THE CRAYFISH AND OTHER CRUSTACEA 87 



exchanges between the blood within and the surrounding 

 medium. 



While the crayfish often consumes vegetable matter as 

 food it is generally carnivorous in its habits, living upon 

 worms and various other living creatures that it may 

 catch, and often devouring dead and partly decayed flesh. 

 The food after being passed to the mouth parts is chewed 

 mainly by the mandibles and is swallowed through a short 

 tube, the esophagus, whence it passes into the stomach. 

 This is a rather large organ lined with chitin which is 

 thickened and hardened in certain places called the ossicles 



FIG. 74. Stomach or "gastric mill" of the crayfish cut through the 

 middle, c, cardiac regions of stomach; d.l, duct from the liver; g, gastro- 

 lith, or calcareous disk secreted by the walls of the stomach; *', intestine; 

 l.t, lateral teeth of grinding apparatus; m.t., median tooth; oe, esophagus; 

 py, pyloric region; v, valve between cardiac and pyloric regions of stomach. 

 (After Hatschek and Cori.) 



which act as a sort of grinding apparatus. The posterior 

 part of the stomach receives the ducts from two large 

 digestive glands, commonly called the liver. These pour 

 into the stomach a digestive fluid which acts upon the 

 ground-up masses of food, making them capable of absorp- 

 tion into the blood. At the posterior part of the stomach 

 there project into the cavity a number of hairs which act 

 as a strainer, allowing only the finely divided food to pass 

 backward into the intestine. The latter is a straight tube 

 extending backward into the abdomen to open at the under 

 side of the base of the telson. 



