222 MANUAL OF ZOOLOGY SECT. 



separation of the teeth is effected partly by the elasticity of 

 the mill, partly by delicate muscles in the walls of the 

 stomach. The pyloric division of the stomach forms a 

 strainer; its walls are thickened and produced into nu- 

 merous setae, which extend quite across the narrow lumen 

 and prevent the passage of any but finely divided particles 

 into the intestine. Thus the stomach has no digestive func- 

 tion, but is merely a masticating and straining apparatus. 

 On each side of the cardiac division is found at certain 

 seasons of the year a plano-convex mass of calcareous 

 matter, the gastrolith. 



The digestion of the food and to some extent the absorp- 

 tion of the digested products are performed by a pair of 

 large glands (Ir), lying one on each side of the stomach 

 and anterior end of the intestine. They are formed of 

 finger-like sacs or cceca, which discharge into wide ducts 

 opening into the small intestine, and are lined with glandu- 

 lar epithelium derived from the endoderm of the embryo. 

 The glands are often called livers, but as the yellow fluid 

 they secrete digests proteids as well as fat, the name hepato- 

 pancreas is often applied to them, or they may be called 

 simply digestive glands. The crayfish is carnivorous, its food 

 consisting largely of decaying animal matter. 



The digestive organs and other viscera are surrounded by 

 a body-cavity, which is in free communication with the blood- 

 vessels and itself contains blood. 



There are well -developed respiratory organs, in the form 

 of gills, contained in a narrow branchial chamber, bounded 

 internally by the proper wall of the thorax (Fig. 127, ef), 

 externally by the gill-cover or pleural region of the carapace 

 (kd). Each gill consists of a stem giving off numerous 

 branchial filaments, so that the whole organ is plume-like. 

 The filaments are hollow, and communicate with two paral- 



