160 THE CRAYFISH. 



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. 

 Remove the abdominal terga, 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 stomodoeum, 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,' or rather gizzard, is a spacious 

 dilatation 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, 

 forming the gastric mill, a complex mecha- 

 nism 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 

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

 from the liver, and its dorsal wall is produced 

 into a small caecum. 



c. The protodceum or intestine is a long narrow 



straight tube running from the hinder end of 

 the mesenteron to the anus. It has a chitinous 



