat of
injury to that tract of collected afferent elements through which it
actually receives the strongest stimulation.


_Staurophora Laciniata._

This is a Medusa about the size of a small saucer which responds to
stimulation of its marginal ganglia, or radial nerve-trunks, by a
peculiar spasmodic movement. This consists in a sudden and violent
contraction of the entire muscle-sheet, the effect of which is
to draw together all the gelatinous walls of the nectocalyx in a
far more powerful manner than occurs during ordinary swimming. In
consequence of this spasmodic action being so strong, the nectocalyx
undergoes a change in form of a very marked and distinctive
character. The corners of the four radial tubes, being occupied by
comparatively resisting tissue, are not so much affected by the
spasm as are other parts of the bell; and they therefore constitute
a sort of framework upon which the rest of the bell contracts, the
whole bell thus assuming the form of an almost perfect square, with
each side presenting a slight concavity inwards. These spasmodic
movements, however, are quite unmistakable when they occur even in
a very minute portion of detached tissue; for, however large or
small the portion may be, when in a spasm it folds upon itself with
the characteristically strong and persistent contraction. I say
_persistent_ contraction, because a spasmodic contraction, besides
being of unusual strength, is also of unusual duration; that is to
say, while an ordinary systolic movement only lasts a short time,
a spasm lasts from six to ten seconds or more, and this whether it
occurs in a large or in a small piece of tissue. Again, the diastolic
movements differ very much in the case of an ordinary locomotor
contraction and in that of a spasm; for while in the former case the
process of relaxation is rapid even to suddenness, in the latter it
is exceedingly prolonged and gradual, occupying some four or five
seconds in its execution, and, from its slow but continuous nature,
presenting a graceful appearance. Lastly, the difference between the
two kinds of contraction is shown by the fact that, while a spasm is
gradually passing off the ordinary rhythmical contractions may often
be seen to be superimposed on it--both kinds of contraction being
thus present in the same tissue at the same time.

Now the point with which we shall be especially concerned is, that
it is only stimulation of _certain parts_ of the organism which has
the effect of throwing it into a spasm. These parts are the margin
(including the tentacles) and the courses of the four radial tubes
(including the manubrium, which in this species is spread over the
radial tubes). This limitation, however, is not invariable; for I
have often seen individuals of this species respond with a spasm
to irritation of the general contractile tissue. Nevertheless,
such response to such stimulation in the case of this species is
exceptional--the usual response to muscular irritation being an
ordinary locomotor contraction, which forms a marked contrast to the
tonic spasm that _invariably_ ensues upon stimulation of the margin,
and _almost_ invariably upon the stimulation of a radial tube.

The first question I undertook to answer was the amount of section
which the excitable tissues of Staurophora laciniata would endure
without losing their power of conducting the spasmodic contraction
from one of their parts to another. This was a very interesting
question to settle, because Staurophora laciniata, like all the other
species of discophorus naked-eyed Medusæ, differs from Aurelia, etc.,
in that the ordinary contraction-waves are very easily blocked by
section. It therefore became interesting to ascertain whether or not
the wave of spasm admitted of being blocked as easily. First, then,
as regards the margin. If this be all cut off in a continuous strip,
with the exception of one end left attached _in situ_, irritation
of any part of the almost severed strip will cause a responsive
spasm of the bell, so soon as the wave of stimulation has time to
reach the latter. I next continued this form of section into the
contractile tissues themselves, carrying the incision round and round
the bell in the form of a spiral, as represented in the case of
Aurelia by Fig. 11, page 70. In this way I converted the whole Medusa
into a ribbon-shaped piece of tissue;[17] and on now stimulating
the marginal tissue at one end of the ribbon, a portion of the
latter would go into a spasm. The object of this experiment was to
ascertain how far into the ribbon-shaped tissue the wave of spasm
would penetrate. As I had expected, different specimens manifested
considerable differences in this respect, but in all cases the degree
of penetration was astonishingly great. For it was the exception to
find cases in which the wave of spasm failed to penetrate from end
to end of a spiral strip caused by a section that had been carried
twice round the nectocalyx; and this is very astonishing when we
remember that the ordinary contraction-waves, whether originated by
stimulation of the contractile tissues or arising spontaneously from
the point of attachment of the marginal strip, usually failed to
penetrate further than a quarter of the way round. Moreover, these
waves of spasm will continue to penetrate such a spiral strip even
after the latter has been submitted to a system of interdigitating
cuts of a very severe description.

  [17] It may be stated that while conducting this mode of section
  of Staurophora laciniata, the animal responds to each cut of the
  contractile tissues with a locomotor contraction (or it may not
  respond at all); but each time the section crosses one of the
  radial tubes, the whole bell in front of the section, and the
  whole strip behind it, immediately go into a spasm.

Now, we have here to deal with a class of facts which physiologists
will recognize as of a perfectly novel character. Why it should be
that the very tenuous tracts of tissue which I have named should
have the property of responding even to a feeble stimulus by issuing
an impulse of a kind which throws the contractile tissues into a
spasm; why it should be that a spasm, when so originated, should
manifest a power of penetration to which the normal contractions of
the tissues in which it occurs bear so small a proportion; why it is
that the contractile tissues should be so deficient in the power of
originating a spasm, even in response to the strongest stimulation
applied to themselves;--these and other questions at once suggest
themselves as questions of interest. At present, however, I am wholly
unable to answer them; though we may, I think, fairly assume that it
is the ganglionic element in the margin, and probably also in the
radial tubes, which responds to direct stimulation by discharging
a peculiar impulse, which has the remarkable effect in question.
For the sake of rendering the matter quite clear, let us employ a
somewhat far-fetched but convenient metaphor. We may compare the
general contractile tissues of this Medusa to a mass of gun-cotton,
which responds to ignition (direct stimulation) by burning with
a quiet flame, but to detonation (marginal stimulation) with an
explosion. In the tissue, as in the cotton, every fibre appears to
be endowed with the capacity of liberating energy in either of two
very different ways; and whenever one part of the mass is made, by
the appropriate stimulus, to liberate its energy in one of these
two ways, all other parts of the mass do the same, and this no
matter how far through the mass the liberating process may have to
extend. Now, employing this metaphor, what we find is that, while the
contractile fibres resemble the cotton fibres in the respects just
mentioned, the ganglion cells resemble detonators, when themselves
directly stimulated. In other words, the ganglion-cells of this
Medusa are able to originate two very different kinds of impulse,
according as they liberate their energy spontaneously or in answer to
direct stimulation, and the muscular tissues respond with a totally
different kind of contraction in the two cases. Possibly, indeed,
direct stimulation of the ganglia is followed by a spasm of the
muscular tissue only because a greater amount of ordinary ganglion
influence is thus liberated than in the case of a merely spontaneous
discharge. If this were the explanation, however, I should not
expect so great a contrast as there is between the facility with
which a spasm may be caused by stimulation of the margin and of the
contractile tissue respectively. The slightest nip of the margin of
Staurophora laciniata, for instance, is sufficient to cause a spasm,
whereas even crushing the contractile tissues with a large pair of
dissecting-forceps will probably fail to cause anything other than an
ordinary contraction. Nevertheless, pricking the margin with a fine
needle usually has the effect of causing only a locomotor contraction.

In conclusion, I may state that anæsthetics have the effect of
blocking the spasmodic wave in any portion of tissue that is
submitted to their influence. It is always observable, however, that
this effect is not produced till after spontaneity has been fully
suspended, and even muscular irritability destroyed as regards direct
stimulation. Up to this stage the certainty and vigour of the spasm
consequent on marginal irritation are not perceptibly impaired;
but soon after this stage the intensity of the spasm begins to
become less, and later still it assumes a _local_ character. It is
important, also, to notice that at this stage the effect of marginal
stimulation is very often that of producing a _general locomotor_
contraction, and sometimes a series of two or three such. During
recovery in normal sea-water all these phases occur in reverse order.




CHAPTER VI.

CO-ORDINATION.

_Covered-eyed Medusæ._


From the fact that in the covered-eyed Medusæ the passage of a
stimulus-wave is not more rapid than that of a contraction-wave,
we may be prepared to expect that in these animals the action of
the locomotor ganglia is not, in any proper sense of the term,
a co-ordinated action; for if a stimulus-wave cannot outrun a
contraction-wave, one ganglion cannot know that another ganglion
has discharged its influence till the contraction-wave, which
results from a discharge of the active ganglion, has reached the
passive one. And this I find to be generally the case; for it may
usually be observed that one or more of the lithocysts are either
temporarily or permanently prepotent over the others, i.e. that
contraction-waves emanate from the prepotent lithocysts, and then
spread rapidly over the swimming-bell, without there being any signs
of co-ordinated or simultaneous action on the part of the other
lithocysts. Nevertheless, in many cases such prepotency cannot, even
with the greatest care, be observed; but upon every pulsation all
parts of the swimming-bell seem to contract at the same instant.
And this apparently perfect co-ordination among the eight marginal
ganglia may continue for any length of time. I believe, however, that
such apparently complete physiological harmony is not co-ordination
properly so called, _i.e._ is not due to special nervous connections
between the ganglia; for, if such were the case, perfectly
synchronous action of this kind ought to be the rule rather than the
exception.

I am therefore inclined to account for these cases of perfectly
synchronous action by supposing that all, or most, of the ganglia
require exactly the same time for their nutrition; that they are,
further, of exactly equal potency in relation to the resistance (or
excitability) of the surrounding contractile tissues; and that,
therefore, the balance of forces being exactly equal in the case
of all, or most, of the ganglia, their rhythm, though perfectly
identical, is really independent. I confess, however, that I am by
no means certain regarding the accuracy of this conclusion, as it
is founded on negative rather than on positive considerations; that
is to say, I arrive at this conclusion regarding the cases in which
such apparent co-ordination is observable only because in other cases
such apparent co-ordination is not observable; and also, I may add,
because my experiments in section have not revealed any evidence
of nervous connections capable of conducting a stimulus-wave with
greater rapidity than a contraction-wave. I therefore consider this
conclusion an uncertain one, and its uncertainty is, perhaps, still
further increased by the result of the following experiments.

If a covered-eyed Medusa be chosen in which perfectly synchronous
action of the ganglia is observable, and if a deep radial incision be
made between each pair of adjacent ganglia--the incisions being thus
eight in number and carried either from the margin towards the centre
or _vice versâ_--it then becomes conspicuous enough that the eight
partially divided segments no longer present synchronous action; for
now one segment and now another takes the initiative in starting a
contraction-wave, which is then propagated to the other segments. And
it is evident that this fact tends to negative the above explanation,
for if the discharges of the ganglia are independently simultaneous
before section, we might expect them to continue so after section.
It must be remembered, however, that the form of section we are
considering is a severe one, and that it must therefore not only
give rise to general shock, but also greatly interfere with the
passage of contraction-waves, and, in general, disturb the delicate
conditions on which, according to the suggested explanation, the
previous harmony depended. Besides, as we shall subsequently see, for
some reason or other segmentation of a Medusa profoundly modifies the
rate of its rhythm. In view of these considerations, therefore, the
results yielded by such experiments must not be regarded as having
any conclusive bearing on the question before us; and as these or
similar objections apply to various other modes of section by which
I have endeavoured to settle this question, I will not here occupy
space in detailing them.

It seems desirable, however, in this connection again to mention
a fact briefly stated in a former chapter, namely, that section
conclusively proves a contraction-wave to have the power, when it
reaches a lithocyst, of stimulating the latter into activity; for it
is not difficult to obtain a series of lithocysts connected in such
a manner that the resistance offered to the passage of the waves by
a certain width of the junction-tissue, is such as just to allow the
residuum of the contraction-wave which emanates from one lithocyst to
reach the adjacent lithocyst, thus causing it to originate another
wave, which, in turn, is just able to pass to the next lithocyst
in the series, and so on, each lithocyst in turn acting like a
reinforcing battery to the passage of the contraction-wave. Now this
fact, I think, sufficiently explains the mechanism of ganglionic
action in those cases where one or more lithocysts are prepotent over
the others; that is to say, the prepotent lithocyst first originates
a contraction-wave, which is then successively reinforced by all
the other lithocysts during its passage round the swimming-bell.
In this way the passage of a contraction-wave is no doubt somewhat
accelerated; for I found, in marginal strips, that the rate of
transit from a terminal lithocyst to the other end of the strip was
somewhat lowered by excising the seven intermediate lithocysts.

  [Illustration: Fig. 24.]

I may here state, in passing, a point of some little interest in
connection with this reinforcing action of lithocysts. When I first
observed this action, it appeared to me a mysterious thing why its
result was always to propagate the contraction-wave in only one
direction--the direction, namely, in which the wave happened to be
passing before it reached the lithocyst. For instance, suppose we
have a strip A D, with a lithocyst at each of the equidistant points
A, B, C, D; and suppose that the lithocyst B originates a stimulus:
the resulting contraction-wave passes, of course, with equal rapidity
in the two opposite directions, B A, B C (arrows _b a_, _b c_): the
contraction-wave _b a_ therefore reaches the lithocyst A at the
same time as the contraction-wave _b c_ reaches the lithocyst C,
and so both A and C discharge simultaneously. What, then, should we
expect to be the result? I think we should expect the wave _b c_ to
continue on its course to D, after having been strengthened at C,
and a _reflex_ wave _a´ b´_ to start from A (owing to the discharge
at A), which would reach B at the same time as a similar _reflex_
wave _c´ b´_ starting from C (owing to the discharge at C); so that
by the time the original wave _b c d_ had reached D, the point B
would be the seat of a collision between the two reflex waves _a´
b´_ and _c´ d´_. And, not to push the supposed case further, it
is evident that if such reflex waves were to occur, the resulting
confusion would very soon require to end in tetanus. As a matter of
fact, these reflex waves do not occur; and the question is, why do
they not? Why is it that a wave is only reinforced in the direction
in which it happens to be travelling--so that if, for instance, it
happens to start from A in the above series, it is successively
propagated by B C in the direction A, B, C, D, and in that direction
only; whereas, if it happens to start from D, it is propagated by
the same lithocysts in the opposite direction, D, C, B, A, and in
that direction only--the wave in the one case terminating at the
lithocyst D, and in the other case at the lithocyst A? Now, although
this absence of reflex waves appears at first sight mysterious,
it admits of an exceedingly simple explanation. I find that the
contractile tissues of the covered-eyed Medusæ cannot be made to
respond to two successive stimuli of minimal, or but slightly more
than minimal intensity, unless such stimuli are separated from one
another by a certain considerable interval of time. Now, when in
the above illustration the contraction-wave starts from A, by the
time it reaches B the portion of tissue included between A and B
has just been in contraction in response to the stimulus from A,
while the portion of tissue included between B and C has not been in
contraction. Consequently, the stimulus resulting from a ganglionic
discharge being presumably of minimal, or but slightly more than
minimal intensity, the tissue included between A and B will not
respond to the discharge of B; while the tissue included between
B and C, not having been just previously in contraction, will
respond. And conversely, of course, if the contraction-wave had been
travelling in the opposite direction.

Seeing that this explanation is the only one possible, and that it
moreover follows as a deductive necessity from my experiments on
stimulation, I think there is no need to detail any of the further
experiments which I made with the view of confirming it. But the
following experiment, devised to confirm this explanation, is of
interest in itself, and on this account I shall state it. Having
prepared a contractile strip with a single remaining lithocyst
at one end, I noted the rhythm exhibited by this lithocyst, and
then imitated that rhythm by means of single induced shocks thrown
in with a key at the other end of the strip. The effect of these
shocks was, of course, to cause the contraction-waves to pass in
the direction opposite to that in which they passed when originated
by the lithocyst. Now I found, as I had expected, that so long as
I continued exactly to imitate the rate of ganglionic rhythm, so
long did the waves always pass in the direction B A--A being the
lithocyst, and B the other end of the strip. I also found that if I
allowed the rate of the artificially caused rhythm to sink slightly
below that of the natural rhythm, after every one to six waves (the
number depending on the degree in which the rate of succession of my
induction shocks approximated to the rate of the natural rhythm)
which passed from B to A, one would pass from A to B.[18]

  [18] When two such waves met, they neutralized each other at
  their line of collision; or perhaps more correctly, the tissue on
  each side of that line, having just been in contraction, was not
  able again to convey a contraction-wave passing in the opposite
  direction to the wave which it had conveyed immediately before.

Of course the only interpretation to be put on these facts is that
every time an artificially started wave reached the terminal ganglion
it caused the latter to discharge; but that the occurrence of a
discharge could not in this case be rendered apparent, because of
the inadequacy of that discharge to start a reflex wave. But that
such discharges always took place was manifest, both _à priori_
because from analogy we may be sure that if there had happened to
be any contractile tissue of appropriate width on the other side of
the ganglion, the discharge of the latter would have been rendered
apparent, and _à posteriori_ because, after the arrival of every
artificially started wave, the time required for the ganglion to
originate another wave was precisely the same as if it had itself
originated the previous wave.

In view of these results, it occurred to me as an interesting
experiment to try the effect on the natural rhythm of exhausting a
ganglion thus situated, by throwing in a great number of shocks at
the other end of the strip. I found that after five hundred single
shocks had been thrown in with a rapidity almost sufficient to
tetanize the strip, immediately after the stimulation ceased, the
natural rhythm of the ganglion, which had previously been twenty
in the minute, fell to fourteen for the first minute, eighteen for
the second, and the original rate of twenty for the third. In such
experiments the diminution of rate is most conspicuous during the
first fifteen or thirty seconds of the first minute. Sometimes
there are no contractions at all for the first fifteen seconds
after cessation of the stimulating process, and in such cases the
natural rhythm, when it first begins, may be as slow as one-half
or even one-quarter its normal rate. All these effects admit of
being produced equally well, and with less trouble, by faradizing
the strip, when it may be even better observed how prolonged may be
the stimulation, without causing anything further than such slight
exhaustion of the ganglion as the above results imply.[19]

  [19] In this description I have everywhere adopted the current
  phraseology with regard to ganglionic action--a phraseology which
  embodies the theory of ganglia supplying interrupted stimulation.
  But although I have done this for the sake of clearness, of
  course it will be seen that the facts harmonize equally well with
  the theory of continuous stimulation, to which I shall allude
  further on.


_Naked-eyed Medusæ._

It would be impossible to imagine movements on the part of so
simple an organism more indicative of physiological harmony than
are the movements of Sarsia. One may watch several hundreds of
these animals while they are swimming about in the same bell-jar
and never perceive, as in the covered-eyed Medusæ, the slightest
want of ganglionic co-ordination exhibited by any of the specimens.
Moreover, that the ganglionic co-ordination is in this case
wonderfully far advanced is proved by the fact of members of this
genus being able to steer themselves while following a light, as
previously described.[20]

  [20] Removing the manubrium does not interfere with this steering
  action; but if any considerable portion of the margin is excised,
  the animal seems no longer able to find the beam of light, even
  though one or more of the marginal bodies be left _in situ_.

In the discophorous species of naked-eyed Medusæ, however, perfectly
co-ordinated action is by no means of such invariable occurrence
as it is in Sarsia; for although in perfectly healthy and vigorous
specimens systole and diastole occur at the same instant over the
whole nectocalyx, this harmoniously acting mechanism is very liable
to be thrown out of gear, so that when the animals are suffering in
the least degree from any injurious conditions, often too slight and
obscure to admit of discernment, the swimming movements are no longer
synchronous over the whole nectocalyx; but now one part is in systole
while another part is in diastole, and now several parts may be in
diastole while other parts are in systole. And as in these animals
very slight causes seem sufficient thus to impair the ganglionic
co-ordination, it generally happens that in a bell-jar containing a
number of specimens belonging to different species, numerous examples
of more or less irregular swimming movements are observable.

Taking, then, the case of Sarsia first, from my previous
observations on the physiological harmony subsisting between the
tentacles, I was led to expect that the co-ordination of the
locomotor ganglia was probably effected by means of the same
tissue-tracts through which the intertentacular harmony was effected,
namely, those situated in the margin of the bell. Accordingly, I
introduced four short radial cuts, one midway between each pair
of adjacent marginal bodies. The co-ordination, however, was not
perceptibly impaired. I therefore continued the radial cuts, and
found that when these reached one-half or two-thirds of the way up
the sides of the inner bell (or contractile sheet), the co-ordination
became visibly affected, and this for the first time.

I also tried the following experiment. Instead of beginning the
radial cuts from the margin, I began them from the apex of the cone;
and I found that however many of such cuts I introduced, and however
far down the cone I carried them, so long as I did not actually sever
the margin, so long did all the divisions of the bell continue to
contract simultaneously.[21] This fact, therefore, proves that the
margin of the bell is alone sufficient to maintain co-ordination.

  [21] This could be particularly well seen if, after the extreme
  apex of the cone had been removed, one of the four radial cuts
  was continued through the margin, and the latter was then spread
  out into a linear form by gently pressing the animal against the
  flat side of the glass vessel in which it was contained. The
  same experiment performed on Aurelia is, of course, attended
  with a totally different result, now one segment and now another
  originating a discharge which then spreads to all the others in
  the form of a contraction-wave.

The next experiment I tried was to make four short radial incisions
in the margin as before described, and then to continue _one_ of
these incisions the whole way up the bell. By careful observation I
could now perceive that all the marginal ganglia did not discharge
simultaneously; for when those situated nearest to the long radial
cut happened to take the initiative, the resulting contraction-wave,
having double the distance to travel which it would have had if the
long radial cut had been absent, could now be followed by the eye in
its very rapid course round the bell. Now, the fact that in this form
of section I was able to detect the passage of a _wave_, proves that
the three short radial sections had destroyed the co-ordinated action
of the marginal ganglia.

From these experiments, then, I conclude that in this genus
ganglionic co-ordination, in the strict sense of the term, is
effected exclusively by means of the marginal nerves. And as these
experiments on Sarsia are exceedingly difficult to conduct, owing
to the very rapid passage of contraction-waves in this genus, it is
satisfactory to find that this conclusion is further supported by the
analogy which the other species of naked-eyed Medusæ afford, and to
the consideration of which we shall now proceed.

The effects of four short radial incisions through the margin of any
species of Tiaropsis, Thaumantias, Staurophora, etc., are usually
very conspicuous. Each of the quadrants included between two adjacent
incisions shows a strong tendency to assume an independent action
of its own. This tendency is sometimes so pronounced as to amount
almost to a total destruction of contractional continuity between
two or more quadrants of the bell; but more usually the effect of
the marginal sections is merely that of destroying excitational
continuity, or at least physiological harmony.

It is an interesting thing that this form of section, although in
actual amount so very slight, is attended with a much more pernicious
influence on the vitality of the organism than is any amount of
section of the general contractile tissues. Thus, if a specimen of
Tiaropsis, for example, be chosen which is swimming about with the
utmost vigour, and if four equidistant radial cuts only just long
enough to sever the marginal canal be made, the animal will soon
begin to show symptoms of enfeeblement, and within an hour or two
after the operation will probably have ceased its swimming motions
altogether. The animal, however, is not actually dead; for if while
lying motionless at the bottom of the vessel it be gently stimulated,
it will respond with a spasm as usual, and perhaps immediately
afterwards give a short and feeble bout of swimming movements. These
surprisingly pernicious results are not so conspicuous in the case
of Sarsia, although in this genus likewise they are sufficiently
well marked to be unmistakable. I here append a table to show the
comparative effects of the operation in question on different
species. The cases may be regarded as very usual ones, though it
often happens that a longer time after the operation must elapse
before the enfeebling effects become so pronounced.

    --------------------+-------------+-----------+-----------+---------
                        |  Number of  |   Number  |  Number   |
                        | contractions| during one|during five|Ultimate
      Name of species.  | during five |   minute  |  minutes  |effects.
                        |   minutes   |   after   |   after   |
                        |   before    | operation.|operation. |
                        | operation.  |           |           |
    --------------------+-------------+-----------+-----------+---------
    Tiaropsis diademata |     57      |     11    |     0     |Permanent
    ---- indicans       |    148      |     23    |     0     |  rest.
    ---- polydiademata  |    102      |     18    |     0     |    "
    ---- oligoplocama   |    131      |     30    |     0     |    "
    Sarsia tubulosa     |    144      |     56    |    14     |    "
    --------------------+-------------+-----------+-----------+---------

This decided effect of so slight a mutilation will not, perhaps,
appear to other physiologists so noteworthy as it appears to me; for
no one who has not witnessed the experiments can form an adequate
idea of the amount of mutilation of any parts, other than their
margins, which the Medusæ will endure without even suffering from
the effects of shock. Another point worth mentioning with regard
to the operation we are considering is, that not unfrequently the
interruptions of the margin, which have been produced artificially,
begin to extend themselves through the nectocalyx in a radial
direction; so that in some cases this organ becomes spontaneously
segmented into four quadrants, which remain connected only by the
apical tissue of the bell. I do not think that this is due to the
mere mechanical tearing of the tissues as a consequence of the
swimming motions, for the latter seem too feeble to admit of their
producing such an effect.

In conclusion, I may state that I have been able temporarily to
destroy the ganglionic co-ordination of Sarsia, by submitting the
animals to severe nervous shock. The method I employed to produce
the nervous shock, without causing mutilation, was to take the
animal out of the water for a few seconds while I laid it on a small
anvil, which I then struck violently with a hammer. On immediately
afterwards restoring the Medusa to sea-water, spontaneity was found
to have ceased, while irritability remained. After a time spontaneity
began to return, and its first stages were marked by a complete want
of co-ordination; soon, however, co-ordination was again restored.
But this experiment by no means invariably yielded the same result.
Spontaneity, indeed, was invariably suspended for a time; but its
first return was not invariably, or even generally, marked by an
absence of co-ordination, even though I had previously struck the
anvil a number of times in succession. I was therefore led to try
another method of producing nervous shock, and this I found a more
effectual method than the one just described. It consisted in
violently shaking the Sarsia in a bottle half filled with sea-water.
I was surprised to find how violent and prolonged such shaking might
be without any part of the apparently friable organism, except
perhaps the tentacles and manubrium, being broken or torn. The
subsequent effects of shock were remarkable. For some little time
after their restoration to the bell-jar, the Sarsia had lost, not
only their spontaneity, but also their irritability, for they would
not respond even to the strongest stimulation. In the course of a
few minutes, however, peripheral irritability returned, as shown
by responses to nipping of the neuro-muscular sheet. The animals
were now in the same condition as when anæsthesiated by caffein or
other central nerve-poison; but in a few minutes later central or
reflex irritability also returned, as shown by single responses to
single nippings of the tentacles. Last of all spontaneity began to
return, and was in some few cases conspicuously marked by a want
of co-ordination, all parts of the margin originating impulses at
different times, with the result of producing a continuous flurried
or shivering movement of the nectocalyx. After a time, however, these
movements became co-ordinated; but in most cases when a swimming bout
had ended and a pause intervened, the next swimming bout was also
inaugurated by a period of shivering before co-ordination became
established. This effect might last for a long time, but eventually
it, too, disappeared, the swimming bouts then beginning with
co-ordinated action in the usual way.




CHAPTER VII.

NATURAL RHYTHM.


It will be convenient here to introduce all the observations that
I have been able to make with regard to the natural rhythm of
the Medusæ. As Dr. Eimer has also made some observations in this
connection, before proceeding with the fresh points having relation
to this subject, I shall consider those to which he alludes.

In Aurelia aurita, as Dr. Eimer noticed, the rate of the rhythm has a
tendency to bear an inverse proportion to the size of the individual.
Size, however, is far from being the only factor in determining the
differences between the rate of the rhythm of different specimens,
the individual variations in this respect being very great even among
specimens of the same size. What the other factors in question may
be, however, I am unable to suggest.

