64 



SCIENCE. 



PHYSIOLOGY OF THE FRESHWATER MEDUSA. 



The structure of this remarkable animal has already been 

 investigated and described by Professors Allman and Lan- 

 kester, with the result of showing that, although constitut- 

 ing a new genus, it is in all respects a true Medusa. After 

 the publication of their papers I began to work out the 

 physiology of the new form, and the following are the 

 results which so far I have obtained. 



The natural movements of the Medusa precisely resem- 

 ble 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 F.) the pauses are frequent, and the rate of the rhytnm 

 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 at that tem- 

 perature the rate of the rhythm is enormously high, some- 

 times rising to three pulsations per second. But by pro- 

 gressively 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 freshwater 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 high 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 murine Medusae which I have examined, it is 

 only a temperature of 100 degrees that is fatal to the fresh- 

 water species. Lastly, while the marine species will endure 

 any degree of cold wiihout loss of life, such is not the case 

 with the freshwater species. Marine Medusas, after having 

 been frozen solid, will, when gradually thawed out, again 

 resume their swimming movements ; but this freshwater 

 Medusa is completely destroyed by freezing. Upon beinu. 

 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 Medusae congregate at the end which re- 

 mains 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 tnus 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 sty, the operation 

 produces immediate, total, and permanent paralysis of the 

 nectoc ilyx, while the severed margin continues to pulsate 

 for two or three davs. 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 .Medusae. Mjre particu arlv, this excitability 

 resembles that of those marine species which sometimes 

 respond to a single stimulation witn two or three successive 

 contractions. 



A point of speciilly physiological interest may be here 

 noticed. In its unmutilated state the freshwater 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 ibe 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 pre- 

 vious!)* describrd as occurring in Tiaropsit indicant {Phil. 

 Trans., vol. clxvii.) Bui further than this, I find a curious 

 difference. For while in '/'. indiums 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 con- 

 nections with the manubrium ; in the freshwater 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 neces- 

 sary 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 move- 

 ments 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 this 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 localiza- 

 tion, 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 cut- 

 ting 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., un- 

 mutilated — margin. 



But to complete this account of the localising 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 cor- 

 rectly localise the seat of irritation, whether or not the margin 

 of the bell has been previously removed. This greater case, 

 so to speak, of localising stimuli in the course of the radial 

 tubes rather than anywhere else in the umbrella except the 

 margin, corresponds with what I found to be the case in 

 /'. indicans, and probably has a direct reference to the dis- 

 tribution of the principal nerve-tracis. 



On the whole, therefore, contrasting this case of localisa- 

 tion with the closely parallel case presented by T. indicans, 

 I should say that the two chiefly differ in the freshwater 

 Medusa, even when unmutilated, not being able to localise 

 so promptly or so certainly : and in the localisation being 

 only performed with reference to the margin and radial 

 tubes, instead of with reference to the whole excitable sur- 

 face of the animal. 



All marine Medusae are very intolerant of fresh water, and 

 therefore as the fresh waicr species most presumably have 

 had marine ancestors, 1 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 de- 

 scribe the effects of fresh water upon the marine species. 5 

 If a naked eyed Medusa which is swimming actively in sea 

 wall 1 is suddenly transferred to fresh water, it will instant- 

 aneously collapse, become motionless, and sink to the 



1 Looking lo the enormous number of marine species of Medusa;, it is 

 mm li more proba' le ill it the freshwater species were derived from them, 

 than thai they were derived from freshwater ancestry. 



• Kor lull account, sec Phil, Trans., vol. clxvii., pp. 744-745- 



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