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._

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.

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.