Jelly-Fish, Star-Fish, and Sea-Urchins: Being a Research on Primitive Nervous Systems

by George John Romanes

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.

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.