POISONS. 245 



rate that I have observed is eighty y)ulsatlons 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 tem- 

 peratures in the one case are compared with those 

 of a series of lower temperatures in the other. 

 Sirailarl}^, while a temperature of 70° is fatal to 

 all the species of marine Medusos 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 an}' degree of cold with- 

 out loss of life, such is not the case with the fresh- 

 water species. Marine Medusip, after having been 

 frozen solid, Avill, when gradually thawed out, again 

 resume their swimming movements; but this fresh- 

 water Medusa is completely destro^^ed 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 Medusae cono-reoate at 

 the end which remains unshaded. Moreover, durino- 

 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-w^ater species. They 

 are themselves non-luminous. 



*'I have tried on about a dozen specimens the 



effect of excising ihe margin of the nectocalyx. In 

 17 



