Imagini ale paginilor

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 nust 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.

* The relation of consciousness to the elaboration of nervecentres throughout the animal kingdom is more fully considered in my work on “Mental Evolution in Animals" (Kegan Paul, Trench & Co. : 1883).



To give a full account of the morphology, development, and classification of the Medusa 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 describing such features in the anatomy of the jelly-fish as will afterwards be found especially to concern us.

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 animais 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 (natural

a mushroom, and that the resemblance

Fig. 1.



* Those who mar 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 Britanuica

Hydrozoa,” together with Professor Haeckel's Report on the Medusæ of the Challenger Expedition.


[ocr errors]


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 noncontractile 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 branched and run in a straight course to the margin of ne 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 communilates by minute apertures with the external medium.

The margin of the umbrella, both in the naked and covered eyed Medusa, 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 “coveredeyed,” 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

[ocr errors]
[ocr errors]
« ÎnapoiContinuați »