SWIMMING ANIMALS. 49 



thus suggest that they are degraded types allied to 

 the original stock from which the Vertebrates them- 

 selves are descended. This is very important as regards 

 the derivation of Vertebrates from aquatic animals ; 

 an origin which we should naturally expect, seeing that 

 fishes breathe by means of gills, and are, therefore, 

 presumed to have had aquatic ancestors. 



With the Fishes, which, with the exception of the 

 Whales, are perhaps, of all animals, the most beauti- 

 fully adapted for rapid motion through the water, we 

 enter the great group of Vertebrates. The contour of 

 an ordinary fish, such as the Perch (Fig. 16), is 

 modelled on those lines best suited for cleaving the 

 water, through which the fish is propelled mainly by 

 the muscular tail with its terminal fin. The fins on 

 the body act chiefly as balances, although aiding to a 

 certain extent in propulsion. These body-fins in all 

 fishes are of two types, namely, paired and median. 

 The number of paired fins is two, the front pair corres- 

 ponding with the fore limbs, and the hinder pair with 

 the hind limbs of quadrupeds. In the Perch (Fig. 16), 

 the front or pectoral pair of fins are seen immediately 

 behind the head; the second or pelvic pair being 

 placed below and slightly behind the pectoral ones. 

 In many other fishes (as in Fig. 2, p. 5) the hinder pair 

 of fins occupy, however, a position corresponding with 

 that of the hind limbs of quadrupeds. The pectoral 

 fins, although assisting to a certain extent in the 

 motion .of the fish through the water, act rather in 

 directing its course than as propellers. Their chief 



