XIII PHYLUM CHORDATA 501 



characterised by the possession of a fusiform, fish-lilce body, 

 tapering backwards to the tail, which is provided with a hori- 

 zontally 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 Whale-bone 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 pinna3 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 movements being restricted, by the 

 nature of the articulations, to antero-posterior movements of 

 flexion and extension. The metacarpal and metatarsal regions 

 are relatively very long. In the Artiodactyla the third and 

 fourth digits of each foot form a symmetrical pair. In the 

 Ruminants vestiges of the second and fifth digits are also commonly 

 present ; but these are usually not functional, never reaching the 

 ground, though in the Reindeer they are better developed than in 

 the others, and have the effect of preventing the foot from so 

 readilysinking in the snow. In the Camelsthethird and fourth digits 

 alone are present. The Giraffes are distinguished from the other 

 Ruminants by the enormous length of the neck. Characteristic of 

 the Ruminants, though absent in the Gamels and some others, are 

 the cephalic appendages known as horns and antlers. The horns 

 of the Hollow-horned Ruminants (Oxen, Sheep, Goats, Antelopes), 

 sometimes developed in both sexes, sometimes only in the males, 

 are horny sheaths supported by bony cores which are outgrowths 

 of the frontal bones. In the Giraffe the horns, which are short 

 and occur in both sexes, are bony structures covered with soft 

 skin, and not at first attached by bony union to the skull, though 



