104 MANUAL OF ANIMAL BIOLOGY 



distal end is definitely broken off. Then a process of regenera- 

 tion begins which soon restores the appendage to the original 

 condition. Autotomy is under the control of the nervous system 

 and is brought about by a series of muscular contractions. In 

 addition to regeneration as a result of autotomy, the Crayfish, 

 particularly when young, possesses regenerative power sufficient to 

 form entire appendages when the original ones are removed. The 

 regenerated appendage, however, does not always have exactly 

 the same structure as the one which was lost. 



C. Organ Systems 

 1. Nutritive System 



The alimentary canal of the Crayfish begins with the mouth 

 which opens between the mandibles on the ventral surface of the 

 third segment. Leading dorsally from the mouth is a short, undif- 

 ferentiated tube, the esophagus, which opens into the stomach 

 almost directly above the mouth. The stomach is a highly spe- 

 cialized organ and is divided into two parts : an anterior portion 

 (cardiac chamber), into the ventral surface of which the esoph- 

 agus opens, and a somewhat smaller posterior portion (pyloric 

 chamber). In the lining of the stomach are a number of peculiar 

 chitinous ossicles which together constitute a complicated food- 

 grinding apparatus, the gastric mill. The muscle layers in the 

 wall of the stomach are able to contract in such a way as to cause 

 the ossicles to work against each other and thus to grind, or masti- 

 cate, the food. Between the cardiac and pyloric portions is a 

 strainer consisting of numerous bristle-like structures. The food 

 must be finely ground in order to get through the strainer in pass- 

 ing from the cardiac into the pyloric portion of the stomach. 



Leading from the posterior end of the pyloric chamber is the tube- 

 like intestine, which continues almost directly posteriorly through 

 the center of the abdomen. It opens ventrally through the anus 

 in the last segment of the body. Large digestive glands, which 

 correspond to those present in higher forms, are located in the 

 cephalothorax, and these produce a secretion which contains cer- 

 tain digestive enzymes. The secretion passes into the pyloric 

 chamber of the stomach through the hepatic ducts. The food of 

 the Crayfish consists both of living and dead animal matter. The 

 process of digestion does not differ fundamentally from that 



