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 temporarily constitute a part 
of its individuality; and as they leave it on the other 
side, their places are made good hy new comers. 