Dr. Eimer also affirms that the duration of the natural pauses,
which in Aurelia habitually alternate with bouts of swimming, bears
a direct proportion to the number and strength of the contractions
that occurred in the previous bout of swimming. I observed that
Sarsia are much better adapted than Aureliæ for determining whether
any such precise relation obtains; for, in the first place, the
strength of the contraction is more uniform, and, in the next place,
the alternation of pauses with bouts of swimming is of a more
decided character in Sarsia than in healthy specimens of Aureliæ. I
further observed that in Sarsia no such precise relation did obtain,
although in a very general way it is true, as might be expected, that
unusually prolonged bouts of swimming were sometimes followed by
pauses of unusual duration. As all the observations are very much the
same, I shall only quote two of them:--

    --------------------------+---------------------------------
            _Sarsia._         |  _Sarsia_ (another specimen).
    -------------+------------+--------------+------------------
     Number of   | Seconds of |  Number of   | Seconds of
     pulsations. |    rest.   |  pulsations. |    rest.
    -------------+------------+--------------+------------------
         54      |     90     |      40      |     60
         20      |     15     |      29      |     90
          9      |     92     |      32      |    132
         51      |     40     |      33      |     92
         38      |     60     |      18      |     59
          1      |     43     |       8      |     63
         63      |     45     |      15      |     35
          1      |     14     |       2      |     85
         60      |     15     |      11      |     63
          6      |     50     |      30      |     33
         38      |     50     |      17      |     81
         22      |     32     |      19      |     67
         25      |     12     |       3      |     65
         56      |     55     |      19      |     36
         65      |     20     |      41      |    123
         42      |     15     |      80      |     23
         35      |     40     |      61      |    150
         76      |     43     |      45      |    145
                 |            |      40      |    120
                 |            |      10      |     97
                 |            |      14      |     35
    -------------+------------+--------------+------------------

These observations may be taken as samples of others which it would
be unnecessary to quote, as it will be seen from the above that there
is no precise relation between the number of the pulsations and
the duration of the pauses. Nevertheless, that there is a general
relation may be seen from some cases in which unusually prolonged
pauses occur. The following instance will serve to show this:--

_Sarsia_ (another specimen).

    Number of pulsations.  |  Seconds of rest.
            38             |         30
            22             |         35
            49             |         40
            30             |         45
            46             |         20
             2             |         15
            24             |        380
           112             |         20
            45             |        185
           894             |         30
             6             |         45
             4             |        140
             2             |        185
            30             |        210
           200             |         60

In this case, the relation between the long pause of 380 seconds
and the subsequent prolonged swimming bout of 112 pulsations is
obvious; also, as the latter was then followed by a short pause of
twenty seconds and another comparatively short bout of forty-five
pulsations, the refreshing influence of the previous 380 seconds rest
may be supposed to have been not quite neutralized by the exhausting
effect of the foregoing 112 pulsations. At any rate, looking to
the general nature of the previous proportions (viz. in their sum
185/211), it is certain that 380/112 leaves a large preponderance in
favour of nutrition, which preponderance is not much modified by
adding the next succeeding proportion, thus, (380 + 20)/(112 + 45) =
400/157. Consequently, the organism may fairly be supposed to have
entered upon the next prolonged period of rest (viz. 185 seconds)
with a large balance of reserve power; so that when to this large
balance there was added the further accumulation due to the further
rest of 185 seconds, we are not surprised to find the next succeeding
swimming bout comprising the enormous number of 894 pulsations.
But this great expenditure of energy seems to have been somewhat
in excess of the energy previously accumulated by the prolonged
rest, for this unusual expenditure seems next to have entailed an
unusually prolonged period of exhaustion. At any rate, it is plainly
observable that the next succeeding proportions are greatly in
favour of repose; for it is not until 360 seconds have elapsed, with
only twelve pulsations in the interval, that energy enough has been
accumulated to cause a moderate bout of thirty pulsations. But next
another long and sustained pause of 240 seconds supervenes, and, the
animal being now fully refreshed with a large surplus of accumulated
energy, the next succeeding swimming bout comprises two hundred
pulsations. Lastly, there succeeded sixty seconds of rest, and here
the observation terminated.[22]

  [22] If the reader takes the trouble to ascertain the average
  proportion between the number of pulsations and the seconds of
  rest in the first observations as far down as the first long
  pause, viz. as above stated, 185/211, and if he then balances
  the succeeding income and expenditure of energy of all the rest
  of the observations, he will find the net result to accord very
  precisely with the proportion he previously obtained. But, as
  already stated, any such precision as this is certainly the
  exception rather than the rule.

  It may here be stated that after the sixty seconds of rest
  above recorded, the animal began another swimming bout. It was
  then immediately bisected, and the subsequent observations are
  detailed in the next footnote.


_Effects of Segmentation on the Rhythm._

We have next to consider Dr. Eimer's observations concerning the
effects on the rhythm of Aurelia which result on cutting the animal
into segments; and here, again, I much regret to say that I cannot
wholly agree with this author. He says he found evidence of a very
remarkable fact, viz. that by first counting the natural rhythm of an
unmutilated Aurelia, and then dividing the animal into two halves,
one of these halves into two quarters, and one of these quarters into
two eighths; the sum of the contractions performed by these four
segments in a given time was equal to the number which had previously
been performed in a similar time by the unmutilated animal. And
not only so, but the number of contractions which each segment
contributed to this sum was a number that stood in direct proportion
to the size of the segment; so that the half contracted half as many
times, the quarter a quarter as many times, and the eighth parts
one-eighth part the number of times that the unmutilated Aurelia had
previously contracted in a period of equal duration. I am glad to
observe that Dr. Eimer does not regard this rule otherwise than as
liable to frequent exception; for, as already observed, I cannot say
that my experiments have tended to confirm it. I am only able to say
that there is general tendency for the smaller segments of an Aurelia
divided in this way to contract less frequently than the larger
segments.

It would be tedious and unnecessary to quote any observations in
this connection; but as these observations brought out very clearly
a fact which I had previously suspected, I may detail one experiment
to illustrate this point. The fact in question is, that the _potency
of the lithocysts_ in any given segment of a divided Aurelia has
more to do with the frequency of its pulsations than has the size
of the segment. As previously mentioned, one or more lithocysts may
often be observed to be permanently prepotent over the others; and
I may here observe that the segmentation experiments just described
have shown the converse to be true, viz. that one or more lithocysts
are often permanently feebler than the others. Well, if a specimen
of Aurelia exhibiting decided prepotency in one or more of its
lithocysts be watched for a considerable length of time, so as to be
sure that the prepotency is not of a merely temporary character, and
if the animal be then divided into segments in such a way that the
prepotent lithocysts shall occupy the smaller segments, it may be
observed, provided time be left for the tissues to recover, that the
segments containing the prepotent lithocysts, notwithstanding their
smaller size, contract more frequently than do the larger segments.
Conversely, if the larger segments happen to contain feeble
lithocysts, their contractions will be but few. I have, indeed, seen
cases in which the lithocysts appeared to be quite functionless, so
far as the origination of stimuli was concerned.

The following observations were made on a healthy specimen of
Aurelia having all its lithocysts in good condition, but prepotency
being well marked in the case of one of them, and also, though in
a lesser degree, in the case of another. I divided the animal so
as to leave one of these two prepotent lithocysts in each of the
eighth-part segments, and the next most powerful lithocysts in the
quadrant segment. In the following description, I shall call the
two eighth-part segments A and B, the former letter designating
the segment containing the most powerful lithocyst. The Aurelia
before being divided manifested for several hours a very regular and
sustained rhythm of thirty-two per minute. After its division, the
various segments contracted at the following rates, in one-minute
intervals:--

    -----------+------------+------------+--------------+--------------
    Time after |            |            |              |
     operation.|Segment 1/2.|Segment 1/4.|Segment 1/8 A.|Segment 1/8 B.
    -----------+------------+------------+--------------+--------------
     1/2 hour. |     20     |     25     |     27       |      15
       1    "  |     20     |     25     |     27       |      15
       2 hours.|     29     |     25     |     27       |      16
       4    "  |     19     |     16     |     27       |      12
    -----------+------------+------------+--------------+--------------

Next morning, the water which contained the segments was somewhat
foul, and this, as is always the case, gave rise to abnormally long
pauses. This effect was much more marked in the case of some of the
segments than in that of others. I therefore observed the segments
over five-minute intervals, instead of one-minute intervals as on the
previous day. The following is a sample of several observations, all
yielding the same general result.

    ----------------------+------------+---------------+---------------
          Segment 1/2.    |Segment 1/4.|Segment 1/8 A. |Segment 1/8 B.
    -----------+----------+------------+---------------+---------------
    Number of  |Seconds of|            |Continued      |Rhythm
    pulsations.|  rest.   |            |persistently to|tolerably
    -----------+----------+            |contract with a|perfect at 78
        12     |    120   |No motion   |nearly perfect |in the 5
         3     |     10   |during the  |rhythm of      |minutes; but
         2     |     20   |hour of     |78 in the 5    |this was
        44     |    130   |observation.|minutes during |occasionally
        12     |     20   |            |the hour       |interrupted by
    -----------+----------+            |of observation.|long pauses of
        73     |   5 min. |            |               |4 or 5 minutes'
               |          |            |               |duration.
 --------------+----------+------------+---------------+----------------
       Average rate       |            |Continuous     |Interrupted
         14-3/5           |No motion.  |rhythm at the  |rhythm at the
       per minute.        |            |rate of 15-3/5 |rate of 15-3/5
                          |            |per minute.    |per minute.
    ----------------------+------------+---------------+----------------

I now transferred all the segments to fresh sea-water, with the
following results:--

   Rhythm during first quarter of an hour immediately after
   transference, in five-minute intervals.

    ----------+-------------+--------------+--------------+-------------
       Time.  |  Segment    |  Segment     |Segment       |Segment
              |    1/2.     |    1/4.        1/8 A.         1/8 B.
    ----------+-------------+--------------+--------------+-------------
    First 5   |             |              |              |
    minutes.  |139          | 0            |83            |20
              |(irregular). |               (regular).     (irregular).
    Second    |             |              |              |
    5 minutes.|  0          | 0            |68     "      | 75
              |             |                               (regular).
    Third     |             |              |              |
    5 minutes.|100          |39            |70     "      | 69     "
              |(regular).   |              |(irregular).  |
    ----------+-------------+--------------+--------------+-------------

   Rhythm two hours after transference (five-minute intervals).

    --------------+---------------+----------------+----------------
     Segment 1/2. |  Segment 1/4. | Segment 1/8 A. | Segment 1/8 B.
    --------------+---------------+----------------+----------------
    82 (regular). | 77 (regular). | 70 (regular).  | 62 (regular).
    --------------+---------------+----------------+----------------



   Rhythm next day (five-minute intervals).

    -------------+--------------+----------------+---------------
    Segment 1/2. | Segment 1/4. | Segment 1/8 A. | Segment 1/8 B.
    -------------+--------------+----------------+---------------
         68      |      55      |       17       |      Dead.
    -------------+--------------+----------------+---------------

Next day all the segments were dead except the largest one, in which
a single lithocyst still continued to discharge at the rate of
twenty-four in five minutes.

Now, with regard to these tables, it is to be observed that during
the first day the prepotent lithocyst in the eighth-part segment
A maintained an undoubted supremacy over all the others, and that
the same is true of the comparatively potent lithocysts in the
quadrant. (This is not the case with segment B; probably the degree
of prepotency of the lithocyst in this case was not sufficient to
counteract the antagonistic influence of the small size of the
segment.) But next day the supremacy of the small segment A was not
so marked; for although its rhythm was more regular in the stale
water than was that of the largest segment, its actual number of
contractions in a given time was just about equal to that of the
largest segment. Again, after transference to fresh sea-water,
the balance began to fall on the side of the larger segments;
for even the quadrant, which in the stale water had ceased its
motions altogether, now held a middle position between that of the
half-segment and the prepotent eighth-part segment. On the next day,
again, the balance fell decidedly in favour of the larger segments,
and the weaker eighth-part segment died. Lastly, next day all the
smaller segments were dead.

Hence the principal facts to be gathered from these observations are,
that as time goes on the rhythm of all the segments progressively
decreases, and that the decrease is more marked in the case of the
smaller than in that of the larger segments. This lesser endurance
of the smaller segments also finds its expression in their earlier
death. Now as these smaller segments started with a greater
proportional amount of ganglionic power than the larger segments,
their lesser amount of endurance can only, I think, be explained by
supposing that the process of starvation proceeds at a rate inversely
proportional to the size of the segment, a supposition which is
rendered probable if we reflect that the smaller the segment the
greater is the proportional area of severed nutrient tubes.[23] And
in this connection it is interesting to observe that, although the
endurance of the smaller segments was less than that of the larger
as regards the deprivation of nutriment, it was greater than that
of the larger segments as regards the deprivation of oxygen. This
is shown by the greater regularity of the rhythm manifested by the
smaller than by the larger segments in the stale water, and the fact
is presumably to be accounted for by the consideration that the
ganglia in the smaller segments were more potent than those in the
larger.

  [23] It may be thought that the greater area of general
  tissue-mass in the larger segments than in the smaller, and
  not the lesser proportional area of tube-section, is the cause
  of the larger segments living longer than the smaller ones. I
  am led, however, to reject this hypothesis, because in Sarsia,
  where segmentation entails a comparatively small amount of
  tube-section, there is no constant rule as to the larger segments
  showing more endurance than the smaller ones--the converse
  case, in fact, being of nearly as frequent occurrence. I can
  only account for this fact by supposing that the endurance of
  the segments of Sarsia is determined by the degree in which the
  three or four minute open tube-ends become accidentally blocked.
  This supposition is the only one I can think of to account for
  the astonishing contrasts as to endurance that are presented
  by different segments of the same individual, and, I may add,
  of different individuals when deprived of their margins and
  afterwards submitted to the same conditions. For instance, a
  number of equally vigorous specimens had their margins removed,
  and were then suspended in a glass cage attached to a buoy in
  the sea. Four days afterwards some of the specimens were putrid,
  while others were as fresh as they were when first operated on.
  Again, as an instance of the experiments in segmentation of
  Sarsia, I may quote an experiment in which a score of specimens
  were divided in all sorts of ways, such as leaving the manubrium
  attached, to one half, or three marginal bodies in one portion
  and the remaining marginal body in the other portion, etc.
  Yet, although it was very exceptional to find the two portions
  presenting an equal degree of endurance, no uniform results
  pointing to the cause of the variations could be obtained. In
  most cases, however, the energy, as distinguished from the
  endurance of the larger segments, was conspicuously greater than
  that of the smaller. (But it is curious that in many cases the
  effects of _shock_ appeared to be more marked in the larger than
  in the smaller segments--the latter, for some time after the
  operation, contracting much more frequently than the former.) To
  show both these effects, one experiment may be quoted. A specimen
  of Sarsia was divided into two parts, of which one was a quadrant.

  Immediately after the operation the results were as follows:--

    ----------------------++----------------------
          Portion 1/4.    ||     Portion 3/4.
    ------------+---------++------------+---------
     Number of  | Minutes || Number of  | Minutes
     pulsations | of rest || pulsations | of rest
    ------------+---------++------------+---------
         20     |    0    ||      0     |    5
          4     |    4    ||     10     |    2
         15     |    5    ||     46     |    1
          6     |    3    ||     23     |    2
    ------------+---------++     49     |    1
         45     |   12    ||    900     |    1
                |         ||    117     |    1
    ------------+---------++------------+---------
                          ||   1145     |   13
    ----------------------++------------+---------

  To show the difference between the _endurance_ of two halves of
  a bisected specimen of Sarsia, I may quote one experiment which
  was performed on the same specimen as the one mentioned in the
  text to show the general relationship between the duration of the
  pauses and that of swimming bouts. (See last footnote.)

  Immediately after bisection.

    ----------------------++----------------------
              1/2 A.      ||         1/2 B.
    ------------+---------++------------+---------
     Number of  | Seconds || Number of  | Seconds
     pulsations | of rest || pulsations | of rest
    ------------+---------++------------+---------
          56    |    10   ||      82    |    180
         150    |   150   ||      51    |     20
          68    |   335   ||      14    |     60
         130    |    30   ||      13    |     50
          46    |    45   ||      46    |     45
           2    |    10   ||      38    |     65
          99    |    66   ||      18    |     45
         103    |   360   ||      23    |     60
          12    |     4   ||      35    |    130
    ------------+---------++     105    |     70
     Pauses now become    ||            |
      longer, and swimming||            |
      bouts shorter.      ||            |
    ----------------------++------------+---------

  Twenty-four hours after the operation.

    ----------------------++----------------------
              1/2 A.      ||         1/2 B.
    ------------+---------++------------+---------
     Number of  | Seconds || Number of  | Seconds
     pulsations | of rest || pulsations | or rest
    ------------+---------++------------+---------
          2     |   363   ||       50   |    20
         12     |   362   ||       81   |    25
          4     |   666   ||       37   |   101
         25     |   300   ||     2400   |    60
    ------------+---------++------------+---------

  But although in the case of Sarsia the leaser endurance of
  the smaller segment than of the larger cannot be regarded as
  a general rule, it may be so regarded, as already stated, in
  the case of Aurelia. The following experiment exemplifies this
  particular rule even more prettily than does the one quoted in
  the text, from the fact that the segments survived the operation
  for a greater number of days.

  An Aurelia having a regular and well-sustained rhythm of twenty
  per minute was divided as already described in the text. In
  five-minute intervals on successive days the average rates of the
  four segments were as follows:--

  Four hours after the operation.

    -----------+-----------+-----------+-------------
     Seg. 1/2. | Seg. 1/4. | Seg. 1/8. | Seg. 1/8 A.
    -----------+-----------+-----------+-------------
        100    |    100    |     85    |    90
    -----------+-----------+-----------+-------------

  Next day.

    -----------+-----------+-----------+-------------
         88    |     90    |     64    |     58
    -----------+-----------+-----------+-------------

  Next Day.

    -----------+-----------+-----------+-------------
         86    |     82    |     62    |     57
    -----------+-----------+-----------+-------------

  Next Day.

    -----------+-----------+-----------+-------------
         59    |     45    |     24    |     20
    -----------+-----------+-----------+-------------

  Next day.

    -----------+-----------+-----------+-------------
          50   |     49    |     20    |     10
    -----------+-----------+-----------+-------------

  Next day.

    -----------+-----------+-----------+-------------
         43    |     33    |     18    |      4
    -----------+-----------+-----------+-------------

  Next Day.

    -----------+-----------+-----------+-------------
         39    |     32    |     19    |    Dead.
    -----------+-----------+-----------+-------------

  Next day.

    -----------+-----------+-----------+-------------
     Seg. 1/2. | Seg. 1/4. | Seg. 1/8. | Seg. 1/8 A.
    -----------+-----------+-----------+-------------
         33    |      7    |    Dead.  |      0
    -----------+-----------+-----------+-------------

  Next day.

    -----------+-----------+-----------+-------------
         28    |    Dead.  |      0    |      0
    -----------+-----------+-----------+-------------

  Next day, the temperature unfortunately rose sufficiently to
  cause the death of the single surviving segment, which otherwise
  would probably have lived for one or two days longer.

With regard, therefore, to the original point under consideration,
I conclude that, although the size of the segments is doubtless one
factor in determining the relative frequency of contraction, there
are at least two other factors quite as important, viz. the relative
potency of the lithocysts, and the length of time that elapses
between performing the operation and observing the rhythm. Hence it
is that in my experience I have found but very few examples of Dr.
Eimer's rule.


_Effects of Other Forms of Mutilation on the Rhythm._

The next point I have to dwell upon is one of some interest. If
the manubrium of Aurelia, or of any other covered-eyed Medusa, be
suddenly cut off at its base, the swimming motions of the umbrella
immediately become accelerated. This acceleration, however, only
lasts for a few minutes, when it gradually begins to decline,
the rate of the rhythm becoming slower and slower, until finally
it comes to rest at a rate considerably less than was previously
manifested by the unmutilated animal. If a circular piece be now cut
out from the centre of the umbrella, the rhythm of the latter again
becomes temporarily quickened; but, as before, gradual slowing next
supervenes. This slowing, however, proceeds further than in the last
case, so that the rate at which the rhythm next becomes stationary
is even less than before. If, now, another circular ring be cut from
the central part of the umbrella--_i.e._ if the previously open ring
into which this organ had been reduced by the former operation be
somewhat narrowed from within--the same effects on the rhythm are
again observable; and so on with every repetition of the operation,
the rate of the rhythm always being quickened in the first instance,
but then gradually slowing down to a point somewhat below the rate
it manifested before the previous operation. It will here suffice to
quote one experiment among many I have made in this connection:--

    An Aurelia manifested a regular and sustained rhythm of        26

    Immediately after removal of manubrium, rhythm rose to         36

    Rate then gradually fell for a quarter of an hour, and became
    stationary at                                                  20

    Circular incision just including ovaries caused rhythm to
    rise to                                                        26

    After gradual fall during quarter of an hour, rhythm became
    stationary at                                                  17

    Another circular incision carried round midway between the
    former one and the margin caused rhythm to rise to             24

    Rate again gradually declined, and in a quarter of an hour was 12

    Another circular incision was carried round as close to the
    margin as was compatible with leaving the physiological
    continuity of all the lithocysts intact. Rhythm rose to        14

    Within a few minutes it fell to                                 6

Excepting the cases where the effects of shock are apparent, some
such series of phenomena as those just recorded are always sure to
ensue when a covered-eyed Medusa is mutilated in the way described,
and this kind of mutilation, besides producing such marked effects
on the _rate_ of the rhythm, also produces an effect in impairing
the _regularity_ of the rhythm. In some specimens the latter effect
is more marked than it is in others. The following series of
observations will serve to give a good idea of this effect:--

An Aurelia manifested a regular and sustained rhythm of 36.
Immediately after the removal of the manubrium, the rate of rhythm
in successive minutes was as follows: 40, 39, 37, 35, 32, 30, 29,
26, 24, 18, 14 (40 seconds' pause), 16, 15, 14, 15, 16 (40 seconds'
pause), 22, 20, 19, 15, 16, 17, 14, 13, 13, 15, 16, 16, 17, 18,
14, 12, 13, 11, 12, 9, 15, 16, 14, 12, 9, etc., the rhythm now
continuing very irregular. An hour after the operation, the following
were the number of contractions given in one-minute intervals, the
observations being taken at intervals of ten minutes: 15, 15, 12, 22,
14, etc.

In this experiment, therefore, as soon as the acceleration and
slowing-stages had been passed, viz. about a quarter of an hour after
the operation, a great disturbance was observable in regularity of
the rhythm; for before the removal of the manubrium, the Medusa had
been swimming for hours with perfect regularity.

Before concluding my description of these experiments, it may
perhaps be as well to mention one other, which was designed to meet
a possible objection to the inferences which, as I shall immediately
argue, these experiments seem to sustain. It occurred to me as a
remote possibility that the slowing and irregularity of the rhythm,
which are observable about a quarter of an hour after the operations
described, might be due to the deprivation of adequate nourishment
suffered by the ganglia, in consequence of the escape of nutrient
matter from the cut ends of the nutrient tubes. Accordingly, instead
of cutting off the manubrium, I tried the effect of momentarily
immersing it in hot water, and found that the subsequent disturbances
of the rhythm were precisely similar to those which result from
removal of the manubrium.

Now, to draw any inferences from such meagre facts as the above would
be hazardous, unless we recognize that in so doing our inferences are
not trustworthy. But, with this recognition, I think there will be no
harm in briefly stating the deductions to which the facts, such as
they are, would seem to point.

Physiologists are undecided as to the extent in which many apparently
automatic actions may not really be actions of a reflex kind. Given
any ganglio-muscular tissue which is rhythmically contracting, how
are we to know whether the action of the ganglia is truly automatic,
or sustained from time to time by stimuli proceeding from other parts
of the organism? In most cases experiments cannot be conducted with
reference to this question, but in the case of the Medusæ they may be
so, and it was with the view of throwing light on this question that
the experiments just described were made. Now in these experiments
the fact is sufficiently obvious that mutilations of any part of the
organism modify the rhythm of the marginal ganglia most profoundly.
That this modification does not proceed from shock, would seem to be
indicated by the facts that the first effect of the mutilation is to
_quicken_ the rhythm; that there is a sort of general proportion to
be observed between the amount of tissue abstracted and the degree of
slowing of the rhythm produced; and that the slowing effects continue
for so long a time. All these facts seem to show that we have here
something other than mere shock to deal with.

A strong suspicion, therefore, arises that the cause of the slowing
of the rhythm which results from removing the manubrium, or a
part of the general contractile tissue of the bell, consists in
the destruction of some influence of an afferent character which
had previously emanated from the parts of the organism which have
been removed, and that the normal rhythm before the operation was
partly due to a continuous reception, on the part of the ganglia,
of this afferent or stimulating influence. In support of this
view are the facts that the first effect of such an operation as
we are considering is greatly to accelerate the rhythm, and that
this acceleration then gradually declines through a period of
about a quarter of an hour. These facts tend to support this view,
because, if it is correct, they are what we might anticipate. If the
manubrium, for instance, while _in situ_ is continually supplying
a gentle stimulus to the marginal ganglia, when it is suddenly cut
off, the nerve-tracts through which this stimulating influence had
previously been conveyed must be cut through; and as it is well
known how irritable nerve-fibres are at their points of section,
it is to be expected that the irritation caused by cutting these
nerve-tracts, and probably also by the action of the sea-water on
their cut extremities, would cause them to stimulate the ganglia
more powerfully than they did before their mutilation. And here I
may state that on several occasions, with vigorous specimens, I have
observed a sudden removal of the manubrium to be followed, not merely
with a quickening of the rhythm on the part of the bell, but with a
violent and long-sustained spasm.

Again, as regards the other fact before us, it is obvious that as
soon as the cut extremities of the nerves begin to die down, and
so gradually to lose their irritability, the effect on the rhythm
would be just what we observe it to be, viz. a gradual slowing till
the rate falls considerably below that which was exhibited by the
unmutilated animal. And even the _irregularity_ which is at this
stage so frequently observable is, I think, what we should expect
to find if this view as to the essentially reflex character of the
natural rhythm is the true one.

If this view is the true one, the question next arises as to the
nature of the process which goes on in the excitable tissues, and
which afterwards acts as a stimulus on the ganglionic tissues. This
question, however, I am quite unable to answer. Whether the process
is one of oxygenation, of chemical changes exerted by the sea-water,
or a process of any other kind, further experiments may be able to
show; but meanwhile I have no suggestion to offer.


_Effects of lessening the Amount of Tissue adhering to a Single
Ganglion._

The above experiments led me to try the effects of cutting out a
single lithocyst of Aurelia, and, after the rhythm of the detached
segment had become regular, progressively paring down the contractile
tissues around the ganglion. I found that this process had no very
marked effect on the rhythm, until the paring reached within an inch
or two of the ganglion: then, however, the effect began to show
itself, and with every successive paring it became more marked.
This effect consisted in slowing the rate of the rhythm, but more
especially in giving rise to prolonged pauses: indeed, if only a very
little contractile tissue was left adhering to the ganglion, the
pauses often became immensely prolonged, so that one might almost
suppose the ganglion to have entirely ceased discharging. But if a
stimulus of any kind were then applied, the rhythmic discharges at
once recommenced. These generally continued for some little time at
a slower rate than that which they had manifested before they were
affected by the paring down of the contractile tissue.


_Effects of Temperature on the Rhythm._

The effects of temperature on the rhythm of Medusæ are very decided.
For instance, a specimen of Sarsia which in successive minutes gave
the following number of pulsations, 16, 26, 0, 0, 26, gave sixty
pulsations during the next minute, while a spirit-lamp was held
under the water in which the Medusa was swimming. If hot water be
added to that in which Sarsia are contained until the whole is
about milk-warm, their swimming motions become frantic. If the same
experiment be performed after the margins of the Sarsia have been
removed, the paralyzed bells remain quite passive, while the severed
margins exhibit the frantic motions just alluded to.

In the case of Aurelia aurita, the characteristic effects of
temperature on rhythm may be better studied than in that of Sarsia,
from the fact that the natural motions are more rhythmical and
sustained in the former than in the latter genus. I have, therefore,
in this connection made more observations on Aurelia than on Sarsia.
The following may be taken as a typical experiment.

A small and active specimen of Aurelia contracted with the greatest
regularity 33 times per minute in water kept at 34°; but on
transference to water kept at 49°, the contractions always became
irregular, in respect (_a_) of not having a perfectly constant
rhythm, and (_b_) of exhibiting frequent pauses, which was never the
case in colder water. The rate of rhythm in the warmer water varied
from 37 to 49; and as in these observations no allowance was made
for the occurrence of the pauses, the actual rate of rhythm during
the swimming motions was about 60 per minute. The following are some
sample observations in the case of this specimen:--

    ----------------------------+---------------------+----------------
    Temperature of water (Fahr.)|Number of pulsations.|Seconds of rest.
    ----------------------------+---------------------+----------------
                   40°          |         41          |        5
                    "           |         49          |        4
    Transferred to 34°          |         33          |        0
         "          "           |         33          |        0
         "          "           |         33          |        0
         "          "           |         33          |        0
    Transferred to 49°          |         45          |        4
         "          "           |         39          |       10
         "          "           |         37          |       15
    Transferred to 34°          |         20          |        0
         "          "           |         30          |        0
         "          "           |         33          |        0
         "          "           |         33          |        0
         "          "           |         33          |        0
         "          "           |         33          |        0
    ----------------------------+---------------------+----------------

This rate continued quite regularly for a quarter of an hour, when
the observation terminated.

