xvi GIANT FISHES, WHALES AND DOLPHINS 



or sickle-shaped, but is never supported by any internal 

 skeleton (Fig. 2). There is no structure corresponding to the 

 anal fin of a fish. 



The fore-limbs of a cetacean are modified to form flippers, 

 which are superficially like the pectoral fins of a fish, but 

 within their solid flatness, instead of the irregularly arranged 

 cartilages or bones of the pectoral fins, may be found the bones 

 that constitute the limb skeleton of a typical land mammal ; 

 upper arm, fore arm, wrist and finger bones are all present 

 (Fig. 3). No external trace of hinder limbs remains, but within 

 the body in the neighbourhood of the vent two small bones, 

 entirely free from the backbone, represent the two halves of the 

 pelvis, and, occasionally associated with them, are small 

 nodules of bone — all that remains of the leg bones. 



In the vast majority of fishes the propulsion of the body is 

 brought about mainly by means of lateral flexions of the 

 muscular tail, aided by movements of the caudal fin. That is 

 to say, the tail is lashed vigorously from side to side in order 

 to drive the fish forward. The cetaceans swim in much the 

 same way, but the movements of the tail are in a different 

 plane. The horizontal position of the flukes is undoubtedly 

 associated with the necessity for the whale to rise to the 

 surface of the water periodically to renew the supplies of air 

 to the lungs. The dorsal and anal fins of a fish, and probably 

 also the back fin of the whale, normally serve as keels, and 

 their purpose is to give stability to the body, but some fishes 

 with fairly long and flexible fins make use of wave-like move- 

 ments of these fins to propel the body at slow or moderate 

 speed. The pelvic fins of a fish assist the dorsal and anal in 

 keeping the body balanced, and appear to function after the 

 manner of bilge keels. The pectoral fins, like the flippers of 

 a cetacean, are probably used mainly for steering and balancing, 

 but also act as brakes to check forward movement. 



The head of a fish or a cetacean, although presenting great 

 diversity in size and appearance, possesses all the usual organs 

 that might be expected in a Verbetrate animal — snout or 

 muzzle, nostrils, eyes, mouth and so on. In a fish the nostrils 

 are scarcely ever used for breathing purposes, and serve merely 

 as organs of smell. In a cetacean the nostrils form a single 

 or double opening called the blowhole, and are situated, almost 



