INVEUTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 657 



Medusa should be called Lhnnocodium Sowerbii. Professor Luiikestcr 

 now writes * that he shall " henceforth speak of the Medusa as 

 Limnocodium Soiverbii, Allman and Lankester." 



Physiology of the Fresh-water Medusa.t — Mr. G. J. Eomanes 

 has worked out the physiology of the new form, and gives an interest- 

 ing account of the results so far obtained. 



The natural movements of the Medusa precisely resemble those of 

 its marine congeners. More particularly, these movements resemble 

 those of the marine sj)ecies which do not swim continuously, bat 

 indulge in frequent pauses. In water at the temperature of that in 

 the Victoria Lily-house the pauses are frequent, and the rate of the 

 rhythm irregular, suddenly quickening and slowing even during the 

 same bout, which has the effect of. giving an almost intelligent appear- 

 ance to the movements. This is especially the case with young 

 specimens. In colder water (65'' to '< 6°) the movements are more 

 regular and sustained, so that, guided by the analogy furnished by 

 experiments on the marine forms, he infers that the temperature of 

 the natural habitat of this Medusa cannot be so high as 8j°. In 

 water at that temj^erature 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, as in the 

 case of the marine species ; and in water at 65° the maximum rate 

 observed was eighty pulsations per minute. As the temperature at 

 which the greatest activity is displayed by the fresh-water species is 

 fatal to all the marine species which he has observed, the effects of 

 cooling are 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 Medusfc, it is 

 only 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 Mcduste, 

 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, tlio 

 animal is seen to have shrunk into a tiny ball, and it never again 

 recovers either its life or its shape. 



The animals seek the sunlight, congregating at the unshaded end 

 of the tank. Moreover, during the daytime they swim about at the 

 surface ; but when the sun goes down they subside, and can no longer 

 be seen, in all these habits resembling many of the sea-water species. 

 They are themselves non-luminous. 



On excising the margin of the nectocalyx, the result corresponded 

 precisely with that which is obtained in the case of marine species, 

 the operation producing immediate, total, and i»crnianent paralysis of 

 the nectocalyx, while the severed margin continues to pulsate for two 

 or three days. 



A point of S2)ecially physiological interest is that in its uniinitilatcd 



♦ 'Nature,' x.\ii. (1880; p. I'Jl. t I-oc. i-it.. \: lltl. 



VOL. 111. - X 



