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 p^'of em, under one of its 

 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 cra3'fish. 



The first step in the process of feeding, therefore, is 

 to reduce the food to such a state, that the sepai'ation 

 of its nutritive parts, or those which can be turned to 

 account, from its innutritions, 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 maj^ 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, 



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