Jelly-fish, star-fish, and sea urchins : being a research on primitive nervous systems

Front matter

t£t IBibliotbcca ^ CAR. I. TABOEIS

THE

INTERNATIONAL SCIENTIFIC SERIES.

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 Medusae and Echinoder>vniata 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,

VI

PREFACE.

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

CIMFTKS PAGB

I. Structure of the Medusa ... ... 10

II. Fundamental Experiments ... ... ... 26

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 aesthetic 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

B 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 “ foi’ced 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

INTRODUCTION.

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 aesthetic, 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 seabreezes, 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 Medusas 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

INTRODUCTION.

o

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 Comatulcv), 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

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

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

INTRODUCTION.

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 bein<x 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 veiy 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 tliin-spun life,”

INTRODUCTION.

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.*

* 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 Medusj2

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

In size, the different species of Meduste 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 wre may say that the form of the animals always resembles that of a mushroom, and that the resemblance

* 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 “ Encyclopaedia Britannica ” on “ Hydrozoa,” together with Professor Haeckel’s Report on the Medusae of the Challenger Expedition.

rig. i.—

Sarsla

(natural

size).

STRUCTURE OP THE MEDUSAE.

extends to a tolerably close imitation by different species of the various forms which are characteristic of different species of mushrooms, from the thimblelike 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 less 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 overlay ed 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 Medusae 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 Medusae, 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

STRUCTURE OF THE MEDUSJE.

present in the Medusae 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 Medusae, so that it was of much importance to ascertain whether or not tho 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 tho numerous investigators, that Professor Huxley — himself one of tho 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 ; Kolliker, “Ueberdie Randkorper der Quallen, Polypen und Strahlthiere,” Froriep’s neue Notizen, bd. xxv., 1843; Yon Beneden, “Memoire sur les Campanulaires de la cote d’Ostende,” “ Mdmoires do FAcademie de Bruxelles,” vol. xvii., 1843; Desor, “ Sur la Gdndration Medusipare des Polypes hydraires,” “Annales d. Scienc. Natur. Zool. ,” ser. iii. t. xii. p. 204 ; Krohn, “ Ueber Podocoryna carnea,”

“ Ai'cliiv. 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 Acaliphae of North America,” “Memoirs of the American Academy of Arts and Sciences,” vol. iii., 1800, vol. iv., 1802 ; Leuekart, “ Archiv. f. Naturgeschichte,” Jahrg. 38, b. ii., 1872 ; Hensen, “ Studien iiber das Geliororgan der Decapoden,” “ Zeitchr. f. wiss. Zool.,” bd. xiii., 1803 ; Semper, “ Reisebericht,” “ Zeitschr. f. wiss. Zool.,” bd. xiii. vol. xiv. ; Claus, “ Bemerkungen iiber Clenophoren und Medusen,” “Zeitschr. f. wiss. Zool.,” bd. xiv., 1804; Allman, “Note on the Structure of Certain Hydroid Medusae,” “Brit. Assoc. Rep.,” 1807; Fritz Muller, “ Polypen und Quallen von S. Catherina,” “ Archiv. f. Naturgesch.,” Jahrg. 25, bd. i., 1859 ; also “ Ueber die Rand-blaschen der Hydroidquallen,” “ Archiv. f. Anatomie und Physiologie,” 1852 ; Haeckel, “Beitrage zur Naturgesch. der Hydromedusen,” 1805; Eimer, “Zoologische Untersuchungen,” Wiirzburg, “ Yerhandlungen 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. Px*ofessor Haeckel, who made his microscopical observations chiefly upon the Geryonidae, 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

STRUCTURE OF THE MEDUS.E.

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 Medusae is given by Drs. Hertwig in their more recently published work on the nervous system and sense-organs of the Medusae, 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 ringnerve ; so that in Haeckel’s researches on Geryonia, whereby both true ganglion-cells and true nerve-fibres were first demonstrated as occurring in the

Medusae, we have a most important step in the histology of these animals. Haeckel’s results in these respects have since been confirmed by Claus, “Grundziige der Zoologie,” 1872; Allman, “A Monograph of the Gymnoblastie or Tubularian Hydroids,” 1871; Harting, “Notices Zoologiques,” Niedlandisches “Archiv. f. Zool.,” bd. ii., Heft 3, 1873 ; F. E. Schulze, “ Ueber den Bau von Syncorzno 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 Medusae. 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 Medusae, 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 ” * 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

* 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.

STRUCTURE OF THE MEDUSAE. 17

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 Geryonid® 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 multipolar. 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 oi 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.

c

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 wo 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 pi’olongations of the epitlielium-cells, and that afterwards these prolongations gradually disappeared, leaving only their remnants to develop into the ordinary ganglion-cells already described.

Beneath the upper nerve-ring lies the lower nerve-ring. It is inserted between the muscletissue 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 maimer with

STRUCTURE OF THE MEDUSAE.

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 obseiwable 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 Medusae, 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 lias called neuro-muscular cells; and, in the tentacles, neuro-muscular cells joined with cells of special sensation (Sinneszellen).