INTRODUCTION xiii 



or, if this is absent, the gill-opening in the true or Bony Fishes, 

 marks the boundary between the head and the trunk. 



In addition to the skin, most fishes are provided with another 

 covering in the form of scales. In the Selachians these scales 

 take the form of small, tooth-like bony structures known as 

 dermal denticles (p. 2), but in the Bony Fishes the scales are 

 usually thin, flexible bony plates, regularly arranged, and 

 overlapping one another like the tiles on a roof. 



The skin of the whales has lost all trace of sebaceous and 

 sweat-glands. Hair, so typical a feature of ordinary mammals, 

 is reduced to a few scattered bristles on the surface of the head 

 and snout, and is more frequently seen in young than in adult 

 specimens. There are never any scales on the body, and the 

 surface of the skin is exceedingly smooth, with a bright glossy 

 sheen when fresh and dry. 



Immediately below the skin in a fish are the muscles, but in 

 the cetacean there is a fibrous layer loaded with oil, known 

 as blubber, in between. This acts mainly as an insulator, to 

 prevent loss of heat, for it must be remembered that, unlike 

 the generality of fishes, the whale is a warm-blooded animal 

 and has to maintain a constant temperature within its body. 



The lateral line is a characteristic feature of most fishes, but 

 nothing of this nature is found in any cetacean. This forms a 

 kind of tunnel beneath the skin, which communicates at 

 regular intervals with the surface by a series of small openings, 

 which in scaly Bony Fishes are situated in a lengthwise row 

 of special scales running along each side of the body. The 

 lateral tunnels are continued on the head, where they form a 

 branching system running in the bones of the skull. On the 

 inner walls of the tunnels, which are filled with a jelly-like 

 mucus, are special organs of sense, which alternate with the 

 openings to the exterior. The function of the lateral line 

 system is as yet imperfectly understood, but it is probably 

 connected with the perception of movements in the water, 

 perhaps enabling the fish to avoid obstacles, and providing it 

 with a hint as to the presence of prey or natural enemies. It 

 may be noted that the internal ear of a fish, which serves as an 

 organ of balance as well as of hearing, is nothing more than a 

 greatly enlarged and specially modified lateral line organ. 



The fins, which provide so characteristic a feature of any 



