EEPEODUCTIVE AND DIGESTIVE SYSTEMS 143 



ii. The vas deferens of each side is a long thick 

 convoluted tube, running from the junction of 

 the anterior and posterior lobes of the testis to 

 the external aperture on the coxopodite of the 

 last thoracic leg. Before the breeding season 

 it will be found filled with an opaque white 

 seminal mass. 



Tease a small piece of the mass in water or glycerine on 

 a slide ; cover, and examine it with low and highjpowers. 



iii. The spermatozoa are flattened spheroidal cells 

 from which project tangentially a number of 

 stiff immobile curved processes. 



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 

 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 outgi'owth of it. The stomatodoeum or 

 anterior portion of the canal and the proctodoeum 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- 

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

 external covering of the body. 



