[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).

[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.

[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.

[4] See "Observations on the Nervous System of Aurelia aurita," Phil. Trans., pt. ii., 1878.

[5] "Die Medusen physiologisch und morphologisch auf ihr Nervensystem untersucht" (TÜbingen, 1878).

[6] I have only met with one individual exception. This occurred in a specimen of Staurophora laciniata, where, after removal of the entire margin, three centres of spontaneity were found to remain in the sheet of contractile tissue lining the nectocalyx.

[7] See Fig. 1.

[8] In the case of the covered-eyed MedusÆ, however, the paralyzed umbrella sometimes responds to a single stimulation with two, and more rarely with three contractions, which are separated from one another by an interval of the same duration as the normal diastole of the unmutilated animal.

[9] As Professor Haeckel observes in his monograph already alluded to, "Die Deutung der Sinnesorgane niederer Thiere gehÖrt ohne Zweifel zu den schwierigsten Objecten der vergleichenden Physiologie und ist der grÖssten Unsicherheit unterworfen. Wir sind gewohnt, die von den Wirbelthieren gewonnenen Anschauungen ohne Weiteres auch auf die wirbellosen Thiere der verschiedenen Kinese zu Übertragen und bei diesen analoge Sinnesempfindungen anzunehmen als wir selbst besitzen ... Noch weniger freilich als die von den meisten Autoren angenommene Deutung der RandblÄschen unserer Medusen als GehÖrorgane kann die von Agassiz und Fritz MÜller vertretene Ansicht befriedigen, dass dieselben Augen seien.... Alle diese VerhÄltnisse sind mit der Deutung der Concretion als 'Linse' und des sie umschliessenden Sinnesganglion als 'Sehnerv' durchaus unvereinbar."

It may not be unnecessary to say that, although the simple experiment above described effectually proves that the marginal bodies have a visual function to subserve, we are not for this reason justified in concluding that these are so far specialized as organs of sight as to be precluded from ministering to any other sense.

[10] The period of latent stimulation merely means the time after the occurrence of an excitation during which a series of physiological processes are taking place, which terminate in a contraction; so that, whether the excitation is of a strong or of a weak intensity, the period of latent stimulation is not much affected. The above question, therefore, was simply this—Does the prolonged delay on the part of these ganglia, in responding to light, represent the time during which the series of physiological processes are taking place in response to an adequate stimulus, or does it represent the time during which light requires to act before it becomes an adequate stimulus?

[11] See "Croonian Lectures," 1875. Philosophical Transactions, vol. 166, part I. pp. 284-6.

[12] It may here be stated that in all the experiments on stimulation subsequently to be detailed, there is no difference to be observed between the behaviour of an entire swimming organ deprived of its ganglia, and that of a portion of any size which may be separated from it.

[13] In a highly interesting paper recently published by Dr. W. H. Gaskell, F.R.S., on "The Innervation of the Heart" (Journ. of Physiol., vol. iv. p. 43, et seq.), it is shown that the experiments in section thus far described yield strikingly similar results when performed upon the heart of the tortoise and the heart of the skate. Dr. Gaskell inclines to the belief that in these cases the contraction-waves are merely muscle-waves. There is one important fact, however, which even here seems to me to indicate that the propagation of the wave is at least in some measure dependent on nervous conduction. This fact is, that after a contraction-wave has been blocked by the activity of a spiral or other form of section, it may again be made to force a passage under the influence of vagus stimulation. Moreover, in a paper still more recently published by Drs. Brunton and Cash on "Electrical Stimulation of the Frog's Heart" (Proc. Roy. Soc., vol. xxxv., No. 227, p. 455, et seq.) it is remarked, "Another interesting consideration is, whether the stimulus which each cavity of the heart transmits to the succeeding one consists in the propagation of an actual muscular wave, or in the propagation of an impulse along the nerves. The observations of Gaskell have given very great importance to the muscular wave occurring in each cavity of the heart of cold-blooded animals as a stimulus to the contraction of the next succeeding cavity. Our observations appear to us to show that, while this in an important factor, it is not the only one in the transmission of stimuli.... We consider that stimuli are also propagated from one chamber of the heart to another through nervous channels: thus we find that irritation of the venus sinus will sometimes produce simultaneous contractions of the auricle and ventricle, instead of the ventricular beat succeeding the auricular in the ordinary way. This we think is hardly consistent with the hypothesis, that a stimulus consists of the propagation of a muscular wave only from the auricle to the ventricle."

