142 THE CRAYFISH 



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 proctodceum 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 

 papillae. 



The anus is a longitudinal slit on the ventral 

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

 proximation of its sides. 



Remove a piece of the intestine, and examine its inner 

 surface under water with a low power of the microscope, to 

 see the ridges and papilla. 



d. The gastric mill is a complex structure made 



up of numerous sclerites, or hardened portions 

 of the cuticle lining the stomach, by means of 

 which the chewing of the food is effected : the 



