SECT, x PHYLUM ARTHROPOD A 221 



into the stomach : when they contract they move the mill in 

 such a way that the three teeth meet in the middle and com- 

 plete the comminution of the food begun by the jaws. The 

 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 

 numerous 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 

 function, 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 

 absorption of the digested products are performed by a pair 

 of large glands (lr.\ lying one on each side of the stomach 

 and anterior end of the intestine. They are formed of 

 finger-like sacs or CCP.CCI, which discharge into wide ducts 

 opening into the small intestine, and are lined with glandular 

 epithelium. 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 ; it does not represent a 

 true ccelome. 



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. 118, <?/.), 



