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

by George John Romanes

I shall now proceed to give an account of my observations on the physiology of this interesting animal, by quoting in extenso my original paper upon the subject (Nature, June 24, 1880). Before doing so, however, I may state that Professors A. Agassiz, Moseley, and others have since informed us that sundry species of sea-water Medusæ have been observed by them living and thriving in the brackish waters of estuaries—a fact which strongly corroborates the inference at the end of the present paper.

"The natural movements of the Medusa precisely resemble those of its marine congeners. More particularly, these movements resemble those of the marine species which do not swim continuously, but indulge in frequent pauses. In water at the temperature of that in the Victoria lily-house (85° Fahr.), the pauses are frequent, and the rate of the rhythm irregular, suddenly quickening and suddenly slowing even during the same bout, which has the effect of giving an almost intelligent appearance to the movements. This is especially the case with young specimens. In colder water (65° to 75°) the movements are more regular and sustained; so that, guided by the analogy furnished by my experiments on the marine forms, I infer that the temperature of the natural habitat of this Medusa cannot be so high as that of the water in the Victoria lily-house. In water of that temperature the rate of the rhythm is enormously high, sometimes rising to three pulsations per second. But by progressively cooling the water, this rate may be progressively lowered, just as in the case of the marine species; and in water at 65°, the maximum rate that I have observed is eighty pulsations per minute. As the temperature at which the greatest activity is displayed by the fresh-water species is a temperature so high as to be fatal to all the marine species which I have observed, the effects of cooling are, of course, only parallel in the two cases when the effects of a series of higher temperatures in the one case are compared with those of a series of lower temperatures in the other. Similarly, while a temperature of 70° is fatal to all the species of marine Medusæ which I have examined, it is only a temperature of 100° that is fatal to the fresh-water species. Lastly, while the marine species will endure any degree of cold without loss of life, such is not the case with the fresh-water species. Marine Medusæ, after having been frozen solid, will, when gradually thawed out, again resume their swimming movements; but this fresh-water Medusa is completely destroyed by freezing. Upon being thawed out, the animal is seen to have shrunk into a tiny ball, and it never again recovers either its life or its shape.

"The animal seeks the sunlight. If one end of the tank is shaded, all the Medusæ congregate at the end which remains unshaded. Moreover, during the daytime they swim about at the surface of the water; but when the sun goes down they subside, and can no longer be seen. In all these habits they resemble many of the sea-water species. They are themselves non-luminous.

"I have tried on about a dozen specimens the effect of excising the margin of the nectocalyx. In the case of all the specimens thus operated upon, the result was the same, and corresponded precisely with that which I have obtained in the case of marine species; that is to say, the operation produces immediate, total, and permanent paralysis of the nectocalyx, while the severed margin continues to pulsate for two or three days. The excitability of a nectocalyx thus mutilated persists for a day or two, and then gradually dies out, thus also resembling the case of the marine naked-eyed Medusæ. More particularly, the excitability resembles that of those marine species which sometimes respond to a single stimulation with two or three successive contractions.

"A point of specially physiological interest may be here noticed. In its unmutilated state the fresh-water Medusa exhibits the power of localizing with its manubrium a seat of stimulation situated in the bell; that is to say, when a part of the bell is nipped with the forceps, or otherwise irritated, the free end of the manubrium is moved over and applied to the part irritated. So far the movement of localization is precisely similar to that which I have previously described as occurring in Tiaropsis indicans (Phil. Trans., vol. clxvii.). But further than this, I find a curious difference. For while in Tiaropsis indicans these movements of localization continue unimpaired after the margin of the bell has been removed, and will be ineffectually attempted even after the bell is almost entirely cut away from its connections with the manubrium, in the fresh-water Medusa these movements of localization cease after the extreme margin of the bell has been removed. For some reason or another the integrity of the margin here seems to be necessary for exciting the manubrium to perform its movements of localization. It is clear that this reason must either be that the margin contains the nerve-centres which preside over these localizing movements of the manubrium, or, much more probably, that it contains some peripheral nervous structures which are alone capable of transmitting to the manubrium a stimulus adequate to evoke the movements of localization. In its unmutilated state this Medusa is at intervals perpetually applying the extremity of its manubrium to one part or another of the margin of the bell, the part of the margin touched always bending in to meet the approaching extremity of the manubrium. In some cases it can be seen that the object of this co-ordinated movement is to allow the extremity of the manubrium—i.e. the mouth of the animal—to pick off a small particle of food that has become entangled in the marginal tentacles. It is therefore not improbable that in all cases this is the object of such movements, although in most cases the particle which is caught by the tentacles is too small to be seen with the naked eye. As it is thus no doubt a matter of great importance in the economy of the Medusa that its marginal tentacles should be very sensitive to contact with minute particles, so that a very slight stimulus applied to them should start the co-ordinated movements of localization, it is not surprising that the tentacular rim should present nerve-endings so far sensitive that only by their excitation can the reflex mechanism be thrown into action. But if such is the explanation in this case, it is curious that in Tiaropsis indicans every part of the bell should be equally capable of yielding a stimulus to a precisely similar reflex action.

