340 THE SEA SHORE 



Mammals are divided into several classes, and one of these 

 Cetacea) includes the fish-like Whales, Porpoises, and Dolphins, all 

 of which are peculiarly adapted to a purely aquatic life. Like most 

 of the fishes, their upper surfaces are of a dark colour, and the lower 

 very light. Their fore limbs are constructed on the same plan as 

 those of the higher mammals, the bones of the arm being attached, 

 to a large shoulder-blade, and the hand formed of four or five well- 

 developed fingers which are enclosed in skin, so that they consti- 

 tute a paddle or flipper well adapted for propulsion through water. 

 There is no collar-bone, however, and the fingers have no nails or 

 claws. There are no hind limbs visible externally, but a rudimentary 

 pelvic girdle forms a part of the internal skeleton. A dorsal fin 

 exists, but this is merely an extension of the skin of the back, and 

 is not supported by either bones or rays. The skin itself has no 

 scales, like that of most fishes, but is smooth and naked; and 

 below it lies a large amount of fat, which, being a very bad 

 conductor of heat, serves to prevent the escape of heat from the 

 body. 



The tails of cetaceans are also mere folds of the skin, supported 

 in the centre by the extremity of the vertebral column ; but unlike 

 the tail fins of fishes, they are expanded horizontally instead of in 

 the vertical plane. This latter is an important adaptive feature of 

 the cetaceans, since the vertical movement of a tail so disposed is 

 exactly what is required to assist the animals as they alternately 

 rise to the surface for air and again descend into the sea in search 

 of their food. 



Among the other external characters of the cetacean we may 

 note the nostrils, which are always situated on the highest point of 

 the head, and are thus the first part exposed when the creature rises 

 to renew its supply of air ; also the ears, which are two small aper- 

 tures behind the eye, without any form of external appendages. 



The skeleton of the cetacean is formed of light spongy bones, 

 saturated with oily matter; and although the animal has no true 

 neck, visible as such externally, it is interesting to note that, in 

 common with all other mammals, even with the long-necked giraffe, 

 it possesses its seven cervical or neck vertebrae. 



Porpoises and Dolphins together form the family Delphinidcc, 

 characterised by having the blow-hole in the form of a crescent with 

 its convexity turned towards the front, and of these the Porpoises 

 constitute the genus Phoccena. 



The Common Porpoise (P. communis) is the species that is so 



