144 THE CRAYFISH 



1. The digestive glands. 



a. The ' liver ' is a large bilobed body, of a brown or 

 greenish colour, lying in the thorax at the sides 

 of the alimentary canal. Each lobe is much 

 branched and hollow, and its secretion enters the 

 mesenteron by a wide bile-duct, which will be 

 seen later on in the dissection. 



2. The alimentary canal. 



Bemove the genital duct and the 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 stomatodoBum, 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 proctodceum or intestine is a long narrow 



straight tube, running from the hinder end of the 