It might naturally be supposed that when the alterations of
temperature between 34° and 49° produce such marked effects on the
rhythm, still greater alterations would be attended with still
greater effects. Such, however, is not the case. Water at 70° or 80°,
for instance, has the effect of permanently _diminishing_ the rate of
the rhythm, after having temporarily raised it for a few seconds. The
following experiment will serve to convey a just estimation of these
facts.

An Aurelia whose rhythm in water at 40° was very regular at
eighteen per minute, was suddenly transferred to water at 80°. In
the immediately succeeding minutes the rhythm was 22, 20, 14. The
latter rate continued for nearly half an hour, when the observation
terminated.

The effect of very warm water, therefore, is to slow the rhythm, as
well, I may add, as to enfeeble the vigour of the contractions. The
case of Medusæ thus differs, in the former respect, from that of the
heart; and I think the reason of the difference is to be found in
the following considerations. Even slight elevations of temperature
are quickly fatal to the Medusæ, so it becomes presumable that
considerable elevations act very destructively on the neuro-muscular
tissues of those animals. This destructive effect of high
temperatures may, therefore, very probably counteract the stimulating
effect which such temperatures would otherwise exert on the natural
rhythm, and hence a point would somewhere be reached at which the
destructive effect would so far overcome the stimulating effect as
to slow the rhythm. That this is probably the true, as it certainly
is the only explanation to be rendered, will, I think, be conceded
when I further state that if an Aurelia be left for some little time
in water at 80°, and then again transferred to water at 30° or 40°,
its original rate of rhythm at the latter temperature does not again
return, but the rhythm remains permanently slowed. And, in favour
of the explanation just offered, it may be further pointed out that
the first effect of sudden immersion in heated water is to _quicken_
the rhythm, it not being for a few seconds, or for even a minute or
two after the immersion, that the rhythm becomes slowed. Lastly, the
slowing takes place gradually; and this is what we should expect if,
as is probable, the destructive effect takes somewhat more time to
become fully developed than does the stimulating effect.

Before leaving the subject of temperature in relation to rhythm, I
must say a few words on the effects of cold. The following may be
regarded as typical experiments.

An Aurelia presenting a regular rhythm of twenty per minute in water
at 45° was placed in water at 19°. Soon after the transference
the rhythm began to slow, and the strength of the contractions
to diminish. Both these phenomena rapidly became more and more
pronounced, till the rhythm fell to ten per minute (still quite
regular), and the contractions ceased to penetrate the muscular
tissue further than an inch or so from the marginal ganglia. Shortly
after this stage pauses became frequent, but mechanical or other
irritation always originated a fresh swimming bout. Next, only one
very feeble contraction was given at long and irregular intervals,
a contraction so feeble that it was restricted to the immediate
vicinity of the lithocyst in which it originated. Soon after this
stage irritability towards all kinds of stimuli entirely ceased,
including even strong spirit dropped on the under surface of the
animal when taken momentarily out of the water. All these stages thus
described were passed through rapidly, the whole series occupying
rather less than five minutes. On now leaving the specimen for ten
minutes and then restoring it to its original water at 45°, all
the above-mentioned stages were passed through in reverse order.
The first faint marginal contraction was confined to the immediate
vicinity of the prepotent lithocyst, and all subsequent contractions
continued to be so for the next three minutes. Rhythm very slow.
Contractions now began to penetrate round the margin, and in eight
minutes from the restoration had gone all the way round, the rate
of their rhythm meanwhile increasing. In two minutes more all the
umbrella was contracting at the rate of fifteen per minute.

In another specimen, subjected to the same conditions, the rate of
recovery was even more rapid, occupying only two minutes altogether;
but in every case the process of recovery is a gradual one, and
differs only in the time it occupies in passing through the various
stages.


_Effects of Freezing Medusæ._

In conclusion, I will describe some rather interesting experiments
that consisted in freezing some specimens of Aurelia into a solid
block of ice. Of course, as sea-water had to be employed, the cold
required was very considerable; but I succeeded in turning out the
Medusæ encased on all sides in a continuous block of sea-water.
By now immersing this block in warm water, I was able to release
the contained specimens, which then presented a very extraordinary
appearance. The thick and massive gelatinous bell of a Medusa is,
as every one knows, chiefly composed of sea-water, which everywhere
enters very intimately into the structure of the tissue. Now,
all this sea-water was, of course, frozen _in situ_, so that the
animals were everywhere and in all directions pierced through by an
innumerable multitude of ice-crystals, which formed a very beautiful
meshwork, pervading the whole substance of their transparent tissues.

These experiments were made in order to ascertain whether the Medusæ,
after having been thus completely frozen, would survive on being
again thawed out, and, if so, whether the freezing process would
exert any permanent influence on the rate of their rhythm. Now in
all the cases the Medusæ, after having been thawed out, presented a
ragged appearance, which was due to the disintegrating effect exerted
by the ice-crystals while forming in the tissues; yet notwithstanding
this mechanical injury superimposed on the physiological effects of
such extreme cold, all the Medusæ recovered on being restored to
sea-water of the normal temperature. The time occupied by the process
of recovery varied in different individuals from a few minutes to
half an hour or more, and it was observable that those specimens
which recovered soonest had the rate of their rhythm least affected
by the freezing. In no case, however, that I observed did the rate
of the rhythm after the freezing return fully to that which had been
manifested before the freezing.


_Effects of Certain Gases on the Rhythm._

_Oxygen._--I will now conclude my remarks on rhythm by very briefly
describing the effects of certain gases. Oxygen forced under
pressure into sea-water containing Sarsia has the effect of greatly
accelerating the rate of their rhythm. The following observation on a
single specimen will serve to render this apparent.

Number of pulsations given by Sarsia in successive five-minute
intervals.

    In ordinary sea-water    472, 527, 470
    In oxygenated sea-water  800
    In ordinary sea-water    268, 350, 430

It will be seen from this observation that the acceleration of the
rhythm due to the oxygenation was most marked; indeed, the pulsations
followed one another so rapidly that it was no easy matter to count
them. It must also be stated that while the animal was under the
influence of oxygen, the duration of the natural pauses between the
swimming bouts was greatly curtailed, the swimming motions, in fact,
being almost quite continuous throughout the five minutes that the
Medusa was exposed to such influence. Lastly, it will be observed
from the above table that the unnatural amount of activity displayed
by the organism while in the oxygenated water entailed on it a
considerable degree of exhaustion, as shown by the fact that even a
quarter of an hour after its restoration to normal water its original
degree of energy had not quite returned.

_Carbonic acid._--As might be expected, this gas has the opposite
effects to those of oxygen. It is therefore needless to say more
about this agent, except that if administered in large doses it
destroys both spontaneity and irritability. Nevertheless, if its
action is not allowed to last too long, the Medusæ will fully recover
on being again restored to normal sea-water.

_Nitrous oxide._--This gas at first accelerates the motions of
Sarsia, but eventually retards them. I omitted, however, to push the
experiment to the stage of complete anæsthesia, which would doubtless
have supervened had the pressure of the gas been sufficiently great.

_Deficient aëration._--It may now be stated that the Medusæ are
exceedingly sensitive to such slight carbonization of the water in
which they are contained as results from their being confined in a
limited body of it for a few hours. The rhythm becomes slowed and
the contractions feeble, while the pauses between the swimming bouts
become more frequent and prolonged. If the water is not changed,
all these symptoms become more marked, and, in addition, the rhythm
becomes very irregular. Eventually the swimming motions entirely
cease; but almost immediately after the animals are restored to
normal sea-water, they recover themselves completely, the rate and
regularity of their rhythm being then quite natural. The suddenness
with which this return to the normal state of things is effected
cannot but strike the observer as very remarkable, and I may mention
that it takes place with equal suddenness at whatever stage in the
above-described process of asphyxiation the transference to normal
sea-water is accomplished.




CHAPTER VIII.

ARTIFICIAL RHYTHM.


If the umbrella of Aurelia aurita has been paralyzed by the
removal of its lithocysts, and if it is then subjected to faradaic
stimulation of minimal intensity, the response which it gives is not
tetanic, but rhythmic. The rate of this artificial rhythm varies
in different specimens, but the limits of variation are always
within those which are observed by the natural rhythm of different
specimens. The artificial rhythm is not in every case strictly
regular; but by carefully adjusting the strength of the current, and
by shifting the electrodes from one part of the tissue to another
until the most appropriate part is ascertained, the artificial rhythm
admits in most cases of being rendered tolerably regular, and in many
cases as strictly regular as is the natural rhythm of the animal. To
show this, I append a tracing of the artificial rhythm (Fig. 25),
which may be taken as a fair sample of the most perfect regularity
that can be obtained by minimal faradaic stimulation.[24]

  [24] This and all the subsequent tracings I obtained by the
  method already described.

  [Illustration: Fig. 25.]

  [Illustration: Fig. 26.]

This artificial rhythm may be obtained with a portion of irritable
tissue of any size, and whether a large or small piece of the tissue
employed be included between the electrodes.

As the fact of this wonderfully rhythmic response to faradaic
irritation was quite unexpected by me, and as it seemed to be a fact
of great significance, I was led to investigate it in as many of its
bearings as time permitted. First, I tried the effect on the rhythm
of progressively intensifying the strength of the faradaic current. I
found that with each increment of the current the rate of the rhythm
was increased, and this up to the point at which the rhythm began to
pass into tetanus due to summation of the successive contractions.
But between the slowest rhythm obtainable by minimal stimulation and
the most rapid rhythm obtainable before the appearance of tetanus,
there were numerous degrees of rate to be observed. I here append
another tracing, to show the effect on the rate of the rhythm of
alterations in the strength of the current (Fig. 26).

It will also be observed from this tracing that, in consequence of
the current having been strengthened slightly beyond the limit within
which strictly rhythmic response was attainable, the curves in the
middle part of the tracing, where the current was strengthened, are
slightly irregular. This irregularity is, of course, due to the first
appearance of tumultuous tetanus. If the faradaic stimulation had in
this case been progressively made still stronger, the irregularity
would have become still more pronounced up to a certain point, when
it would gradually have begun to pass into more persistent tetanus.
But as in this case, instead of strengthening the current still
further, I again weakened it to its original intensity, the rhythm
immediately returned to its original rate and regularity.

Such being the facts, the question arises as to their interpretation.
At first I was naturally inclined to suppose that the artificial
rhythm was due to a periodic variation in the strength of the
stimulus, caused by some slight breach of contact between the
terminals and the tissue on each contraction of the latter. This
supposition, of course, would divest the phenomena in question of
all physiological meaning, and I therefore took pains in the first
instance to exclude it. This I did in two ways: first, by observing
that in many cases (and especially in Cyanæa capillata) the rate of
the rhythm is so slow that the contractions do not follow one another
till a considerable interval of total relaxation has intervened; and
second, by placing the terminals close together, so as to include
only a small piece of tissue between, and then firmly pinning the
tissue all round the electrodes to a piece of wood placed beneath the
Medusæ. In this way the small portion of tissue which served as the
seat of stimulation was itself prevented from moving, and therefore
the rhythmic motions which the rest of the Medusa presented cannot
have been due to any variations in the quality of the contact between
the electrodes and this stationary seat of stimulation.

Any such merely mechanical source of fallacy being thus, I think,
excluded, we are compelled to regard the facts of artificial rhythm
as of a purely physiological kind. The question, therefore, as to
the explanation of these facts becomes one of the highest interest,
and the hypothesis which I have framed to answer it is as follows.
Every time the tissue contracts it must as a consequence suffer a
certain amount of exhaustion, and therefore must become slightly
less sensitive to stimulation than it was before. After a time,
however, the exhaustion will pass away, and the original degree of
sensitiveness will thereupon return. Now, the intensity of faradaic
stimulation which is alone capable of producing rhythmic response,
is either minimal or but slightly more than minimal in relation
to the sensitiveness of the tissue when fresh; consequently, when
this sensitiveness is somewhat lowered by temporary exhaustion, the
intensity of the stimulation becomes somewhat less than minimal
in relation to this lower degree of sensitiveness. The tissue,
therefore, fails to perceive the presence of the stimulus, and
consequently fails to respond. But so soon as the exhaustion is
completely recovered from, so soon will the tissue again perceive
the presence of the stimulus; it will therefore again respond, again
become temporarily exhausted, again fail to perceive the presence
of the stimulus, and again become temporarily quiescent. Now it is
obvious that if this process occurs once, it may occur an indefinite
number of times; and as the conditions of nutrition, as well as those
of stimulation, remain constant, it is manifest that the responses
may thus become periodic.

In order to test the truth of this hypothesis, I made the following
experiments. Having first noted the rate of the rhythm under faradaic
stimulation of minimal intensity, without shifting the electrodes
or altering the intensity of the current, I discarded the faradaic
stimulation, and substituted for it single induction shocks thrown
in with a key. I found, as I had hoped, that the _maximum_ number
of these single shocks which I could thus throw in in a given time
_so as to procure a response to every shock_, corresponded with the
number of contractions which the tissue had previously given during
a similar interval of time when under the influence of the faradaic
current of similar intensity. To make this quite clear, I shall
describe the whole course of one such experiment. The deganglionated
tissue under the influence of minimal faradaic stimulation manifested
a perfectly regular rhythm of thirty contractions per minute, or one
contraction in every two seconds. While the position of the platinum
electrodes and the intensity of the current remained unchanged,
single induction shocks were now administered with a key at any
intervals which might be desired. It was found that if these single
induction stimuli were administered at regular intervals of two
seconds or more, the tissue responded to every stimulus; while if
the stimuli were thrown in more rapidly than this, the tissue did
not respond to every stimulus, but only to those that were separated
from one another by an interval of at least two seconds' duration.
Thus, for instance, if the shocks were thrown in at the rate of one
a second, the tissue only, but always, responded to every alternate
shock. And similarly, as just stated, if any number of shocks were
thrown in, the tissue only responded once in every two seconds. Now,
as this rate of response precisely coincided with the rate of rhythm
previously shown by the same tissue under the influence of faradaic
stimulation of the same intensity, the experiment tended to verify
the hypothesis which it was designed to test.

  [Illustration: Fig. 27.]

I may give one other experiment having the same object and tendency.
Employing single induction shocks of slightly more than minimal
intensity, and throwing them in at twice the rate that was required
to produce a strong response to every shock, I found that midway
between every two strong responses there was a weak response. In
other words, a stimulus of uniform intensity gives rise alternately
to a strong and to a weak contraction, as shown in the appended
tracing (Fig. 27). It will be observed that in this tracing each
large curve represents the whole time occupied by the strong
contraction, the latter beginning at the highest point of the
curve on the left-hand side in each case. The effect of the weak
contraction is that of momentarily interrupting the even sweep of
diastole after the strong contraction, and therefore the result on
the tracing is a slight depression in the otherwise even curve of
ascent. Lest any doubt should arise from the smallness of the curves
representing the weak contractions that the former are in some
way accidental, I may draw attention to the fact that the period
of latent stimulation is the same in the case of all the curves.
To render this apparent, I have placed crosses below the smaller
curves, which show in each case the exact point where the depressing
effect of these smaller curves on the ascending sweeps of the larger
curves first become apparent--_i.e._ the point at which the feeble
contraction begins. Now, what I wish to be gathered from the whole
tracing is this. If the strength of the induction shocks had been
much greater than it was, _all_ the contractions would have become
strong contractions, and tetanus would have been the result. But,
as the strength of the induction shocks was only slightly more than
minimal, the exhaustion consequent on every strong contraction so
far diminished the irritability of the tissue that when, during
the process of relaxation, another shock _of the same intensity_
was thrown in, the stimulus was only strong enough, in relation
to the diminished irritability of the partly recovered tissue, to
cause a feeble contraction. And these facts tend still further to
substantiate the hypothesis whereby I have sought to explain the
phenomena of artificial rhythm.

Now, I think that the strictly rhythmic action of the paralyzed
swimming-bell of Aurelia in answer to constant stimulation is a
fact of the highest significance; for here we have a tissue wholly,
or almost wholly, deprived of its centres of spontaneity, yet
pulsating as rhythmically in answer to artificial stimulation as
it previously did in answer to ganglionic stimulation.[25] Does
not this tend to show that for the production of the natural rhythm
the presence of the ganglionic element is non-essential; that if we
merely suppose the function of this element to be that of supplying a
constant stimulus of a low intensity, without in addition supposing
the presence of any special resistance-mechanism to regulate the
discharges, the periodic sequence of systole and diastole would
assuredly result; and, therefore, that the rhythmical character
of the natural swimming motions is dependent, not on the peculiar
relations of the ganglionic, but on the primary qualities of the
contractile tissue? Or, if we do not go so far as this (and, as I may
parenthetically observe, I am not myself inclined to go so far), must
we not at least conclude that the natural rhythm of these tissues is
not _exclusively_ due to any mechanism whereby the discharges of the
ganglia are interrupted at regular intervals; but that whether these
discharges are supposed to be interrupted or continuous, the natural
rhythm is probably in a large measure due to the same cause as the
artificial rhythm, viz. in accordance with our previous hypothesis,
to the alternate exhaustion and recovery of the excitable tissues?
This much, at least, must be allowed even by the most cautious of
critics, viz. that if, as current views respecting the theory of
rhythm would suppose, it is exclusively the ganglionic element which
in the unmutilated Aurelia causes the rhythm of the swimming motions
by intermittent stimulation, surely it becomes a most unexpected
and unaccountable fact, that after the removal of this element the
contractile tissues should still persist in their display of rhythm
under the influence of constant stimulation. At any rate no one, I
think, will dispute that the facts which I have adduced justify us in
reconsidering the whole theory of rhythm as due to ganglia.

  [25] It will not be forgotten that there are a multitude of
  ganglion-cells distributed throughout the contractile tissues
  of the Medusæ; but forasmuch as these are comparatively rarely
  instrumental in originating stimulation, I think it is probable
  that artificial stimulation acts directly on the contractile
  tissues, and not through the medium of these scattered cells.

As I have already said, I am not inclined to deny that there is
probably some truth in the current theory of rhythm as due to
ganglia; I merely wish to point out distinctly that this theory is
inadequate, and that in order to cover all the facts it will require
to be supplemented by the theory which I now propose. The current
theory of rhythm as due to ganglia attributes the whole of the effect
to the ganglionic element, and thus fails to meet the fact of a
rhythm which is artificially produced after the ganglionic element
has been removed. It also fails to meet a number of other facts of
the first importance; for it is beyond all doubt that rhythmic action
of the strictest kind occurs in an innumerable multitude of cases
where it is quite impossible to suppose anything resembling ganglia
to be present. Not to mention such cases as the Snail's heart, where
the most careful scrutiny has failed to detect the least vestige
of ganglia, but to descend at once to the lowest forms of animal
and vegetable life, rhythmic action may here be said to be the rule
rather than the exception. The beautifully regular motions observable
in some Algæ, Diatomaceæ, and Ocillatoriæ, in countless numbers of
Infusoria, Antherozoids, and Spermatozoa, in ciliary action, and even
in the petioles of Hedysarum gyrano, are all instances (to which many
others might be added) of rhythmical action where the presence of
ganglia is out of the question. Again, in a general way, is it not
just as we recede from these primitive forms of contractile tissue
that we find rhythmic action to become less usual? And, if this is
so, may it not be that those contractile tissues which in the higher
animals manifest rhythmic action are the contractile tissues which
have longest retained their primitive endowment of rhythmicality?
To my mind it seems hard to decide in what respect the beating of
a Snail's heart differs from that of the pulsatile vesicles of the
Infusoria; and I do not think it would be much easier to decide in
what essential respect it differs from the beating of the Mammalian
heart. The mere fact that the presence of ganglia can be proved in
the one case and not in the other, seems to me scarcely to justify
the conclusion that the rhythm is in the one case wholly dependent,
and in the other as wholly independent, of the ganglia. At any rate,
this fact, if it is a fact, is not of so self-evident a character
as to recommend to us the current theory of ganglionic action on _à
priori_ grounds.

Coming, then, to experimental tests, we have already seen that in the
deganglionated swimming organ of Aurelia aurita, rhythmic response is
yielded to constant faradaic stimulation of low intensity. The next
question, therefore, which presents itself in relation to our subject
is as to whether other modes of constant stimulation elicit a similar
response. Now, in a general way, I may say that such is the case,
although I have chosen faradaic stimulation for special mention,
because, in the first place, its effect in producing rhythmic
action is the most certain and precise; and, in the next place, the
effects of administering instantaneous shocks at given intervals
admit of being compared with the effects of constant faradaic
stimulation better than with any other kind of constant stimulation.
Nevertheless, as just stated, other modes of constant stimulation
certainly have a more or less marked effect in producing rhythmic
response. The constant current, during the whole time of its passage,
frequently has this effect in the case of the paralyzed nectocalyx
of Sarsia; and dilute spirit, or other irritant, when dropped on the
paralyzed swimming organ of Aurelia aurita, often gives rise to a
whole series of rhythmical pulsations, the systoles and diastoles
following one another at about the same rate as is observable in the
normal swimming motions of the unmutilated animal.

From this it will be seen that, both in the case of mechanical and
of chemical stimulation, the same tendency to the production of
rhythmic response on the part of the paralyzed tissues of Aurelia
may be observed as in the case of electrical stimulation. The
principal differences consist in the rhythm being much less sustained
in the former than in the latter case. But, by experimenting on
other species of Medusæ, I have been able to obtain, in response to
mechanical and chemical stimulation, artificial rhythm of a much more
sustained character than that which, under such modes of stimulation,
occurs in Aurelia. I have no explanation to offer why it is that some
species, or some tissues, present so much more readiness to manifest
sustained rhythm under certain modes of stimulation, and less
readiness to manifest it under other modes, than do other species or
tissues. Probably these differences depend on some peculiarities in
the irritability of the tissues which it is hopeless to ascertain;
but, in any case, the facts remain, that while Aurelia, Cyanæa, and
the covered-eyed Medusæ generally are the best species for obtaining
artificial rhythm under the influence of faradaic stimulation, some
of the naked-eyed Medusæ are the best species for obtaining it
under the influence of the constant current, and also under that
of mechanical and chemical stimulation. I have already spoken of
this effect of the constant current in the case of Sarsia; I shall
now proceed to describe the effects of mechanical and chemical
stimulation on the same species.

It is but rarely that artificial rhythm can be produced in the
paralyzed nectocalyx of Sarsia by means of mechanical stimulation,
but in the case of the manubrium, a very decided, peculiar, and
persistent rhythm admits of being produced by this means. In this
particular species, the manubrium never exhibits any spontaneous
motion after the ganglia of the nectocalyx have been removed. But if
it be nipped with the forceps, or otherwise irritated, it contracts
strongly and suddenly; it then very slowly and gradually relaxes
until it has regained its original length. After a considerable
interval, and without the application of any additional stimulus,
it gives another single, sudden, though slight contraction, to be
again followed by gradual relaxation and a prolonged interval of
repose, which is followed in turn by another contraction, and so on.
These sudden and well-marked contractions occur at intervals of many
seconds, and show a decided tendency to rhythmic periodicity, though
the rhythm is not always perfectly exact. This intensely slow rhythm,
as the result of injury, may continue for a long time, particularly
if the injury has been of a severe character. There can be no doubt,
therefore, that the mechanical (or other) injury in this case acts
as a source of constant irritation; so that here again we have
evidence of rhythmic action independent of ganglia, and caused by the
alternate exhaustion and recovery of contractile tissues.[26]

  [26] We may pretty safely conclude that ganglia are altogether
  absent in the manubrium of Sarsia, not only because Schultz
  has failed to detect them in this organ microscopically, but
  also because of the complete absence of spontaneity which it
  manifests. I may here mention that this case of the manubrium of
  Sarsia is precisely analogous to another which I have observed
  in a widely different tissue, namely, the tongue of the frog.
  Here, too, the presence of ganglion-cells has never been observed
  microscopically, though specially sought for by Dr. Klein and
  others. Yet, under the influence of mechanical and other modes
  of stimulation, I find that I am able to make the excised organ
  pulsate as rhythmically as a heart.

With regard to artificial rhythm caused by chemical stimuli, by far
the most conspicuous instance that I have observed is that of the
paralyzed nectocalyx of Sarsia. This consists in a highly peculiar
motion of a flurried, shivering character, which is manifested by
this organ when its marginal ganglia have been removed and it is
exposed to the influence of faintly acidulated water. Now, when read
in the light of the foregoing facts, there can be no doubt that
the present one falls into its place very satisfactorily: it is an
additional and very valuable instance of the display of artificial
rhythm under the influence of a constant stimulus of low intensity;
for the shivering motions of the mutilated nectocalyx under these
circumstances are most unmistakably of a rhythmic nature. Viewed from
a little distance, indeed, these motions are not distinguishable
from the natural swimming motions of the unmutilated animal,
except that, not being of quite such a powerful character, they
are not so effective for locomotion. Viewed more closely, however,
it may frequently be seen that the whole bell does not contract
simultaneously, but that, as it were, clouds of contraction pass now
over one part and now over another. Still, whether the contractions
are partial or universal, they are more or less rhythmical. As this
was the only case that had ever been observed of rhythm as due to a
constant chemical stimulus, I studied it with much care, and shall
now give a full description of what appears to me an important body
of physiological facts.

Ten to twenty drops of acetic acid having been added to one thousand
cubic centimetres of sea-water, and the paralyzed bell of Sarsia
having been placed in the mixture, an interval of about half a minute
will elapse before any movement begins. Sooner or later, however, the
artificial rhythm is sure to be induced, and it will then continue
for a variable time--occasionally as long as an hour, and generally
for a considerable number of minutes. After it ceases it may often
be made to recommence, either by adding a few more drops of acid to
the sea-water, or by supplying an additional stimulus to the bell
by nipping it with the forceps. Eventually, however, all movement
ceases, owing to the destruction of irritability by the action of the
acid. By this time the whole inner surface of the bell has become
strongly opalescent, owing to the destructive influence of the acid
on the epithelial cells which overspread the irritable tissues.
The latter fact is worth mentioning, because in no case does the
artificial rhythm set in until this opalescence has begun to show
itself; and as this opalescence is but an optical expression of the
damage which the epithelial coat is undergoing, the explanation of
the time which elapses after the first immersion of the bell in the
acidulated water and the commencement of the artificial rhythm no
doubt is, that during this time the acid has not obtained sufficient
access to the excitable tissues to serve as an adequate stimulus.

During the soaking stage of the experiment--_i.e._ before the
artificial rhythm begins--the excitability of the tissues may be
observed progressively and abnormally to increase; for soon after the
soaking stage begins, in response to a single nip with the forceps
the bell may give two or three locomotor contractions, instead of
a single one, as is _invariably_ the case with a paralyzed bell of
Sarsia in normal water. Later on during the soaking stage, four
or five successive contractions may be yielded in response to a
single mechanical stimulus, and shortly after this a whole bout of
rhythmic contractions may be started by the same means. Indeed, in
some cases the artificial rhythm in acidulated water requires such
a single additional stimulus for its inauguration, the shivering
movements failing to begin spontaneously, but beginning immediately
upon the application of the additional stimulus. Similarly, after
the shivering movements have ceased, a fresh bout may very often
be started by again giving the motionless nectocalyx a single
stimulation. The interpretation of these facts would seem to be that
the general irritability of the excitable tissues is exalted by the
universal and constant stimulus supplied by the acid to an extent
that is just bordering on that which gives rise to rhythmic movement,
so that when the violent contraction is given in response to the
mechanical stimulus, the disturbance serves to start the rhythmic
movement.

If a paralyzed nectocalyx, while manifesting its artificial
rhythm in acidulated sea-water, be suddenly transferred to normal
sea-water, the movements do not cease immediately, but continue for
a considerable time. This fact can easily be explained by the very
probable, and indeed almost necessary, supposition that it takes some
time after the transference to the normal sea-water for the acid to
be washed out from contact with the excitable tissues. Sooner or
later, however, as we should expect, in the normal sea-water the
rhythmic movements entirely cease, and the bell becomes quiescent,
with a normal irritability as regards single stimuli. If it be now
again transferred to the acidulated water, after a short interval
the rhythmic movements will again commence, and so on during several
repetitions of this experiment, until the irritability of the tissues
has finally become destroyed by the influence of the acid.

