THE PROCESS OF FEEDING. 49 
We have learned that the food of the crayfish is made 
up of very diverse substances, both animal and vegetable ; 
but, so far as they are competent to nourish the animal 
permanently, these matters all agree in containing a 
peculiar nitrogenous body, termed protein, under one ofits 
many forms, such as albumen, fibrin, and the like. With 
this may be associated fatty matters, starchy and sac- 
charine bodies, and various earthy salts. And these, 
which are the essential constituents of the food, may be, 
and usually are, largely mixed up with other substances, 
such as wood, in the case of vegetable food, or skeletal 
and fibrous parts, in the case of animal prey, which are 
of little or no utility to the crayfish. 
The first step in the process of feeding, therefore, is 
to reduce the food to such a state, that the separation 
of its nutritive parts, or those which can be turned to 
account, from its innutritious, or useless, constituents, 
may be facilitated. And this preliminary operation is 
the subdivision of the food into morsels of a convenient 
size for introduction into that part of the machinery in 
which the extraction of the useful products is performed. 
The food may be seized by the pincers, or by the 
anterior chelate ambulatory limbs; and, in the former 
case, it is usually, if not always, transferred to the first, 
or second, or both of the anterior pairs of ambulatory 
limbs. These grasp the food, and, tearing it into 
pieces of the proper dimensions, thrust them between 
the external maxillipedes, which are, at the same time, 
E 
