84 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



the urinary fluid, while the sac is a sort of urinary 

 bladder. 



Restricting our attention to the phenomena which have 

 now been described, and to a short period in the life of 

 the crayfish, the body of the animal may be regarded 

 as a factory, provided with various pieces of machinery, 

 by means of which certain nitrogenous and other matters 

 are extracted from the animal and vegetable substances 

 which serve for food, are oxidated, and are then delivered 

 out of the factory in the shape of carbonic acid gas, 

 guanin, and probably some other products, with which 

 we are at present unacquainted. And there is no doubt, 

 that if the total amount of products given out could be 

 accurately weighed against the total amount of materials 

 taken in, the weight of the two would be found to be 

 identical. To put the matter in its most general shape, 

 the body of the crayfish is a sort of focus to which certain 

 material particles converge, in which they move for a 

 time, and from which they are afterwards expelled in new 

 combinations. The parallel between a whirlpool in a 

 stream and a living being, which has often been drawn, is 

 as just as it is striking. The whirlpool is permanent, 

 but the particles of water which constitute it are in- 

 cessantly changing. Those which enter it, on the one 

 side, are whirled around and temporarilj" constitute a part 

 of its individuality ; and as they leave it on the other 

 side, theii' places are made good by new comers. 