Other chemical irritants which I have tried produce substantially
similar effects on the paralyzed bell of Sarsia. I shall, therefore,
only wait to describe the influence of one of these irritants, the
action of which in some respects differs from that of acids, and
which I have found to be one of the most unfailing in its power
to produce the rhythmic movements in question. This irritant is
glycerine, and in order to produce its full effect it requires to be
added to the sea-water in about the proportion of five per cent. The
manifestation of artificial rhythm in solutions of this kind is quite
unfailing. It begins after an exposure of from fifteen to thirty
seconds, and continues for a variable number of seconds. It generally
begins with powerful contractions, of a less shivering character than
those which are produced by acids, and therefore still more closely
resembling the normal swimming motions of the unmutilated animal.
Sometimes, however, the first manifestation of the artificial rhythm
is in the form of a few gentle rhythmic contractions, to be followed
by a few seconds of quiescence, and then by the commencement of the
sustained bout of strong contractions. In either case, when the
bout of strong contractions sets in, the rate of the rhythm becomes
progressively and rapidly increased, until it runs up into incipient
tetanus. The rate of the rhythm still quickening, the tetanus rapidly
becomes more and more pronounced, till at last the bell becomes
quiescent in tonic spasm.[27]

  [27] Sometimes, however, the order of events is slightly
  different, the advent of the spasm being more sudden, and
  followed by a mitigation of its severity, the bell then
  exhibiting what is more usually the first phase of the series,
  namely, the occurrence of the locomotor-like contractions.
  Occasionally, also, rhythmical shivering contractions may be seen
  superimposed on the general tonic contraction, either in a part
  or over the whole of the contractile tissues.

If the bell is still left in the glycerine solution nothing further
happens; the tissues die in this condition of strong systole. But if
the bell be transferred to normal sea-water immediately after, or,
still better, slightly before the tonic spasm has become complete, an
interesting series of phenomena is presented. The spasm persists for
a long time after the transference without undergoing any change,
the length of this time depending on the stage in the severity
and the spasm at which the transference is made. After this time
is passed, the spasm becomes less pronounced than it was at the
moment of transference, and a reversion takes place to the rhythmic
contractions. The spasm may next pass off entirely, leaving only
the rhythmic contractions behind. Eventually these too fade away
into quiescence, but it is remarkable that they leave behind them
a much more persistent exaltation of irritability than is the case
with acid. For in the case of glycerine, the paralyzed bell which
has been exposed to the influence of the irritant and afterwards
become quiescent in normal sea-water, will often continue for hours
to respond to single stimuli with a bout of rhythmic contractions.
This effect of glycerine in producing an extreme condition of exalted
irritability is also rendered apparent in another way; for if,
during the soaking stage of the experiment--_i.e._ before the first
of the rhythmic contractions has occurred--the bell be nipped with
the forceps, the effect may be that of so precipitating events that
the whole of the rhythmic stages are omitted, and the previously
quiescent bell enters at once into a state of rigid tonic spasm. This
effect is particularly liable to occur after prolonged soaking in
weak solutions of glycerine.

As in the case of stimulation by acid, so in that of stimulation by
glycerine, the artificial rhythm never begins in any strength of
solution until the epithelial surface has become opalescent to a
considerable degree.

In the case of stimulation by glycerine, the behaviour of the
manubrium is more unequivocal than it is in the case of stimulation
by acid. I have therefore reserved till now my description of the
behaviour of this organ under the influence of constant chemical
stimulation. This behaviour is of a very marked though simple
character. The manubrium is always the first to respond to the
stimulation, its retraction preceding the first movements of the
bell by an interval of several seconds, so that by the time the bell
begins its rhythmic response the manubrium is usually retracted to
its utmost. The initial response of the manubrium is also rhythmic,
and the rhythm which it manifests--especially if the glycerine
solution be not over-strong--is of the same slow character which
has already been described as manifested by this organ when under
the influence of mechanical stimulation. The rhythm, however, is
decidedly quicker in the former than in the latter case.

Lastly, with regard to the marginal ganglia, it is to be observed
that in the case of all the chemical irritants I have tried,
if unmutilated specimens of Sarsia be immersed in a sea-water
solution of the irritant which is of a sufficient strength to evoke
artificial rhythm in paralyzed specimens, the spontaneity of the
ganglia is destroyed in a few seconds after the immersion of the
animals, _i.e._ in a shorter time than is required for the first
appearance of artificial rhythm. Consequently, whether the specimens
experimented upon be entire or paralyzed by removal of their margins,
the phenomena of artificial rhythm under the influence of chemical
stimulation are the same. But although the spontaneity of the ganglia
disappears before the artificial rhythm sets in, such is not the case
with the reflex activity of the ganglia; for on nipping a tentacle of
the quiescent bell before the artificial rhythm has set in, the bell
will give a single normal response to the stimulation.

Hence, in historical order, on dropping an unmutilated specimen of
Sarsia into a solution of glycerine of the strength named, the usual
succession of events to be observed is an follows. First, increased
activity of the normal swimming motions, to be quickly followed by
a rapid and progressive decrease of such activity, till in about
fifteen seconds after the immersion total quiescence supervenes. Four
or five seconds later the manubrium begins to retract by rhythmical
twitches, the rate of this rhythm rapidly increasing until it ends
in tonic contraction. When the manubrium has just become fully
retracted--or very often a little earlier--the bell suddenly begins
its forcible and well-pronounced rhythmic contractions, which rapidly
increase in their rate of rhythm until they coalesce into a vigorous
and persistent spasm. If the animal be now restored to normal
sea-water, spontaneity will return in a feeble manner; but there is
always afterwards a great tendency displayed by the bell to exhibit
shivering spasms instead of normal swimming movements in response
to natural or ganglionic stimulation. And, as already observed, this
peculiarity of the excitable tissues is also well marked in the case
of the artificial stimulation of deganglionated specimens under
otherwise similar conditions.

One further experiment may here be mentioned. Having split open
the paralyzed bell of Sarsia along the whole of one side from base
to apex of the cone, I suspended the now sheet-like mass of tissue
by one corner in the air, leaving the rest of the sheet to hang
vertically downwards. By means of a rack-work support I now lowered
the sheet of tissue, till one portion of it dipped into a beaker
filled with a solution of glycerine of appropriate strength. After
allowing this portion to soak in the solution of glycerine until it
became slightly opalescent, I dropped the entire mutilated bell, or
sheet of tissue, into another beaker containing sea-water. If the
exposure to the glycerine solution had been of sufficient duration, I
invariably found that in the normal sea-water the rhythmic movements
were performed by the whole tissue-mass quite as efficiently as was
the case in my other experiments, where the whole tissue-mass, and
not merely a portion, had been submitted to the influence of the
irritant. But on now suddenly snipping off the opalescent portion
of the tissue-mass, _i.e._ the portion which had been previously
alone submitted to the influence of the irritant, all movement in
the remainder of the tissue-mass instantly ceased. This experiment
I performed repeatedly, sometimes exposing a large and sometimes a
small portion of the tissue to the influence of the irritant. As I
invariably obtained the same result, there can be no doubt that in
the case of chemical stimulation the artificial rhythm depends for
its manifestation on the presence of a constant stimulus, and is not
merely some kind of obscure fluttering motion which, having been
started by a stimulus, is afterwards kept up independently of any
stimulus.

Such being the case, I naturally expected that if I were to supply
a constant stimulus of a thermal kind, I should also obtain the
phenomena of artificial rhythm. In this, however, my expectations
have not been realized. With no species of Medusa on of artificial
rhythm by immersing the paralyzed animals in heated water. I can only
explain this fact by supposing that the stimulus which is supplied
by the heated medium is of too uniform a character over the whole
extent of the excitable tissues; it would seem that in order to
produce artificial rhythm there must be a differential intensity of
stimulation in different parts of the responding tissue, for no doubt
even the excitatory influence of acidulated water is not of nearly so
uniform an intensity over the whole of the tissue-area as is that of
heated water.

In now quitting the subject of artificial rhythm as it is manifested
by the paralyzed bell of Sarsia, it is desirable again to observe
that sustained artificial rhythm cannot be produced by means of
chemical irritation in the case of any one of the species of
covered-eyed Medusæ that I have met with. In order to evoke any
response at all, stronger solutions of the irritants require to be
employed in the case of the covered than in that of the naked-eyed
Medusæ, and when the responses do occur they are not of so suggestive
a character as those which I thought it worth while so fully to
describe. Nevertheless, even in the covered-eyed Medusæ well marked,
though comparatively brief, displays of artificial rhythm may often
be observed as the result of constant chemical stimulation. Thus,
for instance, in the case of Aurelia, if the paralyzed umbrella be
immersed in a solution of glycerine (ten to twenty per cent.), a few
rhythmic pulsations of normal rate are usually given; but shortly
after these pulsations occur, the tissue begins to go into a tetanus,
which progressively and rapidly becomes more and more pronounced
until it ends in violent tonic spasm. So that the history of events
really resembles that of Sarsia under similar circumstances, except
that the stage of artificial rhythm which inaugurates the spasm is of
a character comparatively less pronounced.

Thus far, then, I have detailed all the facts which I have been able
to collect with reference to the phenomena of artificial rhythm, as
produced by different kinds of constant stimulation. It will not be
forgotten that the interest attaching to these facts arises from the
bearing which they have on the theory of natural rhythm. My belief is
that hitherto the theory of rhythm as due to ganglia has attributed
far too much importance to the ganglionic as distinguished from
the contractile tissues, and I have founded this belief principally
on the facts which have now been stated, and which certainly prove
at least this much: that after the removal of the centres of
spontaneity, the contractile tissues of the Medusæ display a marked
and persistent tendency to break into rhythmic action whenever they
are supplied with a constant stimulus of feeble intensity. Without
waiting again to indicate how this fact tends to suggest that the
natural rhythm of the unmutilated organisms is probably in large
part due to that alternate process of exhaustion and restoration of
excitability on the part of the contractile tissues, whereby alone
the phenomena of artificial rhythm can be explained,[28] I shall go
on to describe some further experiments which were designed to test
the question whether the influences which affect the character of the
natural rhythm likewise, and in the same manner, affect the character
of the artificial rhythm. I took the trouble to perform these
experiments, because I felt that if they should result in answering
this question in the affirmative, they would tend still further to
substantiate the view I am endeavouring to uphold, viz. that the
natural rhythm may be a function of the contractile as distinguished
from the ganglionic tissue. Of the modifying causes in question, the
first that I tried was temperature.

  [28] It is of importance to point out the fact that some of my
  previously stated experiments appear conclusively to prove that
  the natural stimulation which is supplied by the marginal ganglia
  of the Medusæ resembles all the modes of artificial stimulation
  which are competent to produce artificial rhythm in one important
  particular; the _intensity_ of the stimulation which the marginal
  ganglia supply is shown by these experiments to be about the same
  as that which is required to produce artificial rhythm in the
  case of artificial stimulation. In proof of this point, I may
  allude particularly to the observations which are detailed on pp.
  131-136.

Having already treated of the effects of temperature on the natural
rhythm, it will now be sufficient to say that we have seen these
effects to be similar to those which temperature exerts on the rhythm
of ganglionic tissues in general. Now, I find that temperature
exerts precisely the same influence on the artificial rhythm of
deganglionated tissue as it does on the natural rhythm of the
unmutilated animal. To economize space, I shall only quote one of my
observations in a table which explains itself. I also append tracings
of another observation, to render the difference in the rate of the
artificial rhythm more apparent to the eye (Fig. 28).

    Temperature of water  |  Number of contractions
        (Fahr.).          |       per minute.
          25°             |           24
          45°             |           40
          75°             |           60

During the whole progress of such experiments the faradaic
stimulation was, of course, kept of uniform intensity; so that the
progressive acceleration is undoubtedly due to the increase of
temperature alone. With each increment of temperature the rate of
the artificial rhythm increases suddenly, just as it does in the
case of the natural rhythm. Moreover, there seems to be a sort of
rough correspondence between the amount of influence that any given
degree of temperature exerts on the rate of the natural and of the
artificial rhythm respectively. Further, it will be remembered that
in warm water the natural rhythm, besides being quicker, is not so
regular as it is in cold water; thus also it is with the artificial
rhythm. Again, water below 20° or above 85° suspends the natural
rhythm, _i.e._ stops the contractions; and the artificial rhythm
is suspended at about the same degrees. Lastly, just as there are
considerable individual variations in the extent to which the natural
rhythm is affected by temperature, so the artificial rhythm is in
some cases more influenced by this cause than in others, though in
all cases it further resembles the natural rhythm in showing some
considerable degree of modification under such influence.

  [Illustration: Fig. 28.]

On the whole, then, it would be impossible to imagine two cases more
completely parallel than are these of the effects of temperature on
natural and on artificial rhythm respectively; and as it must be
considered in the last degree improbable that all these coincidences
are accidental, I conclude that the effects of temperature on the
natural rhythm of Medusæ (and so, in all probability, on the natural
rhythm of other ganglio-muscular tissues) are for the most part
exerted, not on the ganglionic, but on the contractile element.

In order to test the effects of gases on the artificial rhythm, I
took a severed quadrant of Aurelia, and floated it in sea-water,
with its muscular surface just above the level of the water. Over
the tissue I lowered an inverted beaker filled with the gas the
effects of which I desired to ascertain, and by progressively forcing
the rim of the beaker into the water I could submit the tissue to
various pressures of the atmosphere of the gas I was using. By an
appropriate arrangement the electrodes passed into the interior of
the beaker, and could then be manipulated from the outside, so as to
be properly adjusted on the tissue. In this way I was able to observe
that different gases exerted a marked influence on the rate of the
artificial rhythm.

The following table gives the ratios in the case of one experiment:--

    Rate of artificial rhythm,  |                  |
              in air.           |    In oxygen.    |   In carbonic acid.
                                |                  |
          36 per minute.        |  50 per minute.  |     25 per minute.

It may here be observed that to produce these results, both carbonic
acid and oxygen must be considerably diluted with air, for otherwise
they have the effect of instantaneously inhibiting all response,
even to the strongest stimulation. When this is the case, however,
irritability returns very soon after the tissue is again exposed
to air or to ordinary sea-water. But I desire it to be understood
that the results of my experiments on the influence of oxygen,
both on the natural and on the artificial rhythm, have proved
singularly equivocal; so that as far as this gas is concerned further
observations are required before the above results can be accepted as
certain.

I have still one other observation of a very interesting character
to describe, which is closely connected with the current views
respecting ganglionic action, and may therefore be more conveniently
considered here than in any other part of this treatise. I have
already stated that in no case is the manubrium of a Medusa affected
as to its movements by removal of the periphery of the swimming-bell;
but in the case of Sarsia a very interesting change occurs in the
manubrium soon after the nectocalyx has been paralyzed by excision
of its margin. Unlike the manubriums of most of the other Medusæ,
this organ, in the case of Sarsia, is very highly retractile. In
fresh and lively specimens the appendage in question is carried in
its retracted state; but when the animals become less vigorous--from
the warmth or impurity of the water in which they are confined, or
from any other cause--their manubriums usually become relaxed. The
relaxation may show itself in various degrees in different specimens
subjected to the same conditions, but in no case is the degree of
relaxation so remarkable as that which may be caused by removing
the periphery of the nectocalyx. For the purpose of showing this
effect, it does not signify in what condition as to vigour, etc., the
specimen chosen happens to be in; for whether the manubrium prior to
the operation be contracted or partially relaxed, within half an hour
after the operation it is sure to become lengthened to a considerable
extent.

In order to show the surprising degree to which this relaxation may
proceed, I insert a sketch of a specimen both before and after the
operation. The sketches are of life size, and drawn to accurate
measurement (Figs. 29 and 30).[29]

  [29] I may here mention that the fact of the manubrium of Sarsia
  undergoing this extreme elongation after the removal of the
  marginal ganglia, serves to render the artificial rhythm of the
  organ under the influence of injury, as previously described, all
  the more conspicuous.

  [Illustration: Fig. 29.]

  [Illustration: Fig. 30.]

With regard to this remarkable effect on the manubrium of removing
the margin of the nectocalyx, it is now to be observed that in it
we appear to have very unexceptionable evidence of such a relation
subsisting between the ganglia of the nectocalyx and the muscular
fibres of the manubrium as elsewhere gives rise to what is known as
muscular tonus. This interpretation of the facts cannot, I think, be
disputed; and it fully explains why, in the unmutilated animal, the
degree of elongation on the part of the manubrium usually exhibits
an inverse proportion to the degree of locomotor activity displayed
by the bell. I may here state that I have also observed indications
of muscular tonus in some of the other Medusæ, but for the sake of
brevity I shall now restrict myself to the consideration of this one
case.

To my mind, then, it is an interesting fact that ganglionic tissue,
where it can first be shown to occur in the animal kingdom, has for
one of its functions the maintenance of muscular tonus; but it is
not on this account that I now wish to draw prominent attention to
the fact before us. Physiologists are almost unanimous in regarding
muscular tonus as a kind of gentle tetanus due to a persistent
ganglionic stimulation, and against this opinion it seems impossible
to urge any valid objection. But, in accordance with the accepted
theory of ganglionic action, physiologists further suppose that the
only reason why some muscles are thrown into a state of tonus by
ganglionic stimulation, while other muscles are thrown into a state
of rhythmic action by the same means, is because the resistance to
the passage of the stimulation from the ganglion to the muscle is
less in the former than in the latter case. Here, be it remembered,
we are in the domain of pure speculation: there is no experimental
evidence to show that such a state of differential resistance as
the theory requires actually obtains. Hence we are quite at liberty
to suppose any other kind of difference to obtain, either to the
exclusion of this one or in company with it. Such a supposition I
now wish to suggest, and it is this--that all rhythmical action
being regarded as due (at any rate in large part) to the alternate
exhaustion and restoration of excitability on the part of contractile
tissues, the reason why continuous ganglionic stimulation produces
incipient tetanus in the case of some muscles and rhythmic action,
in the case of others, is either wholly or partly because the
irritability of the muscles in relation to the intensity of the
stimulation is greater in the former than in the latter case. If
this supposition as to differential irritability be granted, my
experiments on paralyzed Aurelia prove that tetanus would result
in the one case and rhythmic action in the other. For it will be
remembered that in these experiments, if the continuous faradaic
stimulation were of somewhat more than minimal intensity, tetanus was
the result; while if such stimulation were but of minimal intensity,
the result was rhythmic action. Now, that in the particular case
of Sarsia the irritability of the tonically contracting manubrium
is higher than that of the rhythmically contracting bell is a
matter, not of supposition, but of observable fact; for not only is
the manubrium more irritable than the bell in response to direct
stimulation of its own substance, but it is generally more so even
when the stimuli are applied anywhere over the excitable tissues
of the bell. And from this it is evident that the phenomena of
muscular tonus, as they occur in Sarsia, tend more in favour of the
exhaustion than of the resistance theory.[30]

  [30] The evidence, however, is not altogether exclusive of the
  resistance theory, for it is quite possible that in addition to
  the high irritability of the manubrium there may be conductile
  lines of low resistance connecting this organ with the marginal
  ganglia. I entertain this supposition because, as explained in my
  Royal Society papers, I see reason to believe that the natural
  swimming movements of Sarsia are probably in part due to an
  intermittent discharge of the ganglia. I think, therefore, that
  in this particular case the ganglia supply a tolerably constant
  stimulation to the manubrium, while it is only at intervals
  that their energy overflows into the bell, and that the higher
  degree at irritability on the part of the manubrium ensures the
  tonic response of this organ at a small cost of nervous energy.
  How far the rhythm of the nectocalyx is to be attributed to the
  resistance mechanism of the ganglia, and how far to the alternate
  exhaustion and recovery of the contractile tissues, I think it is
  impossible to determine, seeing that it is impossible exactly to
  imitate the natural ganglionic stimulation by artificial means.
  But it is, I think, of importance to have ascertained at least
  this much, that in Sarsia the tonus of one organ and the rhythm
  of another, which apparently both received their stimulation from
  the same ganglia, must at any rate in part be attributed to a
  differential irritability of these organs, as distinguished from
  their differential stimulation.

I will now sum up this rather lengthy discussion. The two theories
of ganglionic action may be stated antithetically thus: in both
theories the accumulation of energy by ganglia is supposed to be
a continuous process; but while the resistance theory supposes
the rhythm to be exclusively due to an intermittent and periodic
discharge of this accumulated energy on the part of the ganglionic
tissues, the exhaustion theory supposes that the rhythm is largely
due to a periodic process of exhaustion and recovery on the part
of the responding tissues. Now, I submit that my experiments have
proved the former of these two theories inadequate to explain all the
phenomena of rhythm as it occurs in the Medusæ; for these experiments
have shown that even after the removal of the only ganglia which
serve as centres of natural stimulation, the excitable tissues still
continue to manifest a very perfect rhythm under the influence of any
mode of artificial stimulation (except heat), which is of a constant
character and of an intensity sufficiently low not to produce
tetanus. And as I have proved that the rhythm thus artificially
produced is almost certainly due to the alternate process of
exhaustion and recovery which I have explained, there can scarcely be
any doubt that in the natural rhythm this process plays an important
part, particularly as we find that temperature and gases exert the
same influences on the one rhythm as they do on the other. Again, as
an additional reason for recognizing the part which the contractile
tissues probably play in the production of rhythm, I have pointed
to the fact that in the great majority of cases in which rhythmic
action occurs the presence of ganglia cannot be suspected. For it is
among the lower forms of life, where ganglia are certainly absent,
and where the functions of stimulation and contraction appear to be
blended and diffused, that rhythmic action is of the most frequent
occurrence; and it in obvious with how much greater difficulty the
resistance theory is here beset than is the one I now propose.
Granted a diffused power of stimulation with a diffused power of
response, and I see no essential difference between the rhythmic
motions of the simplest organism and those of a deganglionated Medusa
in acidulated water. Lastly, the facts relating to the tonus of
the manubrium in Sarsia furnish very striking, and I think almost
conclusive, proof of the theory which I have advanced.




CHAPTER IX.

POISONS.


1. _Chloroform._--My observations with regard to the distribution
of nerves in Sarsia led me to investigate the order in which these
connections are destroyed, or temporarily impaired, by anæsthetics.
The results, I think, are worth recording. In Sarsia the following
phases always mark the progress of anæsthesia by chloroform, etc.--1.
Spontaneity ceases. 2. On now nipping a tentacle, pulling the
manubrium, or irritating the bell, a _single_ locomotor contraction
is given in answer to every stimulation. (In the unanæsthesiated
animal a _series_ of such contractions would be the result of such
stimulation.) 3. After locomotor contractions can no longer be
elicited by stimuli, nipping a tentacle or the margin of the bell has
the effect of causing the manubrium to contract. 4. After stimulation
of any part of the nectocalyx (including tentacles) fails to produce
response in any part of the organism, the manubrium will continue its
response to stimuli applied directly to itself.

2. _Nitrite of Amyl._--On Sarsia the effect of this agent is much
the same as that of chloroform--the description just given being
quite as applicable to the affects of the nitrite as to those of
chloroform. Before the loss of spontaneity supervenes, the rate of
the rhythm is increased, while the strength of the pulsations is
diminished.

Tiaropsis diademata, from the fact of its presenting a very regular
rhythm and being but of small size, is a particularly suitable
species upon which to conduct many experiments relating to the effect
of poisons. On this species the nitrite in appropriate (_i.e._ in
very small) doses first causes irregularity and enfeeblement of
the contractions, together with quickening of the rhythm. After a
short time, a gradual cessation of the swimming motions becomes
apparent--these motions dying out more gradually, for example, than
they do under the influence of chloroform. Eventually each pulsation
is marked only by a slight contraction of the muscular tissue in the
immediate neighbourhood of the margin. If the dose has been stronger,
however, well-marked spasmodic contractions come on and obliterate
such gradual working of the poison. In all cases irritability of all
parts of the animal persists for a long time after entire cessation
of spontaneous movements--perhaps for three or four minutes in not
over-poisoned animals; but eventually it too disappears. On being now
transferred to normal sea-water, the process of recovery is slower
than it is after anæsthesiation by chloroform. It is interesting,
moreover, to observe, that just as the power of co-ordination was the
first thing to be affected by the nitrite, so it is the last thing
to return during recovery.

3. _Caffein._--The effects of caffein on Sarsia may be best studied
by immersing the animals in a saturated sea-water solution of the
substance. In such solutions the Medusæ float to the surface, in
consequence of their lower specific gravity. I therefore used shallow
vessels, in order that the margins of the nectocalyces might rest in
the level of the water that was thoroughly saturated. The immediate
effect of suddenly immersing Sarsia in such a solution is very
greatly to increase the rate of the pulsations, and, at the same
time, to diminish their potency. The appearance presented by the
swimming motions is therefore that of a fluttering nature; and such
motions are not nearly so effectual for progression as are the normal
pulsations in unpoisoned water. This stage, however, only lasts for
a few seconds, after which the spontaneous motions begin gradually
to fade away. Soon they altogether cease, though occasionally one
among a number of Sarsia confined in the same saturated solution will
continue, even for several minutes after the first immersion, to give
one or two very feeble contractions at long intervals. Eventually,
however, all spontaneity ceases on the part of all the specimens, and
now the latter will continue for a very long time to be sensitive to
stimulation. At first _several_ feeble locomotor contractions will
be given in response to each stimulus; and as on the one hand these
contractions never originate spontaneously, while, on the other
hand, _paralyzed_ Sarsia never respond to a single stimulus with
more than a single contraction, these multiple responses must, I
think, be ascribed to a state of exalted reflex irritability. After
a long exposure to the poison, however, only a single response is
given to each stimulus; and still later all irritability ceases.
On now transferring the Sarsia to unpoisoned water, recovery is
effected even though the previous exposure has been of immensely long
duration, _e.g._ an hour.

An interesting point with regard to caffein-poisoning of Sarsia is,
that as soon as spontaneity ceases the tentacles and manubrium lose
their tonus and become relaxed to their utmost extent. This is not
the case with anæsthesiation by chloroform, even when pushed to
the extent of suspending irritability. If, however, Sarsia which
have been anæsthesiated to this extent in chloroform be suddenly
transferred to a solution of caffein, the tentacles and manubrium may
soon be seen to relax, and eventually these organs lose their tonus
as completely as if the anæsthesia had from the first been produced
by the caffein. Moreover in this experiment the irritability, which
had been destroyed by the chloroform, returns in the solution
of caffein--provided the latter be not quite saturated--though
spontaneity of course remains suspended throughout.

The effects of graduating the doses of caffein may be stated in
connection with another species, viz. Tiaropsis diademata. In
a weak solution the effects are a quickening of the pulsations
(_e.g._ from 64 to 120 per minute) together with a lessening of
their force. On slightly increasing the dose, the pulsations become
languid, and prolonged pauses supervene. If the dose is again
somewhat strengthened, the pulsations become weaker and weaker, till
they eventually cease altogether. The animal, however, is now in
a condition of exalted reflex irritability; for its response to a
single stimulus consists not merely, as in the unpoisoned animal,
of a single spasm, but also, immediately after this, of a series
of convulsive movements somewhat resembling swimming movements
destitute of co-ordination. If the strength of the solution be now
again increased, a stage of deeper anæsthesiation may be produced, in
which the Medusa will only respond to each stimulation by a single
spasm. In still stronger solutions, the only response is a single
feeble contraction; while in a nearly saturated solution the animal
does not respond at all. But even from a saturated solution Tiaropsis
diademata will recover when transferred to unpoisoned water.

4. _Strychnia._--The species of covered-eyed Medusa which I shall
choose for describing the action of strychnia is Cyanæa capillata,
which is most admirably adapted for experiments with this and some
of the other alkaloid poisons, from the fact that in water kept
at a constant temperature its pulsations are as regular as are
those of a heart. After Cyanæa capillata has been allowed to soak
for ten minutes or so in a weak sea-water solution of strychnia,
unmistakable signs of irregularity in the pulsations supervene.
This irregularity then increases more and more, till at length
it grows into well-marked convulsions. The convulsions manifest
themselves in the form of extreme deviations from the rhythmical
contractions so characteristic of Cyanæa capillata. Instead of the
heart-like regularity with which systole and diastole follow one
another in the unpoisoned animal, we now have periods of violent and
prolonged systole resembling tonic spasm; and when the severity of
this spasm is for a moment abated, it is generally renewed before
the umbrella has had time again to become fully expanded. Moreover,
the spasm itself is not of uniform intensity throughout the time it
lasts; but while the umbrella is in a continuously contracted state,
there are observable a perpetual succession of extremely irregular
oscillations in the strength of the contractile influence. It is
further a highly interesting fact that the convulsions are very
plainly of a _paroxysmal_ nature. After the umbrella has suffered
a prolonged period of convulsive movements, it expands to its full
dimensions, and in this form remains for some time in a state of
absolute quiescence. Presently, however, another paroxysm supervenes,
to be followed by another period of quiescence, and so on for
hours. The periods of quiescence are usually shorter than are those
of convulsion; for while the former seldom last more than forty
seconds or so, the latter may continue uninterruptedly for five or
six minutes. In short, Medusæ, when submitted to the influence of
strychnia, exhibit all the symptoms of strychnia poisoning in the
higher animals. Death, however, is always in the fully expanded form.

