152 THE CRAYFISH. 



greater part of the liver of the left side, and as much of the 

 muscles and cephalic carapace as is necessary to expose the 

 alimentary canal. 



a. The stomodceum, or first portion of the alimentary 



canal, has a chitinous lining. 



i. The mouth is an oval aperture on the ventral 

 surface, between the jaws. 



ii. The oesophagus, is a short wide tube, running 

 vertically upwards from the mouth. 



iii. The stomach is a spacious dilatation of the 

 alimentary canal, occupying the greater part 

 of the head, and extending a short distance 

 into the thorax. It consists of a larger cardiac 

 chamber in front, and a smaller pyloric cham- 

 ber behind, separated from each other by a 

 constriction. Its chitinous lining is thick, and 

 some parts of it are especially hardened, form- 

 ing the gastric mill, a complex mechanism of 

 ' ossicles ' supporting three large and strongly 

 calcified teeth. These structures will be seen 

 at a later stage of the dissection. 



b. The mesenteron or mid-gut, which follows the 



stomach, is very short and devoid of chitinous 

 lining. On each side it receives a wide bile-duct 

 from the liver, and its dorsal wall is produced 

 into a small caecum. 



c. The proctodoeum or intestine is a long narrow 



straight tube, running from the hinder end of the 

 mesenteron to the anus. It has a chitinous lining 

 throughout, and its inner surface presents longi- 

 tudinal and slightly spiral ridges beset with 

 small papillfe. 



The anus is a longitudinal slit on the ventral 

 surface of the telson : it can be closed by ap- 

 proximation of its sides. 



Bemove a piece of the intestine, and examine its inner 