[14] That it can scarcely be electrical induction would seem to be shown by the fact that such effects can only be produced on nerves by strong currents, and also by the fact that the saline tissues of the swimming-bell must short-circuit any feeble electrical currents as soon as they are generated.

[15] I have associated the above theory of nerve-genesis with the name of Mr. Spencer, because it occupies so prominent a place in his "Principles of Psychology." But from what I have said in the text, I think it is clear that the theory, as presented by Mr. Spencer, consists of two essentially distinct hypotheses—the one relating to the formation of nerve-tissue out of protoplasm, and the other to the increase of functional capacity in a nerve-fibre by use (a third hypothesis of Mr. Spencer relating to the formation of ganglion-tissue does not here concern us). The latter hypothesis, however, ought not to be associated with Mr. Spencer's name without explaining that it has likewise occurred to other writers, the first of which, so far as I can ascertain, was Lamarck, who says, "Dans toute action, le fluide des nerfs qui la provoque, subit un mouvement de dÉplacement qui y donne lieu. Or, lorsque cette action a ÉtÉ plusieurs fois rÉpÉtÉe, il n'est pas douteux que le fluide qui l'a exÉcutÉe, ne se soit frayÉ une route, qui lui devient alors d'autant plus facile À parcourir, qu'il l'a effectivement plus souvent franchie, et qu'il n'ait lui-mÊme une aptitude plus grand À suivre cette route frayÉe que celles qui le sont moins." ("Phil. Zool.," tom ii. pp. 318-19.)

[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.

[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.

[18] When two such waves met, they neutralized each other at their line of collision; or perhaps more correctly, the tissue on each side of that line, having just been in contraction, was not able again to convey a contraction-wave passing in the opposite direction to the wave which it had conveyed immediately before.

[19] In this description I have everywhere adopted the current phraseology with regard to ganglionic action—a phraseology which embodies the theory of ganglia supplying interrupted stimulation. But although I have done this for the sake of clearness, of course it will be seen that the facts harmonize equally well with the theory of continuous stimulation, to which I shall allude further on.

[20] Removing the manubrium does not interfere with this steering action; but if any considerable portion of the margin is excised, the animal seems no longer able to find the beam of light, even though one or more of the marginal bodies be left in situ.

[21] This could be particularly well seen if, after the extreme apex of the cone had been removed, one of the four radial cuts was continued through the margin, and the latter was then spread out into a linear form by gently pressing the animal against the flat side of the glass vessel in which it was contained. The same experiment performed on Aurelia is, of course, attended with a totally different result, now one segment and now another originating a discharge which then spreads to all the others in the form of a contraction-wave.

[22] If the reader takes the trouble to ascertain the average proportion between the number of pulsations and the seconds of rest in the first observations as far down as the first long pause, viz. as above stated, 185/211, and if he then balances the succeeding income and expenditure of energy of all the rest of the observations, he will find the net result to accord very precisely with the proportion he previously obtained. But, as already stated, any such precision as this is certainly the exception rather than the rule.

It may here be stated that after the sixty seconds of rest above recorded, the animal began another swimming bout. It was then immediately bisected, and the subsequent observations are detailed in the next footnote.