It seems desirable to supplement these remarks with a few additional
ones on the effects of this poison on the naked-eyed Medusæ. In
the case of Sarsia the symptoms of strychnia poisoning are not
well marked, from the fact that in this species convulsions always
take the form of locomotor contractions. The symptoms, however,
are in some respects anomalous. They are as follows. First of all
the swimming motions become considerably accelerated, periods of
quiescence intervening between abnormally active bouts of swimming.
By-and-by a state of continuous quiescence comes on, during which
the animal is not responsive to tentacular irritation, but remains
so to direct muscular irritation, giving one response to each
direct stimulus. The tentacles and manubrium are much relaxed. In a
sea-water solution just strong enough to taste bitter, this phase
may continue for hours; in fact, till a certain opalescence of the
contractile tissues--which it is a property of strychnia, as of most
other reagents, to produce--has advanced so far as to place the
tissues beyond recovery. If the exposure to such a solution has not
been very prolonged, recovery of the animal in normal water is rapid.
In a specimen exposed for two and a half hours to such a solution,
recovery began in half an hour after restoration to normal water,
but was never complete. In all cases, if the poisoning is allowed to
pass beyond the stage at which response to direct muscular irritation
ceases, the animal is dead.

On Tiaropsis indicans this poison has the effect of causing a general
spasm, which would be undistinguishable from that which in this
species results from general stimulation of any kind, were it not
that there is a marked difference in one particular. For in the case
of strychnia poisoning, the spasm, while it lasts, is not of uniform
intensity over all parts of the nectocalyx; but now one part and now
another part or parts are in a state of stronger contraction than
other parts, so that, as a general consequence, the outline of the
nectocalyx is continually changing its form. Moreover, in addition to
these comparatively slow movements, there is a continual twitching
observable throughout all parts of the nectocalyx. Each individual
twitch only extends over a small area of the contractile tissue; but
in their sum their effect is to throw the entire organ into a sort
of shivering convulsion, which is superimposed on the general spasm.
After a time the latter somewhat relaxes, leaving the former still
in operation, which, moreover, now assumes a paroxysmal nature--the
convulsions consisting of strong shudders and frequent spasms with
occasional intervals of repose.

In the case of Tiaropsis diademata the action of strychnia is very
similar, with the exception that there is no _continuous_ spasm,
although _occasional_ ones occur amid the twitching convulsions.
After a time, however, all convulsions cease, and the animal remains
quiescent. While in this condition its reflex excitability is
abnormally increased, as shown by the fact that even a gentle touch
will bring on, not merely a single responsive spasm, as in the
unpoisoned animal, but a whole series of successive spasms, which are
often followed by a paroxysm of twitching convulsions. The condition
of exalted reflex irritability is thus exceedingly well marked.
Recovery in normal water at this stage is rapid, the motions being at
first characterized by a want of co-ordination, which, however, soon
passes off.

5. _Veratrium._--In Sarsia the first effect of this poison is to
increase the number and potency of the contractions; but its later
effect is just the converse, there being then prolonged periods of
quiescence, broken only by very short swimming bouts consisting of
feeble contractions. The feebleness of the contractions gradually
becomes more and more remarkable, until at last it is with great
difficulty that they can be perceived at all; indeed, the progressive
fading away of the contractions into absolute quiescence is so
gradual that it is impossible to tell exactly when they cease. During
the quiescent stage the animal is for the first time insensible both
to tentacular and to direct stimulation of the contractile tissues.
That the gradual dying out of the strength of the contractions is
not altogether due to the progressive advance of central paralysis,
would seem to be indicated by the fact that contractions in
response to direct stimulation of the contractile tissues are no
more powerful, at any given stage of the poisoning, than are either
responses to tentacular stimulation or the spontaneous contractions.
Still, as we shall immediately see, in the various species of
Tiaropsis, irritability persists after cessation of the spontaneous
contractions. In Sarsia the nervous connections between the tentacles
and manubrium, and also between the tentacles themselves, are not
impaired during the time that the bell is motionless; and even
when the irritability of the bell has quite disappeared as regards
any kind of stimulation, the manubrium and tentacles will continue
responsive to stimuli applied either directly to themselves or to any
part of the neuro-muscular sheet of the bell.

The convulsions due to the action of veratrium are well marked in
the various species of the genus Tiaropsis. They consist of violent
fluttering motions without any co-ordination; but there are no
spasms, as in the case of strychnia poisoning. After the convulsions
have lasted for some time, a quiescent stage comes on, during which
the animal remains responsive to stimulation, though not abnormally
so. Recovery in unpoisoned water is rapid, the movements being at
first marked by an absence of co-ordination.

6. _Digitalin._--The first effect of this poison on Sarsia is to
quicken the swimming motions, and then to enfeeble them progressively
till they degenerate into mere spasmodic twitches. The manubrium and
tentacles are now strongly retracted, while the nectocalyx is drawn
together so as to assume an elongated form. The latter is now no
longer responsive either to tentacular or to direct stimulation; but
the tentacles and manubrium both remain responsive to stimuli applied
either directly to themselves or to the neuro-muscular tissue of the
bell. Death always takes place in very strong systole; and as this is
an exceedingly unusual thing in the case of Sarsia, there can be no
doubt that, in this respect, the action of the digitalin is different
on the Medusæ from what it is on the heart.

On the various species of Tiaropsis, digitalin at first causes
acceleration of the swimming movements, with great irregularity and
want of co-ordination. Next, strong and persistent spasms supervene,
which give the outline of the nectocalyx an irregular form; and every
now and then this unnatural spasm gives place to convulsive swimming
motions. Evidently, however, the spasm becomes quite persistent and
excessively strong. The manubrium of Tiaropsis indicans crouches to
its utmost, and the animal dies in strong systole.

7. _Atropin._--In the case of Sarsia atropin causes convulsive
swimming motions. The systoles next become feeble, and finally cease.
The nectocalyx is now somewhat drawn together in persistent systole,
with the manubrium and tentacles strongly retracted. Muscular
irritability remains after tentacular irritability has disappeared,
but it is then decidedly enfeebled.

In the various species of Tiaropsis the convulsions are strongly
pronounced. They begin as mere accelerations of the natural swimming
motions, but soon grow into well-marked convulsions, consisting of
furious bouts of irregular systoles following one another with the
utmost rapidity, and wholly without co-ordination. Occasionally
these movements are interrupted by a violent spasm, on which strong
shuddering contractions are superimposed.

8. _Nicotin._--On dropping Sarsia into a sea-water solution of
nicotin of appropriate strength, the animal immediately goes into a
violent and continuous spasm, on which a number of rapidly succeeding
minute contractions are superimposed. The latter, however, rapidly
die away, leaving the nectocalyx still in strong and continuous
systole; tentacles and manubrium are retracted to the utmost. Shortly
after cessation of spontaneity, the bell is no longer responsive
to tentacular stimulation, but remains for a considerable time
responsive to direct stimulation of its own substance; eventually,
however, all irritability disappears, while the tentacles and
manubrium relax. On transferring the animal to normal water, muscular
irritability first returns, and then central, as shown by the earlier
response of the bell to direct than to tentacular stimulation; but if
the animal has been poisoned heavily enough to have had its muscular
irritability suspended, it is a long time before central irritability
returns. Soon after central irritability has returned, the animal
begins to show feeble signs of spontaneity, the motions being
exceedingly weak, with long intervals of repose; but the degree of
such feebleness depends on the length of time during which the animal
has previously been exposed to the poison; thus in a specimen which
had been removed from the poison immediately after the disappearance
of reflex irritability had supervened, recovery began in ten minutes
after re-immersion, and was complete in half an hour.

In Tiaropsis the symptoms of nicotin poisoning are also well marked.
When gradually administered, the first effect of the narcotic is a
complete loss of co-ordination in the swimming motions. A slight
increase of the dose brings about a tonic spasm, which differs from
the natural spasm of these animals--(_a_) in being stronger, so that
the nectocalyx becomes bell-shaped rather than square, (_b_) in
being much more persistent, and (_c_) in undergoing variations in
its intensity from time to time, instead of being a contraction of
uniform strength; thus the spasm temporarily affects some parts of
the nectocalyx more powerfully than other parts, so that the organ
may assume all sorts of shapes. Such distortions proceed even further
under the influence of nicotin than under that of strychnine, etc.
Sometimes, for instance, one quadrant will project in the form of a
pointed promontory; at other times two adjacent or opposite quadrants
will thus project, and occasionally all four will do so, the animal
thus becoming star-shaped. Sometimes, again, one quadrant will be
less contracted than the other three, while at other times more or
less slight relaxations affect numerous parts of the bell, its margin
being thus rendered sinuous, though more or less violently contracted
in all its parts. This state of violent spasm lasts for several
minutes, when it gradually passes off, the nectocalyx relaxing into
the form of a deep bowl and remaining quite passive, except that
every now and then one part or another of the margin is suddenly
contracted in a semilunar form. By-and-by, however, even these
occasional twitches cease, and the animal is now insensible to all
kinds of stimulation. Recovery in normal water is gradual, and marked
in its first stage by the occasional retractions of the margin last
mentioned. At about this stage also, or sometimes slightly later, the
animal first becomes responsive to stimulation; and it is interesting
to note that the response is performed, not by giving a general
spasm as would the unpoisoned animal, but by folding in the part
irritated--an action which very much resembles, on the one hand, the
spontaneous convulsive movements just described, and, on the other,
the response which is given to stimulation by the unpoisoned bell
when gently irritated after removal of its margin. After these stages
there supervenes a prolonged period of quiescence, during which the
animal remains normally responsive to stimulation. Spontaneity may
not return for several hours, and, after it does return, the animal
is in most cases permanently enfeebled. Indeed, on all the species
of Medusæ, nicotin, both during its action and in its subsequent
effects, is the most deadly of all the poisons I have tried.

9. _Morphia._--The anæsthesiating effects of morphia are as decided
as are those of chloroform. I shall confine myself to describing the
process of anæsthesiation in the case of Aurelia aurita in an extract
from my notes. "A very vigorous specimen, having twelve lithocysts,
was placed in a strong sea-water solution of morphia. Half a
minute after being introduced commencement of torpidity ensued,
shown by contractions becoming fewer and feebler. In one minute the
feeble impulses emanating from the prepotent lithocyst failed to
spread far through the contractile tissue, appearing to encounter a
growing resistance. Eventually this resistance became so great that
only a very small portion of contractile tissue in the immediate
neighbourhood of the lithocyst contracted, and this in a very slow
and feeble way. Two minutes after immersion even these partial
contractions entirely ceased, and soon afterwards all parts of the
animal were completely dead to stimulation. Recovery in normal water
slower than that after chloroform, but still soon quite complete.
Repeated experiment on this individual four times without injury."

10. _Alcohol._--The solution must be strong to cause complete
intoxication. The first effect on Sarsia is to cause a great increase
in the rapidity of the swimming motions--so much so, indeed, that
the bell has no time to expand properly between the occurrence of
the successive systoles, which, in consequence, are rendered feeble.
These motions gradually die out, leaving the animal quite motionless.
The nectocalyx is now responsive to stimuli applied at the tentacles,
and sometimes two or three contractions will follow such a stimulus,
as if the spontaneity of the animal were slightly aroused by the
irritation. Soon, however, only one contraction is given in response
to every tentacular irritation, and by-and-by this also ceases--the
Medusa being thus no longer responsive to central stimulation. It
remains, however, for a long time responsive to stimulation of the
neuro-muscular sheet; indeed, the strength of the alcohol solution
must be very considerable before loss of muscular irritability
supervenes. It may thus be made to do so, however; and on then
transferring the animal to normal water, recovery begins in from
three minutes to a quarter of an hour. The first contractions are
very feeble, with long intervals of repose; but gradually the animal
returns to its normal state.

The above remarks apply also to Tiaropsis. In Tiaropsis indicans
the manubrium recovers in normal water sooner than the nectocalyx.
Both in Sarsia and Tiaropsis the manubrium and tentacles are
retracted while exposed to alcohol, and, after transference to
normal sea-water, the animals float on the surface, presumably in
consequence of their having imbibed some of the spirit. The period
during which flotation lasts depends, (_a_) on the strength of
the alcohol solution used, and (_b_) on the time of exposure to
its influence. It may last for an hour or more; but in no case is
recovery complete till some time after the flotation ceases.

11. _Curare._--Curare had already been tried upon Medusæ, and was
stated to have produced no effects; it is therefore especially
desirable that I should first of all describe the method of
exhibiting it which I employed.

Having placed the Medusa to be examined in a flat-shaped beaker, I
filled the latter to overflowing with sea-water. I next placed the
beaker in a large basin, into which I then poured sea-water until
the level was the same inside and outside the breaker, _i.e._ until
the two bodies of water all but met over the brim. Having divided the
medusa across its whole diameter, with the exception of a small piece
of marginal tissue at one side to act as a connecting link between
the two resulting halves, I transferred one of these halves to the
water in the basin, leaving the other half still in the beaker--the
marginal tissue which served to unite the two halves being thus
supported by the rim of the beaker. Over the minute portion of the
marginal tissue which was thus of necessity exposed to the air,
I placed a piece of blotting-paper which dipped freely into the
sea-water. Lastly, I poisoned the water in the beaker with successive
doses of curare solution.

The results obtained by this method were most marked and beautiful.
Previous to the administration of the poison both halves of the
Medusa were of course contracting vigorously, waves of contractile
influence now running from the half in the beaker to the half in
the basin, and now _vice versâ_. But after the half in the beaker
had become effectually poisoned by the curare, all motion in it
completely ceased, the other, or unpoisoned half, continuing to
contract independently. I now stimulated the poisoned half by nipping
a portion of its margin with the forceps. Nothing could be more
decided than the result. It will be remembered that when any part
of Staurophora laciniata is pinched with the forceps or otherwise
irritated, the motion of the whole body which ensues is totally
different from that of an ordinary locomotor contraction--all parts
folding together in one very strong and long-protracted systole,
after which the diastole is very much slower than usual. Well, on
nipping any portion of the poisoned half of Staurophora laciniata,
this half remained absolutely motionless, while the unpoisoned half,
though far away from the seat of irritation, immediately ceased its
normal contractions, and folded itself together in the very peculiar
and distinctive manner just described. This observation was repeated
a number of times, and, when once the requisite strength of the
curare solution had been obtained, always with the same result. The
most suitable strength I found to be 1 in 2500, in which solution the
poisoned half required to soak for half an hour.

I also tried the effect of this poison on the covered-eyed Medusæ,
and have fairly well satisfied myself that its peculiar influence is
likewise observable in the case of this group, although not in nearly
so well-marked a manner.

It has further to be stated that when the poisoned half is again
restored to normal sea-water, the effects of curare pass off with
the same rapidity as is observable in the case of the other poisons
which I have tried. Thus, although an exposure of half an hour to the
influence of curare of the strength named is requisite to destroy the
motor power in the case of Staurophora laciniata, half a minute is
sufficient to ensure its incipient return when the animal is again
immersed in unpoisoned water.

It is also to be observed that a very slight degree of
_over_-poisoning paralyzes the transmitting system as well as the
responding one; so that if any one should repeat my observation, I
must warn him against drawing erroneous conclusions from this fact.
Let him use weak solutions with prolonged soaking, and by watching
when the voluntary motions in the poisoned half first cease, he need
experience no difficulty in obtaining results as decided as it is
possible for him to desire.

12. _Cyanide of Potassium._--On Sarsia the first effect is to quicken
the contractions and then to enfeeble them. The animal assumes an
elongated form, as already described under atropin. Spontaneity
ceases very rapidly even in weak solutions; and for an exceedingly
short time after it has done so, the bell continues responsive both
to tentacular and to direct stimulation. For a long time after the
bell ceases to respond to any kind of stimulation, the nervous
connections between the tentacles and between the tentacles and
manubrium remain intact, as also do the nervous connections of these
organs with all parts of the bell. This interesting fact is rendered
apparent, first, by stimulating a tentacle and observing that all the
four tentacles and the manubrium respond; and, second, by irritating
any part of the neuro-muscular sheet of the bell, and observing
that while the latter does not respond both the tentacles and the
manubrium retract. Recovery from this stage occupies several hours.

In the case of Tiaropsis the convulsions are, as usual, more
pronounced, being marked by the occurrence of a gradually increasing
spasm, which differs from a normal spasm in the respects already
described under strychnia. In all the species both of Sarsia and
Tiaropsis, the manubrium and tentacles are retracted during exposure
to this poison.


_Remarks._

The above comprises all the poisons which I have tried, and I think
that all the observations taken together show a wonderful degree of
resemblance between the actions of the various poisons on the Medusæ
and on the higher animals--a general fact which is of interest, when
we remember that in these nerve-poisons we possess, as it were, so
many tests wherewith to ascertain whether nerve-tissue, where it
first appears upon the scene of life, presents the same fundamental
properties as it does in the higher animals. And these observations
show that such is the case. When the physiologist bears in mind that
in Sarsia we have the means of testing the comparative influence
of any poison on the central, peripheral, and muscular systems
respectively,[31] he will not fail to appreciate the significance of
these observations. In reading over the whole list he will meet with
an anomaly here and there; but, on the whole, I do not think he can
fail to be satisfied with the wonderfully close adherence which is
shown by these elementary nervous tissues to the rules of toxicology
that are followed by nervous tissues in general. In one respect,
indeed, there is a conspicuous and uniform deviation from these
rules; for we have seen that in the case of every poison mentioned
more or less complete recovery takes place when the influence of the
poison has been removed, even though this has acted to the extent
of totally suspending irritability. In other words, there is no
poison in the above list which has the property, when applied to
the Medusæ, of destroying life till long after it has destroyed all
signs of irritability. What the cause of this uniform peculiarity may
be is, of course, conjectural; but I may suggest two considerations
which seem to me in some measure to mitigate the anomaly. In the
first place, we must remember that in the Medusæ there are no
nervous centres of such vital importance to the organism that any
temporary suspension of their functions is followed by immediate
death. Therefore, in these animals, the various central nerve-poisons
are at liberty, so to speak, to exert their full influence on all
the excitable tissues without having the course of their action
interrupted by premature death of the organism, which in higher
animals necessarily follows the early attack of the poison on a vital
nerve-centre. Again, in the second place, we must remember that the
method of administering the above-mentioned poisons to the Medusæ was
very different from that which we employ when administering them to
other animals; for, in the case of the Medusæ, the neuro-muscular
tissue is spread out in the form of an exceedingly tenuous sheet, so
that when the animal is soaking in the poisoned water every portion
of the excitable tissue is equally exposed to its influence; and that
the action of a poison is greatly modified by such a difference in
the mode of its administration has been proved by Professor Gamgee,
who found that when a frog's muscle is allowed to soak in a solution
of vanadium, etc., it loses its irritability, while this is not the
case if the poison is administered by means of the circulation.

  [31] The method of comparison consists, as will already have been
  gathered from the perusal of the foregoing sections, in:--first,
  stimulating the tentacles, and observing whether this is followed
  by such a discharge of the attached ganglion as causes the bell
  to contract; next, stimulating the bell itself, to ascertain
  whether the muscular irritability is impaired; and, lastly,
  stimulating either the tentacles or the bell, to observe whether
  the reciprocal connections between tentacles, bell, and manubrium
  are uninjured.

I may further observe that in the case of all poisons I have tried,
the time required for recovery after the animal is restored to
normal water varies immensely. The variations are chiefly determined
by the length of time during which the animal has been exposed to
the influence of the poison, but also, in a lesser degree, by the
strength of the solution employed. To take, for instance, the case
of caffein or chloroform, if Sarsia are transferred to normal water
after they first cease to move, a few seconds are enough to restore
their spontaneity; whereas, if they are allowed to remain in the
poisoned water for an hour, they may not move for one or two hours
after their restoration to unpoisoned water. In consequence of such
great variations occurring from these causes, I was not able to
compare the action of one poison with that of another in respect of
the time required for effects of poisoning to pass away.

I shall conclude all I have to say upon the subject of poisons
by stating the interesting fact, that if any of the narcotic or
anæsthesiating agents be administered to any portion of a contractile
strip cut from the umbrella of Aurelia aurita in the way already
described, the rate of the contraction-waves is first progressively
slowed, and eventually their passage is completely blocked at the
line where the poisoned water begins. Upon now restoring the poisoned
portion of the contractile strip to normal sea-water the blocking is
gradually overcome, and eventually every trace of it disappears.[32]

  [32] In conducting this experiment, care must be taken not to
  exert the slightest pressure on any part of the strip. The method
  I adopted, therefore, was to have a vessel with a very deep
  furrow on each of its opposite lips. Upon filling this vessel
  to the level of these furrows with the poisoned water, and then
  immersing the whole vessel in ordinary sea-water up to the
  level of its brim, some of the poisoned water of course passed
  through the open furrows. The external body of water (_i.e._
  the normal sea-water containing the animal) was therefore made
  proportionally very large, so that the slight escape of poison
  into it did not affect the experiment. On now passing the portion
  of the strip to be poisoned through the opposite furrows, it
  was allowed to soak in the poison while freely floating, and so
  without suffering pressure in any of its parts.

The contractile wave may be blocked by poisons in another way. A
glance at Fig. 11 will show that a circumferential strip cut from
the umbrella of Aurelia aurita is pervaded transversely by a number
of nutrient tubes, which have all been cut through by the section.
At the side of the strip, therefore, furthest from the margin
there are situated a number of open ends of these nutrient tubes.
Now, on injecting any of the narcotic poisons into one of these
open ends, the fluid of course permeates the whole tube, and the
contraction-wave becomes blocked at the transverse line occupied by
the tube as effectually as if the contractile strip had been cut
through at that line.

A glance at Fig. 10, again, will show that each lithocyst is
surrounded by one of these nutrient tubes. Upon injecting this
tube, therefore, in a contractile strip, the effect of the poison
may be exerted on the lithocyst more specially than it could be by
any other method of administration. In view of recent observations
concerning the effects of curare on the central nervous masses of
higher animals, it may be worth while to state that a discharging
lithocyst of Aurelia aurita, when thus injected with curare, speedily
ceases its discharges. This fact alone, however, would not warrant
any very trustworthy conclusions as to the influence of curare upon
discharging centres; for it is not improbable that the paralyzing
effects may here be due to the influence of the poison on the
surrounding contractile tissue.

It is interesting to observe that if the discharging lithocyst be
injected with chloroform, or a not too strong solution of morphia,
it recovers in the course of a night. With alcohol the first effects
of the injection are considerably to accelerate the frequency and to
augment the potency of the discharges; but the subsequent effects
are a gradual diminution in the frequency and the vigour of these
discharges, until eventually total quiescence supervenes. In the
course of a few hours, however, the torpidity wears away, and
finally the medusa returns to its normal state.[33]

  [33] Since the above results on the effects of poisons were
  published in my Royal Society papers, Dr. Krukenberg has
  conducted a research upon "comparative toxicology," in which he
  has devoted the larger share of his attention to the Medusæ.
  While expressing my gratification that when he adopted my methods
  he succeeded in confirming my results, I may observe that the
  criticism which he somewhat bluntly passes upon the latter is not
  merely unwarranted, but based upon a strange misconception of a
  well-known principle in the physiology of nerves and muscles.
  This criticism is that these results as published by me are
  worthless and "a dead chapter in science," because I failed
  to prove that it was the nervous (as distinguished from the
  muscular) elements which were effected by the various poisons. In
  his opinion this distinction can only be made good by employing
  electrical stimulation upon the sub-umbrella tissue when this
  has lost its spontaneity under the influence of poisons: if
  a response ensues which does not ensue when the tissue is
  stimulated mechanically, he regards the fact as proof that the
  muscular tissue remains unaffected while the nervous tissue has
  been rendered functionless.

  Now, in the first place, I have here to show that there is, as I
  have said, a fundamental error touching a well-known principle
  of physiology. So far as there is any difference between the
  excitability of nerve and muscle with respect to mechanical
  and electrical stimulation, it is the precise converse of that
  which Dr. Krukenberg supposes; it is not _muscle_, but _nerve_,
  which is the more sensitive to electrical stimulation--by which
  I understand him to mean the induction shock. The remarkable
  transposition of Dr. Krukenberg's ideas upon this matter does not
  affect the results of his observations upon the action of the
  various poisons; it only renders fatuous his criticism of these
  same results as previously published by me.

  In the next place, I have to observe that in all my experiments
  I tried, as he subsequently tried, both kinds of stimulation,
  and also the constant current; but I soon found that even
  when one went to work with one's ideas upon the subject in a
  non-inverted position, no trustworthy inference could be drawn
  in favour of the muscular elements alone remaining uninjured
  from the bare fact that after the poisoning the neuro-muscular
  tissue often behaved differently towards different kinds of
  stimulation. Further, in the particular case of my experiments
  with curare--against which Dr. Krukenberg's remarks are chiefly
  directed on the ground that I did not prove the paralysis to be
  a merely muscular effect--I succeeded in obtaining very much
  better proof of the poison acting on the nervous elements, to
  the exclusion of the muscular, than I could have obtained by any
  process of inference, however good; that is to say, I obtained
  direct proof. It appears to me that Dr. Krukenberg must have
  failed to understand the English of the following sentences:
  "On nipping any portion of the poisoned half of Staurophora
  laciniata, this half remained _absolutely motionless_, while the
  unpoisoned half, _though far away from the seat of irritation_,
  immediately ceased its normal contractions, and folded itself
  together in the very peculiar and distinctive manner just
  described," _i.e._ "in one very strong and long-protracted
  systole." For the rest, see note on page 232.

  Lastly, while again expressing my satisfaction that on all
  matters of fact, our results are in full harmony, I may be
  allowed to remark that in my opinion his deductions, as embodied
  in his schema of the inferred innervation of Medusæ, are very
  far in advance of anything that is justified by observation.
  (See, for this elaborate schema, in which there are represented
  volitional, motor, reflex, and inhibitory centres, as well
  as a clearly defined system of sensory and motor nerves,
  "Vergleichend-Physiologogische Studien, dritte Abtheiling," p.
  141: Heidelberg, 1880.)


_Physiological Effects of Fresh Water on the Medusæ_

As fresh water exerts a very deadly influence on the Medusæ, this
seems the most appropriate place for describing its action. Such
a description has already been given by Professor L. Agassiz, but
it is erroneous. He writes, "Taking up in a spoonful of sea-water
a fresh Sarsia in full activity, when swimming most energetically,
and emptying it into a tumbler full of fresh water of the same
temperature, the little animal will at once drop like a ball to
the bottom of the glass and remain for ever motionless--killed
instantaneously by the mere difference of the density of the two
media."[34] As regards the appearance presented by Sarsia when
subjected to "this little experiment," the account just quoted is
partly correct; but Professor Agassiz must have been over-hasty
in concluding that, because the animals seemed to be thus "killed
instantaneously," such was really the case. Nothing, indeed, could
be more natural than his conclusion; for not only is the contrast
between the active swimming motions of the Sarsia in the sea-water
and their cessation of all motion in the fresh water very suggestive
of instantaneous death; but a short time after immersion in the
latter their contractile tissues, as Professor Agassiz observed,
become opalescent and whitish. Nevertheless, if he had taken the
precaution of again transferring the Sarsia to sea-water, he would
have found that the previous exposure to fresh water had not had the
effect which he ascribes to it. After a variable time his specimens
would have resumed their swimming motions; and although these might
have had their vigour somewhat impaired, the animals would have
continued to live for an indefinite time--in fact, quite as long as
other specimens which had never been removed from the sea-water. Even
after five minutes' immersion in fresh water, Sarsia will revive
feebly on being again restored to sea-water, although it may be two
or three hours before they do so; they may then, however, live as
long as other specimens. In many cases Sarsia will revive even after
ten minutes' exposure; but the time required for recovery is then
very long, and the subsequent pulsations are of an exceedingly feeble
character. I never knew a specimen survive an exposure of fifteen
minutes.[35] In not a few cases, after immersion in fresh water, the
animal continues to pulsate feebly for some little time; and, in all
cases, irritability of the contractile tissues persists for a little
while after spontaneity has ceased. The opalescence above referred
to principally affects the manubrium, tentacles, and margin of the
nectocalyx. While in fresh water the manubrium and tentacles of
Sarsia are strongly retracted.

  [34] "Mem. American Acad. Arts and Sciences," 1850, p. 229.

  [35] The covered-eyed Medusæ survive a longer immersion than
  the naked-eyed--Aurelia aurita, for instance, requiring from a
  quarter to half an hour's exposure before being placed beyond
  recovery. Moreover, the cessation of spontaneity on the first
  immersion is not so sudden as it is in the case of the naked-eyed
  Medusæ--the pulsations continuing for about five minutes, during
  which time they become weaker and weaker in so gradual a manner
  that it is hard to tell exactly when they first cease.

