486 ZOOLOGY SECT. 



are placed on the ground in walking. The hind-limb is five-toed. 

 The surface is covered with thick skin with sparse hairs. 



The Cetacea (Fig. 1128), among which are the largest of existing 

 Mammals, some reaching a length of 80 or 90 feet, are characterised 

 by the possession of a fusiform, fish-like body, tapering backwards 



:_ 



Fio. 1128. Killer (Orca gladiator). (After True.) 



to the tail, which is provided with a horizontally expanded caudal 

 fin divided into two lobes or " flukes," and a relatively large head, 

 not separated from the body by any distinct neck. A dorsal median 

 fin is usually present. The fore-limbs take the form of flippers, 

 with the digits covered over by a common integument, and devoid 

 of claws ; the hind-limbs are absent. The mouth is very wide ; the 

 nostrils are situated on the summit of the head, and the auditory 

 pinna is absent. Hairs are completely absent, or are represented 

 only by a few bristles about the mouth. In the Whalebone Whales 

 the nostrils have two external slit-like apertures ; in the toothed 

 Whales, Porpoises, and Dolphins, on the other hand, the two nostrils 

 unite to open by a single crescentic valvular aperture. 



In the Sirenia also the body is fish-like, with a horizontal caudal 

 fin, the fore-limbs flipper-like, the hind-limbs absent, and the 

 integument almost hairless. But the body is distinctly depressed, 

 and the head is by no means so large in proportion as in the Cetacea 

 and has a tumid truncated muzzle, not far back from the extremity 

 of which the nostrils are situated. There is no dorsal fin. The eyes 

 are small, the pinnae of the ears absent. The digits are in some cases 

 provided with claws. 



In the Ungulata vera the claws or nails are replaced by thick, 

 solid masses, the hoofs, investing the ungual phalanges and bearing 

 the weight of the body. The number of digits is more or less reduced, 

 and the limbs as a whole are usually specially modified to act as 

 organs of swift locomotion over the surface of the ground, their 