[23] It may be thought that the greater area of general tissue-mass in the larger segments than in the smaller, and not the lesser proportional area of tube-section, is the cause of the larger segments living longer than the smaller ones. I am led, however, to reject this hypothesis, because in Sarsia, where segmentation entails a comparatively small amount of tube-section, there is no constant rule as to the larger segments showing more endurance than the smaller ones—the converse case, in fact, being of nearly as frequent occurrence. I can only account for this fact by supposing that the endurance of the segments of Sarsia is determined by the degree in which the three or four minute open tube-ends become accidentally blocked. This supposition is the only one I can think of to account for the astonishing contrasts as to endurance that are presented by different segments of the same individual, and, I may add, of different individuals when deprived of their margins and afterwards submitted to the same conditions. For instance, a number of equally vigorous specimens had their margins removed, and were then suspended in a glass cage attached to a buoy in the sea. Four days afterwards some of the specimens were putrid, while others were as fresh as they were when first operated on. Again, as an instance of the experiments in segmentation of Sarsia, I may quote an experiment in which a score of specimens were divided in all sorts of ways, such as leaving the manubrium attached, to one half, or three marginal bodies in one portion and the remaining marginal body in the other portion, etc. Yet, although it was very exceptional to find the two portions presenting an equal degree of endurance, no uniform results pointing to the cause of the variations could be obtained. In most cases, however, the energy, as distinguished from the endurance of the larger segments, was conspicuously greater than that of the smaller. (But it is curious that in many cases the effects of shock appeared to be more marked in the larger than in the smaller segments—the latter, for some time after the operation, contracting much more frequently than the former.) To show both these effects, one experiment may be quoted. A specimen of Sarsia was divided into two parts, of which one was a quadrant.

Immediately after the operation the results were as follows:—

To show the difference between the endurance of two halves of a bisected specimen of Sarsia, I may quote one experiment which was performed on the same specimen as the one mentioned in the text to show the general relationship between the duration of the pauses and that of swimming bouts. (See last footnote.)

But although in the case of Sarsia the leaser endurance of the smaller segment than of the larger cannot be regarded as a general rule, it may be so regarded, as already stated, in the case of Aurelia. The following experiment exemplifies this particular rule even more prettily than does the one quoted in the text, from the fact that the segments survived the operation for a greater number of days.

An Aurelia having a regular and well-sustained rhythm of twenty per minute was divided as already described in the text. In five-minute intervals on successive days the average rates of the four segments were as follows:—

Next day, the temperature unfortunately rose sufficiently to cause the death of the single surviving segment, which otherwise would probably have lived for one or two days longer.

[24] This and all the subsequent tracings I obtained by the method already described.

[25] It will not be forgotten that there are a multitude of ganglion-cells distributed throughout the contractile tissues of the MedusÆ; but forasmuch as these are comparatively rarely instrumental in originating stimulation, I think it is probable that artificial stimulation acts directly on the contractile tissues, and not through the medium of these scattered cells.

[26] We may pretty safely conclude that ganglia are altogether absent in the manubrium of Sarsia, not only because Schultz has failed to detect them in this organ microscopically, but also because of the complete absence of spontaneity which it manifests. I may here mention that this case of the manubrium of Sarsia is precisely analogous to another which I have observed in a widely different tissue, namely, the tongue of the frog. Here, too, the presence of ganglion-cells has never been observed microscopically, though specially sought for by Dr. Klein and others. Yet, under the influence of mechanical and other modes of stimulation, I find that I am able to make the excised organ pulsate as rhythmically as a heart.

[27] Sometimes, however, the order of events is slightly different, the advent of the spasm being more sudden, and followed by a mitigation of its severity, the bell then exhibiting what is more usually the first phase of the series, namely, the occurrence of the locomotor-like contractions. Occasionally, also, rhythmical shivering contractions may be seen superimposed on the general tonic contraction, either in a part or over the whole of the contractile tissues.

[28] It is of importance to point out the fact that some of my previously stated experiments appear conclusively to prove that the natural stimulation which is supplied by the marginal ganglia of the MedusÆ resembles all the modes of artificial stimulation which are competent to produce artificial rhythm in one important particular; the intensity of the stimulation which the marginal ganglia supply is shown by these experiments to be about the same as that which is required to produce artificial rhythm in the case of artificial stimulation. In proof of this point, I may allude particularly to the observations which are detailed on pp. 131-136.

