196 THE DOG-FISH 



the ventral surface of the body, just behind the 

 head. 



b, The pelvic fins are smaller than the pectoral, and 

 are placed on the ventral surface of the body, a 

 little way in front of the middle of its length. 

 Their inner borders touch each other, and in 

 the male are fused together posteriorly. 



In the male the inner border of each pelvic 

 fin is separated from the rest of the fin, and 

 specially modified to form a copulating organ or 

 clasper : this is a stout backwardly directed rod, 

 deeply grooved along its inner and dorsal surface. 

 The groove leads forwards into a sac which lies 

 just beneath the skin of the ventral surface, close 

 to the median line, and ends blindly about an 

 inch in front of the pelvic girdle. 



C. External Apertures. 



r \ 



1. Median apertures. 



a. The mouth is a large crescentic opening placed trans- 



versely on the under surface of the head, a short 

 distance from its anterior end. 



b. The cloacal aperture is on the ventral surface of 



the body between the pelvic fins, i.e. in front of 

 the middle of the length of the body. It gives 

 exit to faecal matter from the intestine, and to 

 the renal and genital products. 



2. Paired apertures. 



a. The nostrils are a pair of large circular apertures 



on the under surface of the head in front of the 

 mouth, with which they are connected by wide 

 grooves covered by flaps of skin. 



b. The spiracles are a pair of small apertures on the 



sides of the head, just behind the slit-like open- 

 ings of the eyes. They are really a pair of gill- 

 clefts. 



Pass a seeker down one of the spiracles into the mouth. 



