DIGESTIVE SYSTEM. 151 



from which project tangentially a number of 

 stiff immobile curved processes. 



b. In the female. 



i. The ovary is a saccular organ situated below and 

 at the sides of the pericardial sinus, and 

 extending some distance behind it. Like the 

 testis it consists of two anterior lobes and a 

 posterior one. 



ii. The oviduct of each side is a short wide almost 

 straight tube, running from about the middle 

 of the length of the ovary, with which it is 

 continuous, to the external opening on the 

 coxopodite of appendage XI., the last thoracic 

 leg but two. 



C. The Digestive System. 



The alimentary canal of the crayfish is a tube running in a 

 nearly straight line from the mouth to the anus. Of this tube 

 the middle portion or mesenteron, which is very short, is alone 

 formed from the primitive alimentary tract of the embryo, 

 and the ' liver ' is an outgrowth of it. The stomodceum or 

 anterior portion of the canal and the proctodceum or hind 

 portion, which together form almost the whole length of the 

 canal, are both formed by invagination of the external surface 

 of the body ; and both have a chitinous lining which is con- 

 tinous, at the mouth and anus respectively, with the chitinous 

 external covering of the body. 



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 abdominal terga, the genital duct, and the 



