EEPEODUCTIVE AND DIGESTIVE SYSTEMS 141 



b. In the female. 



i. The ovary is situated below, and at the sides of 

 the pericardial sinus, and extends some dis- 

 tance 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 to the external 

 openiag on the coxopodite of appendage XI., 

 the last thoracic leg but two. 



C. The Digestive System. 



The alimentary canal of a 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 stomatodoeum 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 hning which is con- 

 tinuous, 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 hoUow, 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 genital duct and the greater pa^-t of the liver 

 of the left side, and as much of the muscles as is necessary to 

 expose the alimentary canal. 



a. The stomatodoeum, or first portion of the alimentary 

 canal, has a chitinous lining. 



