220 THE DOG-FISH. 



c. The anal fin is on the ventral surface opposite the 

 interval between the two dorsal fins. 



2. Paired fins, representing the limbs of higher verte- 

 brates. 



a. The pectoral fins are the largest of all the fins, and 



project horizontally outwards from the sides of 

 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 behind. 



In the male a part 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 of the 

 body, close to the median line, and ends blindly 

 about an inch in front of the pelvic girdle. 



C. External Apertures. 



1. Median apertures. 



a. The mouth is a large transverse crescentic opening 



on the under surface of the head, a short dis- 

 tance 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 fsecal 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 



