POISONS. 245 
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 tem- 
peratures 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 Medusze 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 with- 
out loss of life, such is not the ease with the fresh- 
water species. Marine Meduse, 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 Medusze 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. 
“T have tried on about a dozen specimens the 
effect of excising the margin of the nectocalyx. In 
