48 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
aspect, it is Physiology ; so far as Physiology consists in 
the elucidation of complex vital phenomena by deduction 
from the established truths of Physics and Chemistry, or 
from the elementary properties of living matter. 
We have seen that the crayfish is a voracious and 
indiscriminate feeder ; and we shall be safe in assuming 
that, if duly supplied with nourishment, a full-grown 
crayfish will consume several times its own weight of 
food in the course of the year. Nevertheless, the increase 
of the animal’s weight at the end of that time is, at most, 
& small fraction of its total weight; whence it is quite 
clear, that a very large proportion of the food taken into 
the body must, in some shape or other, leave it again. 
In the course of the same period, the crayfish absorbs a 
very considerable quantity of oxygen, supplied by the 
atmosphere to the water which it inhabits ; while it gives 
out, into that water, a large amount of carbonic acid, and 
a larger or smaller quantity of nitrogenous and other ex- 
crementitious matters. From this point of view, the 
crayfish may be regarded as a kind of chemical manu- 
factory, supplied with certain alimentary raw materials, 
which it works up, transforms, and gives out in other 
shapes. And the first physiological problem which offers 
itself to us is the mode of operation of the apparatus 
contained in this factory, and the extent to which the 
products of its activity are to be accounted for by 
reasoning from known physical and chemical principles, 