[29] I may here mention that the fact of the manubrium of Sarsia undergoing this extreme elongation after the removal of the marginal ganglia, serves to render the artificial rhythm of the organ under the influence of injury, as previously described, all the more conspicuous.

[30] The evidence, however, is not altogether exclusive of the resistance theory, for it is quite possible that in addition to the high irritability of the manubrium there may be conductile lines of low resistance connecting this organ with the marginal ganglia. I entertain this supposition because, as explained in my Royal Society papers, I see reason to believe that the natural swimming movements of Sarsia are probably in part due to an intermittent discharge of the ganglia. I think, therefore, that in this particular case the ganglia supply a tolerably constant stimulation to the manubrium, while it is only at intervals that their energy overflows into the bell, and that the higher degree at irritability on the part of the manubrium ensures the tonic response of this organ at a small cost of nervous energy. How far the rhythm of the nectocalyx is to be attributed to the resistance mechanism of the ganglia, and how far to the alternate exhaustion and recovery of the contractile tissues, I think it is impossible to determine, seeing that it is impossible exactly to imitate the natural ganglionic stimulation by artificial means. But it is, I think, of importance to have ascertained at least this much, that in Sarsia the tonus of one organ and the rhythm of another, which apparently both received their stimulation from the same ganglia, must at any rate in part be attributed to a differential irritability of these organs, as distinguished from their differential stimulation.

[31] The method of comparison consists, as will already have been gathered from the perusal of the foregoing sections, in:—first, stimulating the tentacles, and observing whether this is followed by such a discharge of the attached ganglion as causes the bell to contract; next, stimulating the bell itself, to ascertain whether the muscular irritability is impaired; and, lastly, stimulating either the tentacles or the bell, to observe whether the reciprocal connections between tentacles, bell, and manubrium are uninjured.

[32] In conducting this experiment, care must be taken not to exert the slightest pressure on any part of the strip. The method I adopted, therefore, was to have a vessel with a very deep furrow on each of its opposite lips. Upon filling this vessel to the level of these furrows with the poisoned water, and then immersing the whole vessel in ordinary sea-water up to the level of its brim, some of the poisoned water of course passed through the open furrows. The external body of water (i.e. the normal sea-water containing the animal) was therefore made proportionally very large, so that the slight escape of poison into it did not affect the experiment. On now passing the portion of the strip to be poisoned through the opposite furrows, it was allowed to soak in the poison while freely floating, and so without suffering pressure in any of its parts.

[33] Since the above results on the effects of poisons were published in my Royal Society papers, Dr. Krukenberg has conducted a research upon "comparative toxicology," in which he has devoted the larger share of his attention to the MedusÆ. While expressing my gratification that when he adopted my methods he succeeded in confirming my results, I may observe that the criticism which he somewhat bluntly passes upon the latter is not merely unwarranted, but based upon a strange misconception of a well-known principle in the physiology of nerves and muscles. This criticism is that these results as published by me are worthless and "a dead chapter in science," because I failed to prove that it was the nervous (as distinguished from the muscular) elements which were effected by the various poisons. In his opinion this distinction can only be made good by employing electrical stimulation upon the sub-umbrella tissue when this has lost its spontaneity under the influence of poisons: if a response ensues which does not ensue when the tissue is stimulated mechanically, he regards the fact as proof that the muscular tissue remains unaffected while the nervous tissue has been rendered functionless.

Now, in the first place, I have here to show that there is, as I have said, a fundamental error touching a well-known principle of physiology. So far as there is any difference between the excitability of nerve and muscle with respect to mechanical and electrical stimulation, it is the precise converse of that which Dr. Krukenberg supposes; it is not muscle, but nerve, which is the more sensitive to electrical stimulation—by which I understand him to mean the induction shock. The remarkable transposition of Dr. Krukenberg's ideas upon this matter does not affect the results of his observations upon the action of the various poisons; it only renders fatuous his criticism of these same results as previously published by me.

