10 VERTEBRATE ZOOLOGY 



The function of the spiral valve is to effect an increase in the 

 absorptive surface of the intestine. 



Exercise 10. Draw a diagram of the spiral valve. 



Slit open the entire stomach and note the folds in its mucous 

 membrane. 



The Urogenital System. The urinary and the genital organs 

 are closely associated with each other and will be studied together. 

 Entirely remove the digestive system from the body. In an im- 

 mature animal the genital organs cannot be studied. 



The genital organs consist of the genital glands, which are the 

 paired testes or ovaries, and the paired excurrent canals, which in 

 the female are the very large oviducts. The testes or ovaries, as 

 we have already seen, are soft, elongated bodies at the forward 

 end of the abdominal cavity and attached to the dorsal body wall 

 by mesenteries. In the smooth dogfish the two genital glands are 

 grown together. 



The urinary organs are composed of the paired kidneys and the 

 paired excurrent canals. The kidneys are narrow, elongated organs 

 which lie, one on each side of the median line, close against the 

 dorsal body wall, covered ventrally by the peritoneum; they 

 extend almost the entire length of the abdominal cavity. In the 

 male the anterior half of each kidney loses its excretory function 

 and enters into a close relation with the testis. 



The excurrent canals of the kidneys are called the Wolffian 

 ducts. They differ considerably in the two sexes, in the male 

 acting also as the excurrent ducts of the testes. The male Wolffian 

 duct on each side begins at the forward end of the kidney, appear- 

 ing as a very much convoluted tube on the ventral side of it, and 

 passes to the hinder part of the abdominal cavity. Here it joins 

 the other Wolffian duct, and the median urogenital sinus thus 

 formed opens into the cloaca at the end of the urogenital papilla, 

 just behind the opening of the rectum. Note the lateral arm of 

 the urogenital sinus which extends forward from it on each side. 



The anterior and posterior portions of this duct are very dis- 

 tinct from each other. The anterior convoluted portion, which 

 is called the epididymis, is exclusively a sperm duct and does 