Thinking it a curious circumstance that the mere absence of the few
mineral substances which occur in sea-water should exert so profound
and deadly an influence on the neuro-muscular tissues of the Medusæ,
I was led to try some further experiments to ascertain whether it is,
as Agassiz affirms, to the mere difference in density between the
fresh and the sea water, or to the absence of the particular mineral
substances in question, that the deleterious influence of fresh
water is to be ascribed. Although my experiments lead to no very
instructive conclusion, they are, I think, worth stating.

I first tried dissolving chloride of sodium in fresh water till the
latter was of the same density as sea-water. Sarsia dropped into
such a solution continued to live for a great number of hours; but
they were conspicuously enfeebled, keeping for the most part at
the bottom of the vessel, and having the vigour of their swimming
motions greatly impaired. The tentacles and manubrium were strongly
retracted, as in the case of exposure to fresh water, and the tissues
also became slightly opalescent. Thinking that perhaps a fairer test
would be only to add as much chloride of sodium to the fresh water as
occurs in sea-water, I did so; but the results were much the same.
On now adding sulphate of magnesium, however, to the amount normally
present in sea-water, the Sarsia became more active. I next tried the
effects of chloride of sodium dissolved in fresh water to the point
of saturation, or nearly so. The Sarsia, of course, floated to the
surface, and they immediately began to show symptoms of torpidity.
The latter became rapidly more and more pronounced, till spontaneity
was quite suspended. The animals, however, were not dead, nor did
they die for many hours, their irritability continuing unimpaired,
although their spontaneity had so completely ceased. The tentacles
and manubrium were exceedingly relaxed, which is an interesting fact,
as being the converse of that which occurs in water containing too
small a proportion of salt. Lastly, to give the density hypothesis
a still more complete trial, I dissolved various neutral salts and
other substances, such as sugar, etc., in fresh water till it was
of the density of sea-water; but in all cases, on immersing Sarsia
in such solutions, death was as rapid as that which followed their
immersion in fresh water.


_The Fresh-water Medusa._

On June 10, 1880, it was noticed that the fresh water in the large
tank of the lily-house of the Royal Botanical Society, Regent's Park,
was swarming with a small and active species of Medusa, previously
unknown to science--it being, indeed, at that time unknown to
science that any species of Medusa inhabited fresh water, although
it was well known that some of the other Hydrozoa do so. Examination
showed that the new species belonged to the order Trachomedusæ,
and the Petasidæ of Haeckel's classification--its nearest known
relative, according to Professor Ray Lankester, being the genus
Aglauropsis, which occurs on the coast of Brazil. The Medusa was
called Limnocodium ([Greek: limnê], a pond, and [Greek: kôdôn], a
bell) sorbii by Professors Allman and Lankester. I am indebted to the
kindness of Professor Allman for permission to reproduce his drawing
of the animal. (Fig. 31.) It is remarkable that, although this Medusa
has reappeared every June in the same tank, no one has yet succeeded
in tracing its life-history. Nor is it known from what source the
tank first became impregnated with this organism. No doubt the germs
must have been conveyed by the roots or leaves of some tropical
plant that at some time was placed in the tank; but the Botanical
Society has no record of any plant which can be pointed to as thus
having probably served to import the organism.

  [Illustration: Fig. 31.]

I shall now proceed to give an account of my observations on the
physiology of this interesting animal, by quoting _in extenso_ my
original paper upon the subject (_Nature_, June 24, 1880). Before
doing so, however, I may state that Professors A. Agassiz, Moseley,
and others have since informed us that sundry species of sea-water
Medusæ have been observed by them living and thriving in the brackish
waters of estuaries--a fact which strongly corroborates the inference
at the end of the present paper.

"The natural movements of the Medusa precisely resemble those of its
marine congeners. More particularly, these movements resemble those
of the marine species which do not swim continuously, but indulge in
frequent pauses. In water at the temperature of that in the Victoria
lily-house (85° Fahr.), the pauses are frequent, and the rate of
the rhythm irregular, suddenly quickening and suddenly slowing even
during the same bout, which has the effect of giving an almost
intelligent appearance to the movements. This is especially the case
with young specimens. In colder water (65° to 75°) the movements are
more regular and sustained; so that, guided by the analogy furnished
by my experiments on the marine forms, I infer that the temperature
of the natural habitat of this Medusa cannot be so high as that of
the water in the Victoria lily-house. In water of that temperature
the rate of the rhythm is enormously high, sometimes rising to three
pulsations per second. But by progressively cooling the water,
this rate may be progressively lowered, just as in the case of the
marine species; and in water at 65°, the maximum rate that I have
observed is eighty pulsations per minute. As the temperature at which
the greatest activity is displayed by the fresh-water species is a
temperature so high as to be fatal to all the marine species which I
have observed, the effects of cooling are, of course, only parallel
in the two cases when the effects of a series of higher temperatures
in the one case are compared with those of a series of lower
temperatures in the other. Similarly, while a temperature of 70° is
fatal to all the species of marine Medusæ which I have examined,
it is only a temperature of 100° that is fatal to the fresh-water
species. Lastly, while the marine species will endure any degree of
cold without loss of life, such is not the case with the fresh-water
species. Marine Medusæ, after having been frozen solid, will, when
gradually thawed out, again resume their swimming movements; but this
fresh-water Medusa is completely destroyed by freezing. Upon being
thawed out, the animal is seen to have shrunk into a tiny ball, and
it never again recovers either its life or its shape.

"The animal seeks the sunlight. If one end of the tank is shaded, all
the Medusæ congregate at the end which remains unshaded. Moreover,
during the daytime they swim about at the surface of the water; but
when the sun goes down they subside, and can no longer be seen. In
all these habits they resemble many of the sea-water species. They
are themselves non-luminous.

"I have tried on about a dozen specimens the effect of excising the
margin of the nectocalyx. In the case of all the specimens thus
operated upon, the result was the same, and corresponded precisely
with that which I have obtained in the case of marine species; that
is to say, the operation produces immediate, total, and permanent
paralysis of the nectocalyx, while the severed margin continues to
pulsate for two or three days. The excitability of a nectocalyx thus
mutilated persists for a day or two, and then gradually dies out,
thus also resembling the case of the marine naked-eyed Medusæ. More
particularly, the excitability resembles that of those marine species
which sometimes respond to a single stimulation with two or three
successive contractions.

"A point of specially physiological interest may be here noticed.
In its unmutilated state the fresh-water Medusa exhibits the power
of localizing with its manubrium a seat of stimulation situated in
the bell; that is to say, when a part of the bell is nipped with the
forceps, or otherwise irritated, the free end of the manubrium is
moved over and applied to the part irritated. So far the movement of
localization is precisely similar to that which I have previously
described as occurring in Tiaropsis indicans (_Phil. Trans._, vol.
clxvii.). But further than this, I find a curious difference. For
while in Tiaropsis indicans these movements of localization continue
unimpaired after the margin of the bell has been removed, and will
be ineffectually attempted even after the bell is almost entirely
cut away from its connections with the manubrium, in the fresh-water
Medusa these movements of localization cease after the extreme
margin of the bell has been removed. For some reason or another the
integrity of the margin here seems to be necessary for exciting the
manubrium to perform its movements of localization. It is clear that
this reason must either be that the margin contains the nerve-centres
which preside over these localizing movements of the manubrium,
or, much more probably, that it contains some peripheral nervous
structures which are alone capable of transmitting to the manubrium
a stimulus adequate to evoke the movements of localization. In its
unmutilated state this Medusa is at intervals perpetually applying
the extremity of its manubrium to one part or another of the margin
of the bell, the part of the margin touched always bending in to meet
the approaching extremity of the manubrium. In some cases it can
be seen that the object of this co-ordinated movement is to allow
the extremity of the manubrium--_i.e._ the mouth of the animal--to
pick off a small particle of food that has become entangled in the
marginal tentacles. It is therefore not improbable that in _all_
cases this is the object of such movements, although in most cases
the particle which is caught by the tentacles is too small to be
seen with the naked eye. As it is thus no doubt a matter of great
importance in the economy of the Medusa that its marginal tentacles
should be very sensitive to contact with minute particles, so that a
very slight stimulus applied to them should start the co-ordinated
movements of localization, it is not surprising that the tentacular
rim should present nerve-endings so far sensitive that only by their
excitation can the reflex mechanism be thrown into action. But if
such is the explanation in this case, it is curious that in Tiaropsis
indicans every part of the bell should be equally capable of yielding
a stimulus to a precisely similar reflex action.

"In pursuance of this point, I tried the experiment of cutting
off _portions_ of the margin, and stimulating the bell _above the
portions of the margin which I had removed_. I found that in this
case the manubrium did not remain passive as it did when the _whole_
margin of the bell was removed; but that it made ineffectual efforts
to find the offending body, and in doing so always touched some part
of the margin which was still unmutilated. I can only explain this
fact by supposing that the stimulus supplied to the mutilated part is
spread over the bell, and falsely referred by the manubrium to some
part of the sensitive--_i.e._ unmutilated--margin.

"But to complete this account of the localizing movements, it is
necessary to state one additional fact which, for the sake of
clearness, I have hitherto omitted. If any one of the four radial
tubes is irritated, the manubrium will correctly localize the seat of
irritation, whether or not the margin of the bell has been previously
removed. This greater case, so to speak, of localizing stimuli in
the course of the radial tubes than anywhere else in the nectocalyx,
except the margin, corresponds with what I found to be the case
in Tiaropsis indicans and probably has a direct reference to the
distribution of the principal nerve-tracts.

"On the whole, therefore, contrasting this case of localization with
the closely parallel case presented by Tiaropsis indicans, I should
say that the two chiefly differ in the fresh-water Medusa, even when
unmutilated, not being able to localize so promptly or so certainly,
and in the localization being only performed with reference to the
margin and radial tubes, instead of with reference to the whole
excitable surface of the animal.

"All marine Medusæ are very intolerant of fresh water, and,
therefore, as the fresh-water species must presumably have had marine
ancestors,[36] it seemed an interesting question to determine how
far this species would prove tolerant of sea-water. For the sake of
comparison, I shall first briefly describe the effects of fresh water
upon the marine species.[37] If a naked-eyed Medusa which is swimming
actively in sea-water is suddenly transferred to fresh water, it
will instantaneously collapse, become motionless, and sink to the
bottom of the containing vessel. There it will remain motionless
until it dies; but if it be again transferred to sea-water it will
recover, provided that its exposure to the fresh water has not been
of too long duration. I have never known a naked-eyed Medusa survive
an exposure of fifteen minutes; but they may survive an exposure
of ten, and generally survive an exposure of five. But although
they thus continue to live for an indefinite time, their vigour is
conspicuously and permanently impaired; while in the fresh water
irritability persists for a short time after spontaneity has ceased,
and the tentacles and manubrium are strongly retracted.

  [36] Looking to the enormous number of marine species of Medusæ,
  it is much more probable that the fresh-water species was derived
  from them than that they were derived from a fresh-water ancestry.

  [37] For full account, see _Phil. Trans._, vol. clxvii. pp. 744,
  745.

"Turning now to the case of the fresh-water species, when first it is
dropped into sea-water at 85° there is no change in its movements for
about fifteen seconds, although the tentacles may be retracted. But
then, or a few seconds later, there generally occurs a series of two
or three tonic spasms, separated from one another by an interval of
a few seconds. During the next half-minute the ordinary contractions
become progressively weaker, until they fade away into mere twitching
convulsions, which affect different parts of the bell irregularly.
After about a minute from the time of the first immersion all
movement ceases, the bell remaining passive in partial systole.
There is now no vestige of irritability. If transferred to fresh
water after five minutes' exposure, there immediately supervenes a
strong and persistent tonic spasm, resembling rigor mortis, and the
animal remains motionless for about twenty minutes. Slight twitching
contractions then begin to display themselves, which, however, do not
affect the whole bell, but occur partially. The tonic spasm continues
progressively to increase in severity, and gives the outline of the
margin a very irregular form; the twitching contractions become
weaker and less frequent, till at last they altogether die away.
Irritability, however, still continues for a time--a nip with the
forceps being followed by a bout of rhythmical contractions. Death
occurs in several hours in strong and irregular systole.

"If the exposure to sea-water has only lasted two minutes, a similar
series of phenomena is presented, except that the spontaneous
twitching movements supervene in much less time than twenty minutes.
But an exposure of one minute may determine a fatal result a few
hours after the Medusa has been restored to fresh water.

"Contact with sea-water causes an opalescence and eventual
disintegration of the tissues, which precisely resemble the effects
of fresh water upon the marine Medusæ. When immersed in sea-water
this Medusa floats upon the surface, owing to its smaller specific
gravity.

"In diluted sea-water (fifty per cent.) the preliminary tonic spasms
do not occur, but all the other phases are the same, though extended
through a longer period. In sea-water still more diluted (1 in 4
or 6) there is a gradual loss of spontaneity, till all movement
ceases, shortly after which irritability also disappears; manubrium
and tentacles expanded. After an hour's continued exposure, intense
rigor mortis slowly and progressively developes itself, so that at
last the bell has shrivelled almost to nothing. An exposure of a
few minutes to this strength places the animal past recovery when
restored to fresh water. In still weaker mixtures (1 in 8, or 1 in
10) spontaneity persists for a long time; but the animal gradually
becomes less and less energetic, till at last it will only move in a
bout of feeble pulsations when irritated. In still weaker solutions
(1 in 12, or 1 in 15) spontaneity continues for hours, and in
solutions of from 1 in 15, or 1 in 18, the Medusa will swim about for
days.

"It will be seen from this account that the fresh-water Medusa
is even more intolerant of sea-water than are the marine species
of fresh water. Moreover, the fresh-water Medusa is beyond all
comparison more intolerant of sea-water than are the marine species
of brine; for I have previously found that the marine species will
survive many hours' immersion in a saturated solution of salt. While
in such a solution they are motionless, with manubrium and tentacles
relaxed, so resembling the fresh-water Medusa shortly after being
immersed in a mixture of one part sea-water to five of fresh; but
there is the great difference that, while this small amount of salt
is very quickly fatal to the fresh-water species, the large addition
of salt exerts no permanently deleterious influence on the marine
species.

"We have thus altogether a curious set of cross relations. It
would appear that a much less profound physiological change would
be required to transmute a sea-water jelly-fish into a jelly-fish
adapted to inhabit brine, than would be required to enable it to
inhabit fresh water. Yet the latter is the direction in which the
modification has taken place, and taken place so completely that
the sea-water is now more poisonous to the modified species than
is fresh water to the unmodified. There can be no doubt that the
modification was gradual--probably brought about by the ancestors
of the fresh-water Medusa penetrating higher and higher through the
brackish waters of estuaries into the fresh water of rivers--and
it would, I think, be hard to point to a more remarkable case
of profound physiological modification in adaptation to changed
conditions of life. If an animal so exceedingly intolerant of fresh
water as is a marine jelly-fish may yet have all its tissues changed
so as to adapt them to thrive in fresh water, and even die after an
exposure of one minute to their ancestral element, assuredly we can
see no reason why any animal in earth or sea or anywhere else may not
in time become fitted to change its element."[38]

  [38] While these sheets are passing through the press, a paper
  has been read before the Royal Society by Mr. A. G. Bourne,
  describing the hydroid stage of the fresh-water Medusa (_Proc.
  Roy. Soc._, Dec. 11, 1884). He has discovered the hydroids on
  the roots of the _Pontederia_, which have been growing in the
  Lily-tank for several years, and which are therefore probably the
  source from which the tank became impregnated with the Medusæ.




CHAPTER X.

STAR-FISH AND SEA-URCHINS.


_Structure of Star-fish and Sea-Urchins._

We shall now proceed to consider in the organization of the
Echinodermata a type of nervous system which is more highly developed
than that of the Medusæ. In conducting this research, I was joined
by my friend Professor J. Cossar Ewart, to whose unusual skill and
untiring patience the anatomical part of the inquiry is due. But
here, as formerly, I shall devote myself to the physiology of the
subject, as it is not possible within the limits assigned to this
volume to travel further into morphology than is necessary for the
purpose of rendering the experiments intelligible. I shall therefore
begin by seeking to give merely such a general idea of the structure
of the Echinodermata as is necessary for this purpose.

  [Illustration: Fig. 32. Upper surface of a Star-fish
  (_Astropecten_). (From Cassell's "Nat. Hist.")]

As we all know, a Star-fish consists of a central disc and five
radiating arms (Fig. 32). Upon the whole of the upper surface there
occur numerous calcareous nodules embedded in the soft flesh, and
supporting short spines. One of these nodules is much larger than
any of the others, is constant in position, and is called the
madreporic tubercle (Fig. 32, _m_). Continuing our examination of the
upper surface, we may observe, when we use a lens, a number of small
pincer-like organs scattered about between the calcareous nodules,
or attached to the spines; these are known as the pedicellariæ.
Each consists of a stalk serving to support a pair of forceps or
pincers, and the whole being provided with muscles, the stalk is
able to sway about and the pincers to open and shut (Fig. 33). The
entire mechanism is therefore clearly adapted to seizing and holding
on to something; but what it is that these curious organs are thus
adapted to seize, and therefore of what use they are in the economy
of the animal, has long been a standing puzzle to naturalists. I
hope presently to be able to show that we have succeeded in doing
something towards the solution of this puzzle.

  [Illustration: Fig. 33.--Pedicellariæ (magnified). (From
  Cassell's "Nat. Hist.")]

Turning now to the under surface of our Star-fish (Fig. 34), we
observe that the mouth is situated in the centre of the disc, and
that from this mouth as a centre there radiate five grooves or
furrows, which severally extend to the tips of each of the five rays.
On each side of these grooves there are numerous actively moving
membraneous tubes, which may be protruded or retracted by being
filled or emptied with fluid. These are used for crawling, and I
shall therefore call them the feet, or pedicels.

So much, then, for the external surface of a Star-fish. If, now, we
examine the internal structure, we find that the central mouth leads
by a short oesophagus into a central stomach, and that this in turn
communicates with the intestine, which terminates in an orifice on
the dorsal surface. Springing from the intestine at its origin, there
are five tubes, each of which divides into two, and the five pairs of
tubes thus formed extend into the five rays; numerous blind processes
grow out from these tubes, and give rise to glandular structures,
which probably perform the functions of a liver.

  [Illustration: Fig. 34.--Lower surface of common Star-fish.]

When a section is made across the base of one of the arms, the
furrows or grooves before mentioned are seen to be formed of two rows
of plates connected together so as to compose a series of structures
not unlike the couples of an ordinary roof. These so-called
ambulacral plates rest on horizontal spine-bearing plates, from which
other larger plates extend upwards to form the sides of the arms.

  [Illustration: Fig. 35.--The terminal portion of a tube-foot
  (magnified).]

In a living Star-fish the tube-feet or pedicels already mentioned
are seen projecting from each side of the ambulacral groove; and,
with the exception of a few at the tip of each arm, all the tube-feet
terminate in a well-formed sucker, by means of which they can be
firmly fixed to a flat surface (Fig. 35).

If we wish to understand the structure and mechanism of this
locomotor or ambulacral system--which, I may observe in passing, is
of special interest from the fact that as a mechanism it is unique
in the animal kingdom--we must resort to dissection. We then find
that each of the tube-feet is provided in its membraneous walls
with a number of annular or ring-shaped muscular fibres; when these
fibres contract, the fluid contained in the tube is forced back,
while, conversely, when these fibres relax, the fluid runs into the
tube. If the contraction of these fibres is strong, the tube shrinks
up entirely, _i.e._ is retracted within the body of the animal;
but if the contraction of the fibres is not so strong, the tube is
only shortened. If, before its shortening, its terminal expansion,
or sucker, has been applied to any flat surface, the effect of the
shortening is to cause the sucker to adhere to the flat surface,
in consequence of the pressure of the surrounding sea-water being
greater than that of the fluid within the shortened tube. In this
way, by alternately contracting and relaxing the muscular fibres in
the walls of a tube-foot, a Star-fish is able alternately to cause
the terminal sucker to fasten upon and to leave go of any flat
surface upon which the animal may be crawling. In other words, when
the tube-foot is about to form its attachment to a flat surface, it
is fully distended with fluid; but when the terminal sucker touches
the flat surface, this fluid is partly withdrawn, so causing the
sucker to adhere.

  [Illustration: Fig. 36.--Diagram of ambulacral system of a
  Star-fish: _a_, madreporic canal; _b_, inner end; _g_, outer
  end of sinus leading to circular neural vessel; _h_, from which
  radial neural vessels, _l_, arise; _c d_, Polian vesicles; _f_,
  ampullæ; _m_, oral aperture; _n_, madreporic plate.]

When we dissect out one of these tube-feet, we find that at its base,
within the body of the animal, it bifurcates into two branches.
One of these branches passes immediately into a closed sac (Fig.
36, _f_), while the other passes into a large tube (Fig. 36, _k_),
which runs all the way from base to tip of the ray, receiving in
its course similar branches from all the tube-feet in the ray.
This common or radial tube itself opens into a circular tube (Fig.
36, _e_) surrounding the mouth of the animal (Fig. 36, _m_). This
circular tube therefore receives five radial tubes--one from each
of the five rays--and is likewise in communication with a number of
membraneous sacs (Fig. 36, _c_, _d_), resembling in their structure
(though larger in size) those which occur at the base of each of the
tube-feet. The function both of these sacs and of those at the base
of each tube-foot is the same, namely, that of acting as reservoirs
of the fluid when this is expelled from the tube-feet. Moreover, all
these membraneous sacs are provided with ring-shaped muscular fibres
in their membraneous walls, which therefore serve as antagonists
to the ring-shaped muscles which occur in the membraneous walls of
the tube-feet; that is to say, when the muscles of the reservoirs
contract (Fig. 36, _c_, _d_, _f_), the pressure in the tube-feet is
increased, and when these muscles relax, that pressure is diminished.
The animal is thus furnished with the means of varying the head of
pressure in its tube-feet, either locally or universally.

The circular tube surrounding the mouth communicates at one point
with a calcareous tube (Fig. 36, _a_), which runs straight to the
dorsal surface of the animal, and there terminates in the madreporic
tubercle, to which I have already directed attention (Fig. 32, _m_,
and Fig. 36, _m_). Thus it will be seen that all the pedicels of all
the rays are in communication, by means of a closed system of tubes,
with this madreporic tubercle. It has therefore been surmised that
the function of this tubercle is that of acting as a filter to the
sea-water which in large part constitutes the fluid that fills the
ambulacral system. We have been able to prove that this surmise is
correct; for we found that if we injected any part of the ambulacral
system with coloured fluid--maintaining the injection for several
hours at as great a pressure as the tubes would stand without
rupturing--the coloured fluid found its way up the calcareous tube to
the madreporic tubercle, on arriving at which it slowly oozed through
the porous substance of which that tubercle consists.

Such, then, is the so-called ambulacral system of the Star-fish.
Passing over another system of vessels which I need not wait to
describe (Fig. 36, _g_, _h_, _l_), we come next to the nervous
system. This is disposed on a very simple plan. It consists of a
pentagonal ring surrounding the mouth, from which a nerve-trunk
passes into each of the five rays, to run along the ambulacral groove
as far as the extreme tip of the ray, where it ends in a small red
pigmented spot, about which I shall have more to say presently. Each
of these five radial nerves gives off in its course a number of
delicate branches to the tube-feet.


_Modifications of the Star-fish Type._

So much, then, for the structure of the common Star-fish. I must next
say a few words on the remarkable modifications which this structure
undergoes in different members of the Star-fish group.

In some species the size of the central disc is increased so as to
fill up the interspaces between the rays, the whole animal being thus
converted into the form of a pentagon. In other species, again, the
reverse process has taken place, the rays having become relatively
longer, and being at the same time very active; they look like five
little snakes joined together by a circular disc (Fig. 37). Again, in
another species the rays have begun to branch, these branches again
to branch, and so on till the whole animal looks like a mat. But the
most extreme modifications are attained in the sea-cucumbers and
lily-stars (Fig. 38). Without, however, waiting to consider these,
I shall go a little more particularly into the modification of
Star-fish structure which is presented by the sea-urchin, or Echinus
(Fig. 39).

  [Illustration: Fig. 37.--A Brittle-star. (From Cassell's "Nat.
  Hist.")]

  [Illustration: Fig. 38.--A Lily-star. (From Cassell's "Nat.
  Hist.")]

Externally, the animal presents the form of an orange, and is
completely covered with a large number of hard calcareous spines,
on which account it derives its scientific name of Echinus, or
hedgehog (the spines have been removed from the larger portion of
the specimen represented in Fig. 39). In the living animal these
spines are fully movable in all directions, each being mounted on a
ball-and-socket joint, and provided with muscles at its base. On the
external surface, besides the spines, we meet with pedicellariæ (Fig.
33 magnified), and also with the madreporic tubercle (Fig. 39, _m_).
The pedicellariæ in their main features resemble those which occur
in the Star-fish, though considerably larger, and the ambulacral
system is constructed upon the same plan. If we shave off the spines
and pedicellariæ (Fig. 39), we find that we come to a hard shell,
which, if we break, we find to be hollow and filled with fluid (Fig.
40). The fluid closely resembles sea-water, but is, nevertheless,
richly corpusculated; it coagulates when exposed to the air, and
otherwise shows that it is something more than mere sea-water. If we
look closely into the shell which has been deprived of its spines, we
find that it is composed of a great number of small hexagonal plates
(Fig. 41), the edges of which fit so closely together that the whole
shell is converted into a box, which, when the animal is alive, is
water-tight, as we have proved by submitting the contained fluid to
hydrostatic pressure, under which circumstances there is no leakage
until the pressure is sufficient to burst the shell. Nevertheless,
if we look closely at the dried shell of an Echinus, we shall see
that it is not an absolutely closed box; for we shall see that the
hexagonal plates are so arranged as to give rise to five double rows
of holes or pores (Fig. 41), which extend symmetrically from pole
to pole of the animal (Fig. 39). It is through these holes that the
tube-feet are protruded; so that if we imagine a pentagonal species
of Star-fish to be curved into the shape of a hollow spheroid, and
then converted into a calcareous box with holes left for its feet to
come through, we should have a mental picture of an Echinus. It would
only be necessary to add the curious apparatus of teeth (Figs. 40 and
42), which occurs in the Echinus, to increase the size of the spines
and pedicellariæ, and to make a few other such minor alterations; but
in all its main features an Echinus is merely a Star-fish with its
five rays calcified and soldered together so as to constitute a rigid
box.

  [Illustration: Fig. 39.--An Echinus, partly denuded of its
  spines. (From Cassell's "Nat. Hist.")]

  [Illustration: Fig. 40.--Showing interior of Echinus shell. (From
  Cassell's "Nat. Hist.")]

  [Illustration: Fig. 41.--A portion of the external shell of an
  Echinus denuded of spines and slightly magnified, showing the
  arrangement of the plates, the balls in the ball-and-socket
  joints of the spines, and the holes through which the ambulacral
  feet are protruded. (From Cassell's "Nat. Hist.")]

  [Illustration: Fig. 42.--Teeth of Echinus (from Cassell's "Nat.
  Hist.")]

This echinoid type itself varies considerably among its numerous
constituent species as to size, shape, length and thickness of the
spines, etc.; but I need not wait to go into these details. Again,
merely inviting momentary attention to the developmental history of
these animals, I may remark that the phases of development through
which an individual Echinoderm passes are not less varied and
remarkable than are the permanent forms eventually assumed by the
sundry species.


_Natural Movements._

Turning now to the physiology of the Star-fish group, I shall begin
by describing the natural movements of the animals.

Taking the common Star-fish as our starting-point, I have already
explained the mechanism of its ambulacral system. The animals usually
crawl in a determinate direction, and when in the course of their
advance the terminal feet of the advancing ray--which are used, not
as suckers, but as feelers, protruded forwards--happen to come into
contact with a solid body, the Star-fish may either continue its
direction of advance unchanged, or may turn towards the body which
it has touched. Thus, for instance, while crawling along the floor
of a tank, if the terminal feet at the end of a ray happen to touch
a perpendicular side of the tank, the animal may either at once
proceed to ascend this perpendicular side, or it may continue its
progress along the floor, feeling the perpendicular side with the
end of its rays perhaps the whole way round the tank, and yet not
choosing, as it were, to ascend. In the cases where it does ascend
and reaches the surface of the water, a Star-fish very often performs
a number of peculiar movements, which we may call acrobatic (Fig.
43). On reaching the surface, the animal does not wish to leave its
native element--in fact, cannot do so, because its sucking feet can
only act under water--and neither does it wish again to descend into
the levels from which it has just ascended. It, therefore, begins
to feel about for rocks or sea-weeds at the surface, by crawling
along the side of the tank, and every now and then throwing back its
uppermost ray or rays along the surface of the water to feel for any
solid support that may be within reach. If it finds one, it may
very likely attach its uppermost rays to it, and then, letting go
its other attachments, swing from the one support to the other. The
activity and co-ordination manifested in these acrobatic movements
are surprising, and give to the animal an almost intelligent
appearance.

