134 PRACTICAL ZOOLOGY 



maxillae or auxiliary jaws and maxillipeds or foot jaws holding 

 the food while it is being crushed by the true jaws or mandibles 



(Fig. 75, 3)- 



The food is not thoroughly ground up, however, until it has 

 passed through the oesophagus (Fig. 75, 20} into the stomach 

 (Fig. 75, 21). Here it encounters a number of tooth-like 

 structures which are moved by powerful muscles and form 

 the gastric mill. After being ground up in the gastric mill the 

 food is mixed with digestive juices poured into the stomach by 

 two digestive glands. 



Absorption and Circulation. The digested food is absorbed 

 by the intestinal walls and passes into the blood surrounding the 

 intestine; and the undigested food matter is cast out through the 

 anal opening (Fig. 75, 6). The blood into which the digested 

 food passes resembles that of insects (see p. 14), but besides 

 transporting food and waste products, it must also carry 

 oxygen and carbon dioxide, as does human blood. There is a 

 well-developed heart (Fig. 75, 29} which pumps blood into six 

 arteries leading to various parts of the body. As in insects, 

 the body cavity in which the vital organs lie is filled with blood 

 which passes out of the ends of the arteries. Circulation is com- 

 pleted by the entrance of the blood into the heart again. 



Respiration. The crayfish breathes very differently from 

 insects. It is a typical aquatic animal, and its respiratory sys- 

 tem consists of gills resembling those of a fish. These gills or 

 branchiae are attached to the bases of the legs and lie within 

 the branchial chambers. These chambers are formed by an ex- 

 tension of the exoskeleton on each side of the thorax, which pro- 

 tects the delicate filamentous gills from injury. A constant 

 stream of fresh water is forced through these chambers from be- 

 hind forward by the movements of the oarlike part of the second 

 maxillae. The gill filaments are supplied with circulating blood 

 which takes up some of the oxygen that is mixed with the water 

 and gives off carbon dioxide to the water. 



Reproduction. Often crayfishes are caught which have 



