334 THE CRAYFISH less. 



thorax ; a; gullet ; em, dorsal muscles ; ///t, ventral muscles • g-, brain ; 

 A, heart ; M, large intestine ; /r, left digestive gland ; mt/, small intes- 

 tine ; o, right lateral ostium of heart ; oa, ophthalmic artery ; oaa, dorsal 

 abdominal artery ; a, gullet ; //. 1-5, pleopods ; //. 6, uropod ; />s, 

 pyloric division of stomach ; s. a, sternal artery ; / (near heart), testis ; 

 / (below anus) telson ; ttaa, ventral abdominal artery ; z>. d, vas defer- 

 ens ; vdoy male genital aperture. 

 (From Lang, after Huxley.) 



thus constitutes the innermost layer of those cavities. It is 

 proved by development that the small intestine, which has 

 no chitinous lining, is the only part of the enteric canal 

 developed from the enteron of the embryo ; the gullet and 

 stomach arise from the stomoda2um, the large intestine from 

 the proctodaeum. Thus a very small portion of the enteric 

 epithelium is endodermal (see Fig. 83, a). 



In the cardiac division of the stomach the chitinous 

 lining is thickened and calcified in certain parts, so as to 

 form a comple.x articulated framework, the gastric /«///, on 

 which are borne a median and two lateral teeth, strongly 

 calcified and projecting into the cavity of the stomach. 

 Two pairs, of strong muscles arise from the carapace, and 

 are inserted into the stomach : when they contract they 

 move the mill in such a way that the three teeth meet in 

 the middle line and complete 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 r its walls are thickened and pro- 

 duced into numerous setse, 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 strain- 

 ing apparatus. On each side of the cardiac division is 

 found, at certain seasons of the year, a plano-convex mass 

 of calcareous matter, ihe gastrolith or "crab's-eye." 