  [Illustration: Fig. 43.--Natural movements of a Star-fish on
  reaching the surface of water.]

In Astropecten the ambulacral feet have become partly rudimentary,
inasmuch as they have lost their terminal suckers (Fig. 44). These
Star-fish, therefore, assist themselves in locomotion by the muscular
movements of their rays, while they use their suckerless feet to run
along the ground somewhat after the manner of centipedes. It is to
be noticed, however, that although the feet have lost their suckers,
the Star-fish is still able to make them adhere to solid surfaces in
a comparatively inefficient manner, by constricting the tube on one
side after it has brought this side into opposition with the solid
surface (Fig. 45).

  [Illustration: Fig. 44.--A pedicel of Astropecten (magnified),
  showing the absence of any terminal sucker.]

  [Illustration: Fig. 45.--The same, showing the method of
  extemporizing a sucker.]

In the Brittle-stars the ambulacral feet have been still more reduced
to rudiments, and are of no use at all, either as suckers or for
assisting in locomotion. These Star-fish have, therefore, adopted
another method of locomotion, and one which is a great improvement
upon the slow crawling of other members of the Star-fish group. The
rays of the Brittle-stars are very long, flexible, and muscular, and
by their combined action the animal is able to shuffle along flat
horizontal surfaces. When it desires to move rapidly, it uses two
of its opposite arms upon the horizontal floor with a motion like
swimming (Fig. 46); at each stroke the animal advances with a leap
or bound about the distance of two inches, and as the strokes follow
one another rapidly, the Star-fish is able to travel at the rate of
six feet per minute. A common Star-fish, on the other hand, with
its slow crawling method of progression, can only go two inches per
minute. Some of the Comatulæ, in which the muscularity of the rays
has proceeded still further, are able actually to swim in the water
by the co-ordinated movements of their rays.[39]

  [39] In this case the locomotion of a Star-fish comes to be
  performed on the same plan or method as that of a Jelly-fish--the
  five rays performing, by their co-ordinated action, the same
  function as a swimming-bell. It is a curiously interesting fact,
  that although no two plans or mechanisms of locomotion could
  well be imagined as more fundamentally distinct than those
  which are respectively characteristic of these two groups of
  animals, nevertheless in this particular case and in virtue of
  special modification, a Star-fish should have adopted the plan or
  mechanism of a Jelly-fish.

  [Illustration: Fig. 46.--Natural movements of a Brittle-star when
  proceeding along a solid horizontal surface.]

The Echinus crawls in the same way as the common Star-fish; but
besides its long suckers it also uses its spines, which by their
co-ordinated action push the animal along. The suckers, moreover, in
being protruded from all sides of a globe instead of from the under
side of a flat organism, are of much more use as feelers than they
are in the Star-fish. Therefore, while advancing, the feet facing
the direction of advance are always kept extended to their fullest
length, in order to feel for any object which the animal may possibly
be approaching. When a perpendicular surface is reached, the Echinus
may either ascend it or not, as in the case of the Star-fish. While
walking, the animal keeps pretty persistently in one direction of
advance. If it be partly rotated by the hand, it does not continue
in the same direction, but continues its own movements as before;
so that, for instance, if it is turned half round, it will proceed
in a direction opposite to that in which it had previously been
going. When at rest, some of the feet are used as anchors, and others
protruded as feelers.

Regarded from the standpoint of the evolutionist, we have here an
interesting series of gradations. At one end of the series we have
the Echinus with its rays all united in a box-like rigid shell. At
the other end of the series we have the Brittle-stars and Comatulæ
with their highly muscular and mobile rays. Midway in the series
we have Astropecten and the common Star-fish, where the rays
are flexible and mobile, though not nearly so much so as in the
Brittle-stars. Now, the point to observe is, that in correlation
with this graduated difference in the mobility of the rays, there
is a correspondingly graduated difference in the development of the
ambulacral system of suckers. For in Echinus this system is seen in
its most elaborate and efficient form; in the common Star-fish the
suckers are still the most important organs of locomotion, though
the muscularity of the rays has begun to tell upon the development
of the specially ambulacral system, the suckers not being so long or
so powerful as they are in Echinus. Lastly, the Brittle-stars and
Comatulæ have altogether discarded the use of their sucking feet in
favour of the much more efficient organs of locomotion supplied by
their muscular rays; and, as a consequence, their feet have dwindled
into useless rudiments, while the rays have become limb-like in their
activity.

  [Illustration: Fig. 47.--Natural righting movements of common
  Star-fish.]

There is only one other point in connection with the natural
movements of the Echinodermata which it is necessary for me to touch
upon. All the species when turned upon their backs are able again
to right themselves; but seeing, as I have just observed, that the
organs of locomotion in the different species are not the same,
the methods to which these species have to resort in executing
the righting manoeuvre are correspondingly diverse. Thus, the
Brittle-stars can easily perform the needful manoeuvre by wriggling
some of their snakelike arms under the inverted disc, and heaving
the whole body over by the mere muscularity of these organs. The
common Star-fish, however, experiences more difficulty, and executes
the manoeuvre mainly by means of its suckers. That is to say, it
twists round the tip of one or more of its rays (Fig. 47) until the
ambulacral feet there situated are able to get a firm hold of the
floor of the tank (_a_); then, by a successive and similar action
of the ambulacral feet further back in the series, the whole ray
is twisted round (_b_), so that the ambulacral surface of the end
is applied flat against the floor of the tank (_c_). The manoeuvre
continuing, the semi-turn or spiral travels progressing all the
way down the ray. Usually two or three adjacent rays perform this
manoeuvre simultaneously; but if, as sometimes happens, two opposite
rays should begin to do so, one of them soon ceases to continue the
manoeuvre, and one or both of the rays adjacent to the other takes it
up instead, so assisting and not thwarting the action. The spirals of
the co-operating rays being invariably turned in the same direction
(Fig. 47, _a_, _b_, and _c_), the result is, when they have proceeded
sufficiently far down the rays, to drag over the remaining rays,
which then abandon their hold of the bottom of the tank, so as not to
offer any resistance to the lifting action of the active rays. The
whole movement does not occupy more than half a minute. As a general
rule, the rays are from the first co-ordinated to effect the righting
movement in the direction in which it is finally to take place--the
rays which are to be the active ones alone twisting over, and so
twisting that all their spirals turn in the same direction.

A Star-fish (Astropecten) which is intermediate between the
Brittle-star and the common Star-fish, in that its ambulacral feet
are partly aborted (having lost their suckers, as shown in Fig.
44) and its rays more mobile than those of the common Star-fish,
rights itself after the manner shown in Fig. 48, where the animal is
represented as standing on the tips of four of its rays, while the
fifth one is just about to be thrown upwards and over the others,
in order to carry with it the two adjacent rays, and so eventually
to overbalance the system round the fulcrum supplied by the tips of
the other two rays, and thus bring the animal down upon its ventral
surface.

  [Illustration: Fig. 48.--Righting movements of Astropecten.]

  [Illustration: Fig. 49.]

But it is in the case of Echinus that these righting movements become
most interesting, from the fact that they are so much more difficult
to accomplish than they are in the case of the Star-fishes. For while
a Star-fish is provided with flat, flexible, and muscular rays,
comprising a small and light mass in relation to the motive power,
an Echinus is a rigid, non-muscular, and globular mass, whose only
motive power available for conducting the manoeuvre is that which is
supplied by its relatively feeble ambulacral feet. It is, therefore,
scarcely surprising that unless the specimens chosen for these
observations are perfectly fresh and vigorous, they are unable to
right themselves at all; they remain permanently inverted till they
die. But if the specimens are fresh and vigorous, they are sooner
or later sure to succeed in righting themselves, and their method
of doing so is always the same. Two, or perhaps three, adjacent
rows of suckers are chosen out of the five, as the rows which are
to accomplish the task (Fig. 49). As many feet upon the rows as can
reach the floor of the tank are protruded downwards and fastened
firmly to the floor; their combined action then serves to tilt the
globe slightly over in their own direction, the anchoring feet on
the other or opposite rows meanwhile releasing their hold of the
tank to admit of this tilting (Fig. 50). The effect of this tilting
is to enable the next feet in the active ambulacral rows to touch
the floor of the tank, and, when they have established their hold,
they assist in increasing the tilt; then the next feet in the series
lay hold, and so on, till the globe slowly but steadily rises upon
its equator (Fig. 51). The difficulty of raising such a heavy mass
into this position by means of the slender motive power available
can be at once appreciated on witnessing the performance, so that
one is surprised, notwithstanding the co-ordination displayed by all
the suckers, that they are able to accomplish the work assigned to
them. That the process is in truth a very laborious one is manifest,
not only from the extreme slowness with which it takes place, but
also because, as already observed, in the case of not perfectly
strong specimens complete failure may attend the efforts to reach the
position of resting on the equator--the Echinus, after rearing up a
certain height, becoming exhausted and again falling back upon its
ab-oral pole. Moreover, in some cases it is interesting to observe
that when the equator position has been reached with difficulty, the
Echinus, as it were, gives itself a breathing space before beginning
the movement of descent--drawing in all its pedicels save those
which hold it securely in the position to which it has attained, and
remaining in a state of absolute quiescence for a prolonged time. It
then suddenly begins to protrude all its feet again, and to continue
its manoeuvre. At any time during such a period of rest, a stimulus
of any kind will immediately determine a recommencement of the
manoeuvre.

  [Illustration: Fig. 50.]

  [Illustration: Fig. 51.]

It will be perceived that as soon as the position just described has
been attained, gravity, which had hitherto been acting in opposition
to the righting movement, now begins to favour that movement. It
might, therefore, be anticipated that the Echinus would now simply
let go all its attachments and allow itself to roll over into its
natural position But an Echinus will never let go its attachments
without some urgent reason, seeming to be above all things afraid
of being rolled about at the mercy of currents; and therefore in
this case it lets itself down almost as slowly as it raised itself
up. So gently, indeed, is the downward movement effected, that an
observer can scarcely tell the precise moment at which the righting
is concluded. Therefore, in the downward movement, the feet, which
at the earlier part of the manoeuvre were employed successfully in
rearing the globe upon its equator, are now employed successfully in
preventing its too rapid descent (Fig. 52).

  [Illustration: Fig. 52.]

Several interesting questions arise with reference to these righting
movements of Echinus. First of all we are inclined to ask what it is
that determines the choice of the rows of feet which are delegated
to effect the movements. As the animal has a geometrical form of
perfect symmetry, we might suppose that when it is placed upon its
pole, all the five rows of feet would act in antagonism to one
another; for there seems nothing more to determine either the action
or the inaction of one row rather than another. Indeed, if there
were any moral philosophers among the Echinoderms, they might point
with triumph to the fact of their being able to right themselves as
an irrefutable argument in favour of the freedom of the Echinoderm
will. "We are in form," they might say, "perfectly geometrical, and
our feet-rows are all arranged with perfect symmetry; therefore there
is no reason, apart from the sovereign freedom of our choice, why we
should ever use one set of feet rather than another in executing this
important movement." And indeed, I do not see how these Echinoderm
philosophers could be answered by any of the human philosophers,
who, with less mathematical data and with less physiological reason,
employ analogous arguments to prove the freedom of the human will.
Physiologists, however, would give these Echinoderm philosophers
the same answer that they are in the habit of giving to the human
philosophers, viz. that although the physiological conditions are
very nicely balanced, they are never _so_ nicely balanced as to leave
positively nothing to determine which rows of feet--that is to say,
which sets of nerves--shall be used. And in this connection I may
observe that on making a number of trials it becomes apparent in the
case of certain individual specimens that they manifested a marked
tendency to rotate always in the same direction, or to use the same
set of foot-rows for the purpose of righting themselves. In these
individual specimens, therefore, we must conclude that the foot-rows
thus employed are selected because of some slight accidental
prepotency or superiority over the others; the animal has, as it
were, thus much individual _character_ as the result of a slight
prepotency of some of its nerve-centres over the others.

Another question of still more interest arises out of these righting
movements, namely, that as to their prompting cause. This question,
however, I shall defer till later on, since it cannot be answered
without the aid of experiments as distinguished from observation.


_Stimulation._

In now quitting our observations on the natural movements of the
Echinodermata, and beginning an account of the various experiments
which we have tried upon these animals, I shall first take the
experiments in stimulation.

All the Echinodermata seek to escape from injury. Thus, for instance,
if a Star-fish or an Echinus is advancing continuously in one
direction, and if it be pricked or otherwise irritated on any part
of an excitable surface facing the direction of advance, the animal
immediately reverses that direction. There is one point of special
interest concerning these movements of response to stimulation. The
form of the animals and the distribution of the nervous system being,
as I have before said, of geometrical regularity, it follows that
by applying two stimuli simultaneously on two different aspects
of the animal, the combined result of these two stimuli is that of
furnishing a very pretty instance in physiology of the physical
principle of the parallelogram of forces. Thus, for instance, if two
stimuli of equal intensity be applied simultaneously at the opposite
sides of a globular Echinus, the animal begins to walk in a direction
at right angles to an imaginary line joining these two points. And,
generally, wherever the two points of simultaneous stimulation may
be situated, the direction of the animal's advance is the diagonal
between them. As showing in more detail how very delicate is the
physiological balancing of stimuli which may be produced in these
organisms, and consequently the manner in which we are able to
play, as it were, upon their geometrically disposed nervous systems
in illustration of the mechanical principle of the composition of
forces, I shall quote a series of observations.

"1. Scraped with a scalpel the equator of an Echinus at two points
opposite to each other--animal crawled at right angles to the line of
injury.

"2. Similarly scraped at the ab-oral pole--no effect. There was no
reason why injury here should determine escape in one direction
rather than in another.

"3. Scraped similarly near the oral pole, and half-way between pole
and equator--little or no effect.

"4. Scraped in rapid succession five equatorial and equidistant
injuries--Echinus crawled actively in one determinate direction; the
equal and equidistant injuries all round the globe neutralized one
another.

"5. Scraped a band of uniform width all the way round the
equator--same result as in 4.

"6. Band of injury in same specimen was then widened in the
side facing the direction of crawling--no effect. Still further
widened--slight change of direction, and, after a time, persistent
crawling away from the widest part of the injured zone. Repeated this
experiment on other specimens by scraping round the whole equator,
and simultaneously making one part of the zone of injury wider than
the rest--same result; the animal crawled away from the _greatest
amount_ of injury.

"7. Scraped on one side of the equator, and, after the animal had
been crawling in a direct line from the source of irritation for a
few minutes, similarly scraped equator on the opposite side--animal
reversed its direction of crawling; it crawled away from the stimulus
_supplied latest_.

"8. Scraped a number of places on all aspects of the animal
indiscriminately--direction of advance uncertain and discontinuous,
with a strong tendency to rotation upon vertical axis."

These observations show conclusively that the whole external surface,
not only of the soft and fleshy Star-fish, but even of the hard
and rigid Echinus, is everywhere sensitive to stimulation. Closer
observation shows that this sensitiveness, besides being so general,
is highly delicate. For if any part of the external surface of an
Echinus is lightly touched with the point of a needle, all the
feet, spines, and pedicellariæ within reach of that part, and even
beyond it, immediately converge and close in upon the needle, grasp
it, and hold it fast. This simultaneous movement of such a little
forest of prehensile organs is a very beautiful spectacle to witness.
In executing it the pedicellariæ are the most active, the spines
somewhat slower, and the feet very much slower. The area affected
is usually about half a square inch, although the pedicellariæ even
far beyond this area may bend over towards the seat of stimulation,
which, however, from their small size they are not able to reach.

And here we have proof of the function of the pedicellariæ--proof
which we consider to be important, because, as I have before said,
the use of these organs has so long been a puzzle to naturalists. In
climbing perpendicular or inclined surfaces of rock, covered with
waving sea-weeds, it must be of no small advantage to an Echinus to
be provided on all sides with a multitude of forceps, all mounted on
movable stalks, which instantaneously bring their grasping forceps
to bear upon and to seize a passing frond. The frond being thus
arrested, the spines come to the assistance of the pedicellariæ,
and both together hold the Echinus to the support furnished by
the sea-weed. Moreover the sea-weed is thus held steady till the
ambulacral feet have time also to establish their hold upon it with
their sucking discs. That the grasping and arresting of fronds of
sea-weed in this way for the purposes of locomotion constitute an
important function of the pedicellariæ, may at once he rendered
evident experimentally by drawing a piece of sea-weed over the
surface of a healthy Echinus in the water. The moment the sea-weed
touches the surface of the animal, it is seen and felt to be seized
by a number of these little grasping organs, and--unless torn away by
a greater force than is likely to occur in currents below the surface
of the sea--it is held steady till the ambulacral suckers have time
to establish their attachments upon it. Thus there is no doubt that
the pedicellariæ are able efficiently to perform the function which
we regard as their chief function. We so regard this function, not
merely because it is the one that we observe these organs chiefly
to perform, but also because we find that their whole physiology
is adapted to its performance. Thus their multitudinous number and
ubiquitous situation all over the external surface of the animal
is suggestive of their being adapted to catch something which may
come upon them from any side, and which may have strings and edges
so fine as to admit of being enclosed by the forceps. Again, the
instantaneous activity with which they all close round and seize a
moving body of a size that admits of their seizing it, is suggestive
of the objects which they are adapted to seize being objects
which rapidly brush over the surface of the shell, and therefore
objects which, if they are to be seized at all, must be seized
instantaneously. Lastly, we find, on experimenting upon pedicellariæ,
whether _in situ_ or when separated from the Echinus, that the
clasping action of the forceps is precisely adapted to the function
which we are considering; for not only is the force exerted by the
forceps during their contraction of an astonishing amount for the
size of the organ (the serrated mandibles of the trident pedicellariæ
holding on with a tenacity that can only have reference to some
objects liable to be dragged away from their grasp), but it is very
suggestive that this wonderfully tenacious hold is spontaneously
relaxed after a minute or two. This is to say, the pedicellariæ
tightly fix the object which they have caught for a time sufficient
to enable the ambulacral suckers to establish their connections with
it, and then they spontaneously leave go; their grasp is not only so
exceedingly powerful while it lasts, but it is as a rule timed to
suit the requirements of the pedicels.[40]

  [40] A further proof that this is at least one of the functions
  of the pedicellariæ is furnished by a simple experiment. If an
  Echinus is allowed to attach its feet to a glass plate held just
  above its ab-oral pole, and this plate be then raised in the
  water so that the Echinus is freely suspended in the water by
  means of its feet alone, the animal feels, as it were, that its
  anchorage is insecure, and actively moves about its unattached
  feet to seek for other solid surfaces. Under such circumstances
  it may be observed that the pedicellariæ also become active, and
  especially so near the surface of attachment as if seeking for
  pieces of sea-weed. If a piece is presented to them, they lay
  hold upon it with vigour.

  Of course the pedicellariæ may also have other functions to
  perform, and in a Star-fish Mr. Sladen has seen them engaged in
  cleaning the surface of the animal; but we cannot doubt that at
  least in Echinus their main function is that which we have stated.

Concerning the physiology of the pedicellariæ little further remains
to be said. It may be stated, however, that the mandibles, which
are constantly swaying about upon their contractile stalks as if
in search for something to catch, will snap at an object only if it
touches the inner surface of one or more of the expanded mandibles.
Moreover, in the larger pedicellariæ, a certain part of the inner
surface of the mandibles is much more sensitive to contact than is
the rest of that surface; this part is a little pad about one-third
of the way down the mandible: a delicate touch with a hair upon
this part of any of the three mandibles is certain to determine an
immediate closure of all the three. It is obvious that there is an
advantage in the sensitive area, or zone, being placed thus low
enough down in the length of the mandibles to ensure that the whole
apparatus will not close upon an object till the latter is far enough
within the grasp of the mechanism to give this mechanism the best
possible hold. If, for instance, the tips of the mandibles were the
most sensitive parts, or even if their whole inner surfaces were
uniformly sensitive, the apparatus would be constantly closing upon
objects when these merely brushed past their tips, and therefore
closing prematurely for the purpose of grasping. But, as it is, the
apparatus is admirably adapted to waiting for the best possible
chance of getting a secure hold, and then snapping upon the object
with all the quickness and tenacity of a spring-trap.

Another point worth mentioning is that if, after closure, any one or
more of the mandibles be gently stroked on its outer surface near
the base, all the mandibles are by this stimulation usually, though
not invariably, induced again to expand. This is the only part of
the whole organ the stimulation of which thus exerts an inhibitory
influence on the contractile mechanism. If there is any functional
purpose served by such relaxing influence of stimulating this
particular part of the apparatus, we think it can only be as follows.
When a portion of sea-weed brushes this particular part, it must be
well below the tips of the mandibles, and therefore in a position
where it, or some over-lying portion, may soon pass between the
mandibles, if the latter are open; hence when touched in this place
the mandibles, if closed, open to receive the sea-weed, should any
part of it come within their cavity.

Turning next to experiments in stimulation with reference to the
spines, I may observe that we have found these organs to be,
physiologically considered, highly remarkable and interesting, from
the fact that they display co-ordinated action in a degree which
entitles them to be regarded as a vast multitude of limbs. Thus,
for instance, if an Echinus be taken out of the water and placed
upon a table, it is no longer able to use its feet for the purpose
of locomotion, as their suckers are only adapted to be used under
water. Yet the animal is able to progress slowly by means of the
co-ordinated action of its spines, which are used to prop and push
the globe-like shell along in some continuous direction. If, while
the animal is thus slowly progressing, a lighted match be held near
it, facing the direction of advance, as soon as the animal comes
close enough to feel the heat, all the spines begin to make the
animal move away in the opposite direction. Moreover, as showing
the high degree in which the action of the spines is co-ordinated,
I may mention that there is an urchin-like form of Echinoderm,
which is called Spatangus, and which differs from the Echinus in
having shorter feet and longer spines. When, therefore, a Spatangus
is inverted, it is unable to right itself by means of its feet, as
these are too short to admit of being used for this purpose; but,
nevertheless, the animal is able to right itself by means of the
co-ordinated action of its long spines, these being used successively
and laboriously to prop and push the animal over in some one definite
direction. The process takes a very long time to accomplish, and
there are generally numerous failures, but the creature perseveres
until it eventually succeeds.

Coming now to stimulation with reference to the feet, we find that
when a drop of acid, or other severe stimulation, is applied to any
part of a row of protruded pedicels, the entire row is immediately
retracted, the pedicels retracting successively from the seat of
irritation--so that if the latter be in the middle point of the
series, two series of retractions are started, proceeding in opposite
directions simultaneously; the rate at which they travel is rather
slow. This process of retraction, however, although so complete
within the ray irritated, does not extend to the other rays. But if
the stimulus be applied to the centre of the disc, upon the oral
surface of the animal, all the feet in all the rays are more or
less retracted--the process of retraction radiating serially from
the centre of stimulation. The influence of the stimulus, however,
diminishes perceptibly with the distance from the centre. Thus, if
weak acid be used as the irritant, it is only the feet near the
bases of the rays that are retracted; and even if very strong acid
be so used, it is only the feet as far as one-half or two-thirds of
the way up the rays that are fully retracted--the remainder only
having their activity impaired, while those near the tip may not
be affected at all. If the drop of acid be placed on the dorsal,
instead of the ventral surface of the disc, the effect on the feet
is found to be just the converse; that is, the stimulus here applied
greatly increases the activity of the feet. Further experiments show
that this effect is produced by a stimulus applied anywhere over
the dorsal aspect of the animal; so that, for instance, if a drop
of acid be placed on the skin at the edge of a ray, and therefore
just external to the row of ambulacral feet, the latter will be
stimulated into increased activity; whereas, if the drop of acid
had been placed a very small distance past the edge of the ray, so
as to touch some of the feet themselves, then the whole row would
have been drawn in. We have here rather an interesting case of
antagonism, which is particularly well marked in Astropecten, on
account of the active writhing movements which the feet exhibit when
stimulated by an irritant placed on the dorsal surface of the animal.
It may be added that in this antagonism the inhibitory function
is the stronger; for when the feet are in active motion, owing to
an irritant acting on the dorsal surface, they may be reduced to
immediate quiescence--_i.e._ retracted--by placing another irritant
on the ventral surface of the disc. Similarly, if retraction has
been produced by placing the irritant on the ventral surface of the
disc, activity cannot be again induced by placing another drop of the
irritant on the dorsal surface.

Now, if we regard all these facts of stimulation taken together, it
becomes evident that the external organs of an Echinoderm--feet,
spines, and pedicellariæ--are all highly co-ordinated in their
action; and therefore the probability arises that they are all held
in communication with one another by means of an external nervous
plexus. Accordingly we set to work on the external surface of the
Echinus to see whether we could obtain any evidence of such a plexus
microscopically. This we succeeded in doing, and afterwards found
that Professor Lovèn had already briefly mentioned such a plexus as
having been observed by him. The plexus consists of cells and fibres,
closely distributed all over the surface of the shell, immediately
under the epidermal layer of cells (Figs. 53, 54, 55), and it sends
fibres all the way up the feet, spines, and pedicellariæ. As it
seemed to us important to investigate the physiological properties
of this plexus, Professor Ewart and I made a number of further
experiments, an account of which will now lead us on to the next
division of our subject, or that of section.

  [Illustration: Fig. 53. External nerve-plexus of Echinus.]

  [Illustration: Fig. 54. Structure of a nerve-trunk of Echinus.]

  [Illustration: Fig. 55. Nerve-cells lying among the muscular
  fibres at the base of a spine in Echinus.]


_Section._

1. _Star-fish._--Single rays detached from the organism crawl as fast
and in as determinate a direction as do the entire animals. They also
crawl up perpendicular surfaces, and sometimes away from injuries;
but they do not invariably, or even generally, seek to escape from
the latter, as is so certain to be the case with entire animals.
Lastly, when inverted, separated rays right themselves as quickly as
do the unmutilated organisms.

Dividing the nerve in any part of its length has the effect, whether
or not the ray is detached from the animal, of completely destroying
all physiological continuity between the pedicels on either side
of the line of division. Thus, for instance, if the nerve be cut
across half-way up its length, the row of pedicels is at once
physiologically bisected, one-half of the row becoming as independent
of the other half as it would were the whole ray divided into two
parts: that is to say, the distal half of the row may crawl while
the proximal half is retracted, or _vice versâ_; and if a drop of
acid be placed on either half, the serial contraction of the pedicels
in that half stops abruptly at the line of nerve-division. As a
result of this complete physiological severance, when a detached ray
so mutilated is inverted, it experiences much greater difficulty
in righting itself than it does before the nerve is divided. The
line of nerve injury lies flat upon the floor of the tank, while
the central and distal portions of the ray, _i.e._ the portions on
either side of that line, assume various movements and shapes.
The central portion is particularly apt to take on the form of an
arch, in which the central end of the severed ray and the line of
nerve-section constitute the points of support (tetanus?) (Fig. 56),
or the central end may from the first show paralysis, from which it
never recovers. The distal end, on the other hand, usually continues
active, twisting about in various directions, and eventually
fastening its tip upon the floor of the tank to begin the spiral
movement of righting itself. This movement then continues as far as
the line of nerve-injury, where it invariably stops (Fig. 56). The
central portion may then be dragged over into the normal position,
or may remain permanently inverted, according to the strength of
pull exerted by the distal portion; as a rule, it does not itself
assist in the righting movement, although its feet usually continue
protruded and mobile. Thus, the effect of a transverse section of
the nerve in a ray is that of completely destroying physiological
continuity between the pedicels on either side of the section.

  [Illustration: Fig. 56. Movements performed by a detached ray of
  a Star-fish, in which the central nerve-trunk is divided.]

