246 THE DOG-FISH 



course, unite to form a wide thin-walled tube, the 

 ureter, which runs back along the inner side and 

 dorsal surface of the vesicula seminalis to open into 

 the urino-genital sinus ; just before reaching the 

 sinus the ureter is joined by the fifth or most 

 posterior duct, from the hinder end of the kidney. 

 The number and arrangement of the ureters are sub- 

 ject to individual variation. 

 Ths metanephric ducts are most readily followed by in- 

 jecting them with a coloured fluid from the ureter, which is 

 a large wide tube, readily found along the inner side of the 

 mesonephric duct. 



8. The cloaca is the shallow depression between the pelvic 



fins into which, in the male, the rectum opens in front 

 and the uriao-genital sinus behind. 



9. The pronepliric or Miillerian ducts are rudimentary in 



the male, their anterior ends alone being present. 

 A small pocket-like sac in the suspensory ligament of 

 the Uver, between it and the pericardium, represents 

 the abdominal opening of the oviducts of the female. 

 Prom this sac the ducts can be traced a short dis- 

 tance round the sides of the oesophagus. 



10. The claspers (see p. 198). 



VIII. DISSECTION OF THE NEEVOUS SYSTEM.' 



For this purpose a dog-fish should be taken which has been 

 hardened in spirit, the roof of the skull having been previously 

 removed so as to allow the spirit free access to the brain. 

 The specimen must be moistened from time to time to prevent 

 the nerves drying up, or else should be dissected in water or 

 weak spirit. 



The nervous system consists of, (1) a central portion, the 

 brain and spinal cord, lodged within the skull and vertebral 

 column respectively ; and (2) a peripheral portion, the nerves 

 themselves, which run from the brain and spinal cord to all 

 parts of the body. 



