236 THE DOG-FISH 



developed, compact, laterally compressed, and lobu- 

 lated mass of a brownish colour, extending back as 

 far as the level of the cloaca. 



Dissect off the peritoneum from the ventral surface of one 

 of the kidneys, so as to expose it along its whole length. At 

 the hinder end dissect away the metanephros from the body- 

 wall, so as to see its full size. 



4. The mesonephric or Wolffian ducts are a pair of narrow 



tubes lying on the ventral surface of the kidneys along 

 their whole length. Their posterior ends are enlarged 

 and unite to form a tubular sac, the urinary sinus, 

 about half an inch in length : this opens into the 

 cloaca at the apex of a small papilla placed in the 

 posterior margin of the opening of the oviducts. 



The mesonephric ducts are most readily followed from 

 behind forwards. Slit one up along its ventral wall, and follow 

 it back to its opening into the cloaca. 



5. The metanephric ducts or ureters are slender tubes, 



usually four to six in number on each side, which 

 run back from the enlarged metanephros^o open into 

 the dorsal surface of the urinary sinus. The anterior 

 one is much longer than the others. 



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

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

 the sinus 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 lying along 

 the dorsal wall of the body-cavity, and in some cases 

 connected with each other in the median plane. 

 Like the ovary they are attached to the body-wall by 