The only other experiments in nerve-section to which the simple
anatomy of a Star-fish exposes itself is that of dividing the
nerve-ring in the disc; or, which is virtually the same thing, while
leaving this intact, dividing all the nerves where they pass from
it into the rays. In specimens mutilated by severing the nerves at
the base of each of the five rays, or by dividing the nerve-ring
between all the rays, the animal loses all power of co-ordination
among its rays. When a common Star-fish is so mutilated it does not
crawl in the same determinate manner as an unmutilated animal, but,
if it moves at all, it moves slowly and in various directions. When
inverted, the power of effecting the righting manoeuvre is seen to
be gravely impaired, although eventually success is always achieved.
There is a marked tendency, as compared with unmutilated specimens,
to a promiscuous distribution of spirals and doublings, so that
instead of a definite plan of the manoeuvre being formed from the
first, as is usually the case with unmutilated specimens, such a plan
is never formed at all; among the five rays there is a continual
change of un-coördinated movements, so that the righting seems to
be eventually effected by a mere accidental prepotency of some of
the righting movements over others. Appended is a sketch of such
un-coördinated movement, taken from a specimen which for more than
an hour had been twisting its rays in various directions (Fig. 57).
Another sketch is appended to show a form of bending which specimens
mutilated as described are very apt to manifest, especially just
after the operation. When placed upon their dorsal surface, they
turn up all their rays with a peculiar and exactly similar curve in
each, which gives to the animal a somewhat tulip-like form (Fig.
58). This form is never assumed by unmutilated specimens, and in
mutilated ones, although it may last for a long time, it is never
permanent. In detached rays this peculiar curve is also frequently
exhibited; but if the nerve of such a ray is divided at any point
in its length, the curve is restricted to the distal portion of the
ray, and it stops abruptly at the line of nerve-section. When entire
Star-fish are mutilated by a section of each nerve-trunk half-way up
each ray, and the animal is then placed upon its back, the tetanic
contraction of the muscles in the rays before mentioned as occurring
under this form of section in detached rays, has the effect, when now
occurring in all the rays, of elevating the disc from the floor of
the tank. This opisthotonous-like spasm is not, however, permanent;
and the distal ends of the rays forming adhesions to the floor of the
tank, thy animal eventually rights itself, though much more slowly
than unmutilated specimens. After it has righted itself, although it
twists about the distal portions of the rays, it does not begin to
crawl for a long time, and when it does so, it crawls in a slow and
indeterminate manner. Star-fish so mutilated, however, can ascend
perpendicular surfaces.

  [Illustration: Fig. 57. Un-coördinated movements of a Star-fish,
  in which the nerves of all the rays have been divided.]

  [Illustration: Fig. 58. Form frequently assumed by Star-fish
  under similar circumstances.]

The loss of co-ordination between the rays caused by division of the
nerve-ring in the disc is rendered most conspicuous in Brittle-stars,
from the circumstance that in locomotion and in righting so much here
depends upon co-ordinated muscular contraction of the rays. Thus,
for instance, when a Brittle-star has its nerve-ring severed between
each ray, an interesting series of events follows. First, there is a
long period of profound shock--spontaneity, and even irritability,
being almost suspended, and the rays appearing to be rigid, as if in
tetanic spasm. After a time, feeble spontaneity returns--the animal,
however, not moving in any determinate direction. Irritability also
returns, but only for the rays immediately irritated, stimulation
of one ray causing active writhing movements in that ray, but not
affecting, or only feebly affecting, the other rays. The animal,
therefore, is quite unable to escape from the source of irritation,
the aimless movements of the rays now forming a very marked contrast
to the instantaneous and vigorous leaping movements of escape which
are manifested by unmutilated specimens. Moreover, unmutilated
specimens will vigorously leap away, not only from stimulation of
the rays, but also from that of the disc; but those with their
nerve-ring cut make no attempts to escape, even from the most violent
stimulation of the disc. In other words, the disc is entirely severed
from all physiological connection with the rays.

If the nerve-ring be divided at two points, one on either side of a
ray, that ray becomes physiologically separated from the rest of
the organism. If the two nerve-divisions are so placed as to include
two adjacent rays--_i.e._ if one cut is on one side of a ray and
the other on the further side of an adjacent ray--then these two
rays remain in physiological continuity with one another, although
they suffer physiological separation from the other three. When a
Brittle-star is completely divided into two portions, one portion
having two arms and the other three, both portions begin actively to
turn over on their backs, again upon their faces, again upon their
backs, and so on alternately for an indefinite number of times. These
movements arise from the rays, under the influence of stimulation
caused by the section, seeking to perform their natural movements of
leaping, which however end, on account of the weight of the other
rays being absent, in turning themselves over. An entire Brittle-star
when placed on its back after division of its nerve-ring is not able
to right itself, owing to the destruction of co-ordination among its
rays. Astropecten, under similar circumstances, at first bends its
rays about in various ways, with a preponderant disposition to the
tulip form, and keeps its ambulacral feet in active movement. But
after half an hour, or an hour, the feet generally become retracted
and the rays nearly motionless--the animal, like a Brittle-star,
remaining permanently on its back. In this, as in other species, the
effect of dividing the nerve-ring on either side of a ray is that of
destroying its physiological connection with the rest of the animal,
the feet in that ray, although still remaining feebly active, no
longer taking part in any co-ordinated movement--that ray, therefore,
being merely dragged along by the others.

Under this division it only remains further to be said, that section
of the nerve-ring in the disc, or the nerve-trunks of the rays,
although, as we have seen, so completely destroying physiological
continuity in the rows of ambulacral feet and muscular system of the
animal, does not destroy physiological continuity in the external
nerve-plexus; for however much the nerve-ring and nerve-trunks may
be injured, stimulation of the dorsal surface of the animal throws
all the ambulacral feet and all the muscular system of the rays
into active movement. This fact proves that the ambulacral feet and
the muscles are all held in nervous connection with one another by
the external plexus, without reference to the integrity of the main
nerve-trunks.

2. _Echini._--_Section of external surface of shell._--If a
cork-borer be applied to the external surface of the shell of an
Echinus, and rotated there till the calcareous substance of the
shell is reached, and therefore a continuous circular section of
the over-lying tissues effected, it is invariably found that the
spines and pedicellariæ within the circular area are physiologically
separated from the contiguous spines and pedicellariæ, as regards
local reflex excitability. That is to say, if any part of this
circular area be stimulated, all the spines and pedicellariæ within
that area immediately respond to the stimulation in the ordinary
way; while none of the spines or pedicellariæ surrounding the area
are affected. Similarly, if any part of the shell external to the
circumscribed area be stimulated, the spines and pedicellariæ within
that area are not affected. These facts prove that the function which
is manifested by these appendages of localizing and gathering round
a seat of stimulation, is exclusively dependent upon the external
nerve-plexus. It is needless to add that in this experiment it does
not signify of what size or shape or by what means the physiological
island is made, so long as the destruction of the nervous plexus by
a closed curve of injury is rendered complete. In order to ascertain
whether, in the case of an unclosed curve of injury, any irradiation
of a stimulus would take place round the ends of the curve, we made
sundry kinds of section. It is, however, needless to describe these,
for they all showed that, after injury of a part of the plexus, there
is no irradiation of the stimulus round the ends of the injury. Thus,
for instance, if a short straight line of injury be made, by drawing
the point of a scalpel over the shell, say along the equator of the
animal, and if a stimulus be afterwards applied on either side of
that line, even quite close to one of its ends, no effect will be
exerted on the spines or pedicellariæ on the other side of the line.
This complete inability of a stimulus to escape round the ends of an
injury, forms a marked contrast to the almost unlimited degree in
which such escape takes place in the more primitive nervous plexus of
the Medusæ.

Although the nervous connections on which the spines and
pedicellariæ depend for their function of localizing and closing
round a seat of stimulation are thus shown to be completely destroyed
by injury of the external plexus, other nervous connections, upon
which another function of the spines depends, are not in the smallest
degree impaired by such injury. The other function to which I allude
is that which brings about the general co-ordinated action of all
the spines for the purposes of locomotion. That this function is not
impaired by injury of the external plexus is proved by the fact that
if the area within a closed line of injury on the surface of the
shell be strongly irritated, all the spines over the whole surface
begin to manifest their peculiar bristling movements, and by this
co-ordinated action rapidly move the animal in a straight line of
escape from the source of irritation; the injury to the external
plexus, although completely separating the spines enclosed by it
from their neighbouring spines as regards what may be called their
local function of seizing the instrument of stimulation, nevertheless
leaves them in undisturbed connection with all the other spines in
the organism as regards what may be called their universal function
of locomotion.

Evidently, therefore, this more universal function must depend
upon some other set of nervous connections; and experiment shows
that these are distributed over all the _internal_ surface of the
shell. Our mode of experimenting was to divide the animal into two
hemispheres, remove all the internal organs of both hemispheres
(these operations producing no impairment of any of the functions of
the pedicels, spines, or pedicellariæ), and then to paint with strong
acid the inside of the shell--completely washing out the acid after
about a quarter of a minute's exposure. The results of a number of
experiments conducted on this method may be thus epitomized:--

The effect of painting the back or inside of the shell with strong
acid (_e.g._ pure HCl) is that of at first strongly stimulating the
spines into bristling movements, and soon afterwards reducing them to
a state of quiescence, in which they lie more or less flat, and in a
peculiarly confused manner that closely resembles the appearance of
corn when "laid" by the wind. The spines have now entirely lost both
their spontaneity and their power of responding to a stimulus applied
on the external surface of the shell--_i.e._ their local reflex
excitability, or power of closing in upon a source of irritation.
These effects may be produced over the whole external surface of the
shell, by painting the whole of the internal surface; but if any
part of the internal surface be left unpainted, the corresponding
part of the external surface remains uninjured. Conversely, if all
the internal surface be left unpainted except in certain lines or
patches, it will only be corresponding lines and patches on the
external surface that suffer injury. It makes no difference whether
these lines or patches be painted in the course of the ambulacral
feet, or anywhere in the inter-ambulacral spaces.

The above remarks, which have reference to the spines, apply equally
to the pedicellariæ, except that their spontaneity and reflex
irritability are not destroyed, but only impaired.

Some hours after the operation it usually happens that the
spontaneity and reflex irritability of the spines return, though in a
feeble degree, and also those of the pedicellariæ, in a more marked
degree. This applies especially to the reflex irritability of the
pedicellariæ; for while their spontaneity does not return in full
degree, their reflex irritability does--or almost in full degree.

These experiments, therefore, seem to point to the conclusions--1st,
that the general co-ordination of the spines is dependent on the
integrity of an internal nerve-plexus; 2nd, that the internal plexus
is everywhere in intimate connection with the external; and 3rd, that
complete destruction of the former, while profoundly influencing the
functions of the latter, nevertheless does not wholly destroy them.

Professor Ewart therefore undertook carefully to examine the internal
surface of the shell, to see whether any evidence of this internal
nervous plexus could be found microscopically, and, after a great
deal of trouble, he has succeeded in doing so. But as he has not yet
published his results, I shall not forestall them further than to say
that this internal plexus spreads all over the inside of the shell,
and is everywhere in communication with the external plexus by means
of fibres which pass between the sides of the hexagonal plates of
which the shell of the animal is composed. Thus we can understand
how it is that when a portion of the external plexus is isolated from
the rest of that plexus as a result of the cork-borer experiment, the
island still remains in communication with the nerve-centres which
preside over the co-ordination of the spines, as proved by the fact
of the Echinus using its spines to escape from irritation applied to
the area included within the circle of injury to the external plexus
produced by the cork-borer.

Now, where are these nerve-centres situated? We have just seen
that we have evidence of the presence of such centres somewhere
in an Echinus, seeing that all the spines exhibit such perfect
co-ordination in their movements. Where, then, are these centres?

Seeing that in a Star-fish the rays are co-ordinated in their action
by means of the pentagonal ring in the disc, analogy pointed to the
nervous ring round the mouth of an Echinus as the part of the nervous
system which most probably presides over the co-ordinated action
of the spines. Accordingly, we tried the effect of removing this
nervous ring, and immediately obtained conclusive proof that this was
the centre of which we were in search; for as soon as the nervous
ring was removed, the Echinus lost, completely and permanently,
all power of co-ordination among its spines. That is to say, after
this operation these organs were never again used by the animal for
the purposes of locomotion, and no matter how severe an injury we
applied, the Echinus, when placed on a table, did not seek to escape.
But the spines were not wholly paralyzed, or motionless. On the
contrary, their power of spontaneous movement continued unimpaired,
as did also their power of closing round a seat of irritation on
the external surface of the shell. The same remark applies to the
pedicellariæ, and the explanation is simple. It is the external
nervous plexus which holds all the spines and pedicellariæ in
communication with one another as by a network; so that when any part
of this network is irritated, all the spines and pedicellariæ in the
neighbourhood move over to the seat of irritation. On the other hand,
it is the internal plexus which serves to unite all the spines to the
nerve-centre which surrounds the mouth, and which alone is competent
to co-ordinate the action of all the spines for the purposes of
locomotion.

It remains to consider whether the ambulacral feet exhibit any
general co-ordinated action, and, if so, whether this likewise
depends upon the same nerve-centre.

The fact already mentioned, that during progression an Echinus
uses some of its feet for crawling and others for feeling its
way, is enough to suggest that all the feet are co-ordinated by a
nerve-centre. But in order to be quite sure about the fact of there
being a general co-ordination among all the feet, we tried the
following experiments.

I have already described the righting movements which are performed
by an Echinus when the animal is inverted, and it will be remembered
that in this animal the manoeuvre is effected by means of the feet
alone. At first sight this might almost seem sufficient to prove
the fact of a general co-ordination among the feet; but further
reflection will show that it is not so. For the feet being all
arranged in regular series, when one row begins to effect the
rotation of the globe, it may very well be that its further rotation
in the same direction is due only to the fact that the slight tilt
produced by the pulling of the first feet in the series A, B, C gives
the next feet in the series D, E, F an opportunity of reaching the
floor of the tank; their adhesions being established, they would tend
by their pulling to increase still further the tilt of the globe,
thus giving the next feet in the series an opportunity of fastening
to the floor of the tank, and so on. In order, therefore, to see
whether these righting movements were due to nervous co-ordination
among the feet, or merely to the accident of the serial arrangement
of the feet, we tried the experiments which I shall now detail.

First of all we took an Echinus, and by means of a thread suspended
it upside-down in a tank of water half-way up the side of the tank,
and in such a way that only the feet on one side of the ab-oral pole
were able to reach the perpendicular wall of the tank. These feet as
quickly as possible established their adhesions to the perpendicular
wall, and, the thread being then removed, the Echinus was left
sticking to the side of the tank in an inverted position by means of
the ab-oral ends of two adjacent feet-rows (Fig. 59). Under these
circumstances, as we should expect from the previous experiments, the
animal sets about righting itself as quickly as possible. Now, if
the righting action of the feet were entirely and only of a serial
character, the righting would require to be performed by rearing the
animal upwards; the effect of foot after foot in the same rows being
applied in succession to the side of the tank, would require to be
that of rotating the globular shell against the side of the tank
towards the surface of the water, and therefore against the action
of gravity. This is sometimes done, which proves that the energy
required to perform the feat is not more than a healthy Echinus can
expend. But much more frequently the Echinus adopts another device,
and the only one by which it is possible for him to attain his
purpose without the labour of rotating upwards: he rotates laterally
and downwards in the form of a spiral. Thus, let us call the five
feet-rows, 1, 2, 3, 4, and 5 (Figs. 59, 60, 61), and suppose that
1 and 2 are in use near their ab-oral ends in holding the animal
inverted against the perpendicular side of a tank. The downward
spiral rotation would then be effected by gradually releasing the
outer feet in row 1, and simultaneously attaching the outer feet in
row 2 (_i.e._ those nearest to row 3, and furthest from row 1), as
far as possible to the outer side of that row. The effect of this is
to make the globe roll far enough to that side to enable the inner
feet of row 3 (_i.e._ those nearest to row 2), when fully protruded,
to touch the side of the tank. They establish their adhesions, and
the residue of feet in row 1, now leaving go their hold, these new
adhesions serve to roll the globe still further round in the same
direction of lateral rotation, and so the process proceeds from row
to row; but the globe does not merely roll along in a horizontal
direction, or at the same level in the water, for each new row that
comes into action takes care, so to speak, that the feet which it
employs shall be those which are as far below the level of the feet
in the row last employed as their length when fully protruded (_i.e._
their power of touching the tank) renders possible. The rotation of
the globe thus becomes a double one, lateral and downwards, till
the animal assumes its normal position with its oral pole against
the perpendicular tank wall. So considerable is the rotation in the
downward direction, that the normal position is generally attained
before one complete lateral, or equatorial, rotation is completed.

  [Illustration: Fig. 59.]

  [Illustration: Fig. 60.]

  [Illustration: Fig. 61

  Figs. 50, 60, and 61 are righting movements of Echinus on a
  perpendicular surface.]

The result of this experiment, therefore, implies that the righting
movements are due to something more than the merely successive action
of the series of feet to which the work of righting the animal may
happen to be given. The same conclusion is pointed to by the results
of the following experiment.

A number of vigorous Echini were thoroughly shaved with a scalpel
over the whole half of one hemisphere, _i.e._ the half from the
equator to the oral pole. They were then inverted on their ab-oral
poles. The object of the experiment was to see what the Echini which
were thus deprived of the lower half of three feet-rows would do
when, in executing their righting manoeuvres, they attained to the
equatorial position and then found no feet wherewith to continue the
manoeuvre. The result of this experiment was first of all to show us
that the Echini invariably chose the unmutilated feet-rows wherewith
to right themselves. Probably this is to be explained, either by
the general principle to which the escape from injury is due--viz.
that injury inflicted on one side of an Echinoderm stimulates into
increased activity the locomotor organs of the opposite side,--or by
the consideration that destruction of the lower half of a row very
probably induces some degree of shock in the remaining half, and so
leaves the corresponding parts of the unmutilated rows prepotent
over the mutilated one. Be this as it may, however, we found that
the difficulty was easily overcome by tilting the animal over upon
its mutilated feet-rows sufficiently far to prevent the unmutilated
rows from reaching the floor of the tank. When held steadily in this
position for a short time, the mutilated rows established their
adhesions, and the Echinus was then left to itself. Under these
circumstances an Echinus will always continue the manoeuvre along the
mutilated feet-rows with which it was begun, till the globe reaches
the position of resting upon its equator, and therefore arrives at
the line where the shaved area commences. The animal then remains
for hours in this position, with a gradual but continuous motion
backwards, which appears to be due to the successive slipping of the
spines--these organs in the righting movements being always used
as props for the ambulacral feet to pull against while rearing the
globe to its equatorial position, and in performing this function on
a slate floor the spines are liable often to slip. The only other
motion exhibited by Echini thus situated is that of a slow rolling
movement, now to one side and now to another, according to the
prepotency of the pull exerted by this or that row of ambulacral
feet. Things continue in this way until the slow backward movement
happens to bring the animal against some side of the tank, when the
uninjured rows of ambulacral feet immediately adhere to the surface
and rotate the animal upwards or horizontally, until it attains the
normal position. But if care be taken to prevent contact with any
side of the tank, the mutilated Echinus will remain propped on its
equator for days; it never adopts the simple expedient of reversing
the action of its mutilated feet-rows, so as to bring the globe again
upon its ab-oral pole and get its unmutilated feet-rows into action.

From this we may conclude that the righting movements of the pedicels
are due, not to the merely serial action of the pedicels, but to
their co-ordination by a nerve-centre acting under a stimulus
supplied by a sense of gravity; for if the movements of the pedicels
were merely of a serial character, we should not expect that the
equatorial position, having been attained under these circumstances,
should be permanently maintained. We should not expect this, because
after a while the pedicels, which are engaged in maintaining the
globe in its equatorial position, must become exhausted and relax
their hold, when those next behind in the series would lay hold
of the bottom of the tank, and so on, the rotation of the globe
thus proceeding in the opposite direction to that in which it
had previously taken place. On the other hand, if the righting
movements of the pedicels are due to co-ordination proceeding from a
nerve-centre acting under a sense of gravity, we should expect the
animal under the circumstances mentioned to remain permanently reared
upon its equator; for this would allow that the nerve-centre was
always persistently, though fruitlessly, endeavouring to co-ordinate
the action of the absent feet.

Further, as proof that the ambulacral feet of Echinus are under
the control of some centralizing apparatus when executing the
righting manoeuvre, we may state one other fact. When the righting
manoeuvre. is nearly completed by the rows engaged in executing
it, the lower feet in the other rows become strongly protruded and
curved downwards, in anticipation of shortly coming into contact with
the floor of the tank when the righting manoeuvre shall have been
completed (see Fig. 52, p. 280). This fact tends to show that all
the ambulacral feet of the animal are, like all the spines, held in
mutual communication with one another by some centralizing mechanism.

But the best proof of all that the feet in executing the righting
manoeuvre are under the influence of a co-ordinating centre, is one
that arose from an experiment suggested to me by Mr. Francis Darwin,
and which I shall now describe. Mr. Darwin having kindly sent the
apparatus which his father and himself had used in their experiments
on the geotropism of plants, it was employed thus. A healthy Echinus
was placed in a large bottle filled to the brim with sea-water, and
having been inverted on the bottom of the bottle, it was allowed in
that position to establish its adhesions. The bottle was then corked
and mounted on an upright wheel of the apparatus whereby, by means of
clockwork, it could be kept in continual slow rotation in a vertical
plane. The object of this was to ascertain whether the continuous
rotation in a vertical plane would prevent the animal from righting
itself (because confusing the nerve-centres which, under ordinary
circumstances, could feel by their sense of gravity which was up and
which was down), or would still allow the animal to right itself
(because not interfering with the serial action of the feet). Well,
it was found that this rotation of the whole animal in a vertical
plane entirely prevented the righting movements during any length
of time that it might be continued, and that these movements were
immediately resumed as soon as the rotation was allowed to cease.
This, moreover, was the case, no matter what phase of the righting
manoeuvre the Echinus might have reached at the moment when the
rotation began. Thus, for instance, if the globe were allowed to have
reached the position of resting on its equator before the rotation
was commenced, the Echinus would remain motionless, holding on with
its equatorial feet, so long as the rotation was kept up.

Therefore, there can be no question that the ambulacral feet are all
under the influence of a co-ordinating nerve-centre, quite as much
as are the spines. But, on the other hand, experiments show that
the centre in this case is not of so localized a character as it
is in the case of the spines; for when the nerve-ring is cut out,
the co-ordination of the feet, although impaired, is not wholly
destroyed. Take, for instance, the case of the righting manoeuvre.
The effect of cutting out the nerve-ring is that of entirely
destroying the ability to perform this manoeuvre in the case of the
majority of specimens; nevertheless about one in ten continue able
to perform it. Again, if an Echinus is divided into two hemispheres
by an incision carried from pole to pole through any meridian, the
two hemispheres will live for days, crawling about in the same manner
as entire animals; if their ocular plates are not injured, they
seek the light, and when inverted they right themselves. The same
observations apply to smaller segments, and even to single detached
rows of ambulacral feet. The latter are, of course, analogous to
the single detached rays of a Star-fish, so far as the system of
ambulacral feet is concerned; but, looking to the more complicated
apparatus of locomotion (spines and pedicellariæ), as well as to
the rigid consistence and awkward shape of the segment--standing
erect, instead of lying flat--the appearance presented by such a
segment in locomotion is much more curious, if not surprising, than
that presented by the analogous part of a Star-fish under similar
circumstances. It is still more surprising that such a fifth-part
segment of an Echinus will, when propped up on its ab-oral pole (Fig.
62), right itself (Fig. 63) after the manner of larger segments
or entire animals. They, however, experience more difficulty in
doing so, and very often, or indeed generally, fail to complete the
manoeuvre.

  [Illustration: Figs. 62 and 63.--Righting and ambulacral
  movements of severed segments of Echinus.]

On the whole, then, we may conclude that the nervous system of an
Echinus consists (1) of an external plexus which serves to unite
all the feet, spines, and pedicellariæ together, so that they
all approximate a point of irritation situated anywhere in that
plexus; (2) of an internal nervous plexus which is everywhere in
communication through the thickness of the shell with the external,
and the function of which is that of bringing the feet, spines,
and probably also the pedicellariæ into relation with the great
co-ordinating nerve-centre situated round the mouth; (3) of central
nervous matter which is mainly gathered round the mouth, and there
presides exclusively over the co-ordinated action of the spines, and
in large part also over the co-ordinated action of the feet, but
which is further in part distributed along the courses of the main
nerve-trunks, and so secures co-ordination of feet even in separated
segments of the animal.


_Special Senses._

Before concluding, I must say a few words on the experiments whereby
we sought to test for the presence in Echinoderms of the special
senses of sight and smell.

We have found unequivocal evidence of the Star-fish (with the
exception of the Brittle-stars) and the Echini manifesting a strong
disposition to crawl towards, and remain in, the light. Thus, if a
large tank be completely darkened, except at one end where a narrow
slit of light is admitted, and if a number of Star-fish and Echini
be scattered over the floor of the tank, in a few hours the whole
number, with the exception of perhaps a few per cent., will be found
congregated in the narrow slit of light. The source we used was
diffused daylight, which was admitted through two sheets of glass,
so that the thermal rays might be considered practically excluded.
The _intensity_ of the light which the Echinoderms are able to
perceive may be very feeble indeed; for in our first experiments we
boarded up the face of the tank with ordinary pinewood, in order to
exclude the light over all parts of the tank except at one narrow
slit between two of the boards. On taking down the boards we found,
indeed, the majority of the specimens in or near the slit of light;
but we also found a number of other specimens gathering all the way
along the glass face of the tank that was immediately behind the
pine-boards. On repeating the experiment with blackened boards, this
was never found to be the case; so there can be no doubt that in the
first experiments the animals were attracted by the faint glimmer of
the white boards, as illuminated by the very small amount of light
scattered from the narrow slit through a tank, all the other sides
of which were black slate. Indeed, towards the end of the tank,
where some of the specimens were found, so feeble must have been the
intensity of this glimmer, that we doubt whether even human eyes
could have discerned it very distinctly. Owing to the prisms at our
command not having sufficient dispersive power for the experiments,
and not wishing to rely on the uncertain method of employing coloured
glass, we were unable to ascertain how the Echinoderms might be
affected by different rays.

On removing with a pointed scalpel the eye-spots from a number of
Star-fish and Echini, without otherwise injuring the animals, the
latter no longer crawled towards the light, even though this were
admitted to the tank in abundance; but they crawled promiscuously
in all directions. On the other hand, if only one out of the five
eye-spots were left intact, the animals crawled towards the light as
before. It may be added that single detached rays of Star-fish and
fifth-part segments of Echini crawl towards the light in the same
manner as entire animals, provided, of course, that the eye-spot is
not injured.

The presence of a sense of smell in Star-fish was proved by keeping
some of these animals for several days in a tank without food, and
then presenting them with small pieces of shell-fish. The Star-fish
immediately perceived the proximity of food, as shown by their
immediately crawling towards it. Moreover, if a small piece of the
food were held in a pair of forceps and gently withdrawn as the
Star-fish approached it, the animal could be led about the floor
of the tank in any direction, just as a hungry dog could be led
about by continually withdrawing from his nose a piece of meat as
he continually follows it up. This experiment, however, was only
successful with Star-fish which had been kept fasting for several
days; freshly caught Star-fish were not nearly so keen in their
manifestations, and indeed in many cases did not notice the food at
all.

Desiring to ascertain whether the sense of smell were localized in
any particular organs, as we had found to be the case with the sense
of sight, I first tried the effect of removing the five ocelli. This
produced no difference in the result of the above experiment with
hungry Star-fish, and therefore I next tried the effect of cutting
off the tips of the rays. The Star-fish behaving as before, I then
progressively truncated the rays, and thus eventually found that the
olfactory sense was equally distributed throughout their length. The
question, however, still remained whether it was equally distributed
over both the upper and the lower surfaces. I therefore tried the
effect of varnishing the upper surface. The Star-fish continued to
find its food as before, which showed that the sense of smell was
distributed along the lower surface. I could not try the converse
experiment of varnishing this surface, because I should thereby have
hindered the action of the ambulacral feet. But by another method
I was able nearly as well to show that the upper surface does not
participate in smelling. This method consisted in placing a piece of
shell-fish upon the upper surface and allowing it to rest there. When
this was done, the Star-fish made no attempt to remove the morsel
of food by brushing it off with the tips of its rays, as is the
habit of the animal when any irritating substance is applied to this
surface. Therefore I conclude that the upper or dorsal surface of a
Star-fish takes no part in ministering to the sense of smell, which
by the experiment of varnishing this surface, and also by that of
progressively truncating the rays, is proved to be distributed over
the whole of the ventral or lower surface of the animal. For I must
add that severed rays behave in all these respects like the entire
organisms, although they are disconnected from the mouth and disc.

As this chapter has already extended to so great a length, I omit
from it any account of some further experiments which I tried
concerning the effects of nerve-poisons upon the Echinodermata. A
full record of these experiments may be found in the publications of
the Linnean Society.


FINIS.




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Transcriber's note:
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End of the Project Gutenberg EBook of Jellyfish, Starfish, and Sea-Urchins:
Being a Research on Primitive Nervous Systems, by G. J. Romanes

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