4)8 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, 

 a 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 ofi'ers 

 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. 