In the next place, I have to observe that in all my experiments I tried, as he subsequently tried, both kinds of stimulation, and also the constant current; but I soon found that even when one went to work with one's ideas upon the subject in a non-inverted position, no trustworthy inference could be drawn in favour of the muscular elements alone remaining uninjured from the bare fact that after the poisoning the neuro-muscular tissue often behaved differently towards different kinds of stimulation. Further, in the particular case of my experiments with curare—against which Dr. Krukenberg's remarks are chiefly directed on the ground that I did not prove the paralysis to be a merely muscular effect—I succeeded in obtaining very much better proof of the poison acting on the nervous elements, to the exclusion of the muscular, than I could have obtained by any process of inference, however good; that is to say, I obtained direct proof. It appears to me that Dr. Krukenberg must have failed to understand the English of the following sentences: "On nipping any portion of the poisoned half of Staurophora laciniata, this half remained absolutely motionless, while the unpoisoned half, though far away from the seat of irritation, immediately ceased its normal contractions, and folded itself together in the very peculiar and distinctive manner just described," i.e. "in one very strong and long-protracted systole." For the rest, see note on page 232.

Lastly, while again expressing my satisfaction that on all matters of fact, our results are in full harmony, I may be allowed to remark that in my opinion his deductions, as embodied in his schema of the inferred innervation of MedusÆ, are very far in advance of anything that is justified by observation. (See, for this elaborate schema, in which there are represented volitional, motor, reflex, and inhibitory centres, as well as a clearly defined system of sensory and motor nerves, "Vergleichend-Physiologogische Studien, dritte Abtheiling," p. 141: Heidelberg, 1880.)

[34] "Mem. American Acad. Arts and Sciences," 1850, p. 229.

[35] The covered-eyed MedusÆ survive a longer immersion than the naked-eyed—Aurelia aurita, for instance, requiring from a quarter to half an hour's exposure before being placed beyond recovery. Moreover, the cessation of spontaneity on the first immersion is not so sudden as it is in the case of the naked-eyed MedusÆ—the pulsations continuing for about five minutes, during which time they become weaker and weaker in so gradual a manner that it is hard to tell exactly when they first cease.

[36] Looking to the enormous number of marine species of MedusÆ, it is much more probable that the fresh-water species was derived from them than that they were derived from a fresh-water ancestry.

[37] For full account, see Phil. Trans., vol. clxvii. pp. 744, 745.

[38] While these sheets are passing through the press, a paper has been read before the Royal Society by Mr. A. G. Bourne, describing the hydroid stage of the fresh-water Medusa (Proc. Roy. Soc., Dec. 11, 1884). He has discovered the hydroids on the roots of the Pontederia, which have been growing in the Lily-tank for several years, and which are therefore probably the source from which the tank became impregnated with the MedusÆ.

[39] In this case the locomotion of a Star-fish comes to be performed on the same plan or method as that of a Jelly-fish—the five rays performing, by their co-ordinated action, the same function as a swimming-bell. It is a curiously interesting fact, that although no two plans or mechanisms of locomotion could well be imagined as more fundamentally distinct than those which are respectively characteristic of these two groups of animals, nevertheless in this particular case and in virtue of special modification, a Star-fish should have adopted the plan or mechanism of a Jelly-fish.

[40] A further proof that this is at least one of the functions of the pedicellariÆ is furnished by a simple experiment. If an Echinus is allowed to attach its feet to a glass plate held just above its ab-oral pole, and this plate be then raised in the water so that the Echinus is freely suspended in the water by means of its feet alone, the animal feels, as it were, that its anchorage is insecure, and actively moves about its unattached feet to seek for other solid surfaces. Under such circumstances it may be observed that the pedicellariÆ also become active, and especially so near the surface of attachment as if seeking for pieces of sea-weed. If a piece is presented to them, they lay hold upon it with vigour.

Of course the pedicellariÆ may also have other functions to perform, and in a Star-fish Mr. Sladen has seen them engaged in cleaning the surface of the animal; but we cannot doubt that at least in Echinus their main function is that which we have stated.