"In pursuance of this point, I tried the experiment of cutting off portions of the margin, and stimulating the bell above the portions of the margin which I had removed. I found that in this case the manubrium did not remain passive as it did when the whole margin of the bell was removed; but that it made ineffectual efforts to find the offending body, and in doing so always touched some part of the margin which was still unmutilated. I can only explain this fact by supposing that the stimulus supplied to the mutilated part is spread over the bell, and falsely referred by the manubrium to some part of the sensitive—i.e. unmutilated—margin.

"But to complete this account of the localizing movements, it is necessary to state one additional fact which, for the sake of clearness, I have hitherto omitted. If any one of the four radial tubes is irritated, the manubrium will correctly localize the seat of irritation, whether or not the margin of the bell has been previously removed. This greater case, so to speak, of localizing stimuli in the course of the radial tubes than anywhere else in the nectocalyx, except the margin, corresponds with what I found to be the case in Tiaropsis indicans and probably has a direct reference to the distribution of the principal nerve-tracts.

"On the whole, therefore, contrasting this case of localization with the closely parallel case presented by Tiaropsis indicans, I should say that the two chiefly differ in the fresh-water Medusa, even when unmutilated, not being able to localize so promptly or so certainly, and in the localization being only performed with reference to the margin and radial tubes, instead of with reference to the whole excitable surface of the animal.

"All marine Medusæ are very intolerant of fresh water, and, therefore, as the fresh-water species must presumably have had marine ancestors,[36] it seemed an interesting question to determine how far this species would prove tolerant of sea-water. For the sake of comparison, I shall first briefly describe the effects of fresh water upon the marine species.[37] If a naked-eyed Medusa which is swimming actively in sea-water is suddenly transferred to fresh water, it will instantaneously collapse, become motionless, and sink to the bottom of the containing vessel. There it will remain motionless until it dies; but if it be again transferred to sea-water it will recover, provided that its exposure to the fresh water has not been of too long duration. I have never known a naked-eyed Medusa survive an exposure of fifteen minutes; but they may survive an exposure of ten, and generally survive an exposure of five. But although they thus continue to live for an indefinite time, their vigour is conspicuously and permanently impaired; while in the fresh water irritability persists for a short time after spontaneity has ceased, and the tentacles and manubrium are strongly retracted.

"Turning now to the case of the fresh-water species, when first it is dropped into sea-water at 85° there is no change in its movements for about fifteen seconds, although the tentacles may be retracted. But then, or a few seconds later, there generally occurs a series of two or three tonic spasms, separated from one another by an interval of a few seconds. During the next half-minute the ordinary contractions become progressively weaker, until they fade away into mere twitching convulsions, which affect different parts of the bell irregularly. After about a minute from the time of the first immersion all movement ceases, the bell remaining passive in partial systole. There is now no vestige of irritability. If transferred to fresh water after five minutes' exposure, there immediately supervenes a strong and persistent tonic spasm, resembling rigor mortis, and the animal remains motionless for about twenty minutes. Slight twitching contractions then begin to display themselves, which, however, do not affect the whole bell, but occur partially. The tonic spasm continues progressively to increase in severity, and gives the outline of the margin a very irregular form; the twitching contractions become weaker and less frequent, till at last they altogether die away. Irritability, however, still continues for a time—a nip with the forceps being followed by a bout of rhythmical contractions. Death occurs in several hours in strong and irregular systole.