CLASSIFICATION. 57 



may be either double or single, can be completely closed 

 at will, and are only opened, at such times as the creatures 

 come to the surface to breathe, when a column of water is 

 generally thrown up by the rush of expired air let loose 



Fi&.20. The Bridled Dolphin. (From True, Bull. U.S. Nat. Museum.} 



shortly before the head reaches the surface. In all their 

 internal structures, as well as in the mode of production 

 and nourishing of their young, cetaceans conform strictly 

 to the ordinary mammalian type ; and we accordingly see 

 that their assumption of a fish -like form is, with the 

 exception of the loss of the hind limbs and the modifi- 

 cation of the front pair into flippers, mainly superficial. 

 In departing from the fish-type in having the expansion 

 of the tail- fin horizontal instead of vertical, the necessity 

 of having an organ capable of bringing them rapidly 

 to the surface has been the inducing cause ; while in 

 order to prevent their blood from being reduced below 

 the proper temperature by the chill of the surrounding 

 water, the whole body is invested with a thick layer 

 of oily fat, commonly known as the blubber, beneath the 

 skin. 



Existing cetaceans are divisible into two great groups, 

 distinguished as the whalebone whales and the toothed 

 whales ; the latter group including sperm-whales, together 



