242 THE DOG-FISH 



ends are enlarged to form a pair of tubular sacs, the 



urinary sinuses, which run back side by side, unite 



together posteriorly, and open into the cloaca at the 



apex of a small papilla in the posterior margin of the 



opening of the oviducts. 



Find the opening at the apex of the urinary papilla : pass 



a seeker through the opening and forwards along the urina/ry 



sinus of one side : slit up the ventral wall of the sinus along 



its whole length. 



5. The metanephric ducts or ureters are slender tubes, 

 usually four to six in number on each side, which 

 run from the enlarged metanephros to open into the 

 dorsal surface of the urinary sinus by four to six 

 slit-like apertures lying along the septum between 

 the two sinuses. The anterior ureter is much longer 

 than the others. 

 Find the ureters, lying on the ventral surface of the meta- 

 nephros, and trace them bach to the urinary sinus. Slit up 

 the sinus, unless this has already been done, and examine its 

 dorsal wall to see the openings of the ureters. 



6. The cloaca is a shallow depression between the pelvic 

 fins. In the female there are three openings into 

 it : the rectum in front, the genital aperture in the 

 middle, and the urinary sinus behind. 



B. The Male. 



1. The testes are a pair of soft elongated bodies lyin^long 



the dorsal wall of the abdominal cavity, and in the 

 adult united with each other at their hinder ends. 

 Like the ovary they are attached to the body-waU. by 

 delicate folds of peritoneum which are very readily 

 torn, so that in many specimens the testes appear to 

 lie free in the abdominal cavity. 



2. The vasa efferentia are a number of very slender ducts 



which arise on each side from the dorsal surface of 

 the testis, and run forwards in the fold of peritoneum 

 to the mesonephros, which they enter at its anterior 

 end. 



