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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_.