xxii. 6 SLOTHS 599 



tween the relative rate of growth of the faee in these three genera, and 

 the differing final forms result mainly, though not wholly, from the 

 differences in absolute size (Figs. 377 and 378). In all ant-eaters the 

 face becomes relatively longer as the animal increases in size, and the 

 enormous snout of the great ant-eater is produced by a relative growth- 

 rate only slightly higher than that found in Tamandua and Cyclopes. 

 This is an excellent example of the way in which the proportions of an 

 organ will vary in animals of different sizes if its growth is allometric, 



Fig. 379. Two-toed sloth, Choloepus. (From a photograph in Scott, 



Land Mammals, copyright 1913, 1937 by the American Philosophical 



Society and used with the permission of the Macmillan Company.) 



that is to say, relatively faster or slower than that of the body as a 

 whole (p. 737). 



The hard palate is prolonged backwards in Myrmecophaga by union 

 of the pterygoids, a condition found also in some armadillos (Dasypus). 



The great ant-eater is a fine animal, over 6 ft long, with a long hairy 

 coat, including a very bushy tail and with a black stripe edged with 

 brown at the shoulder. It has a long thin tongue for collecting ants, 

 and enormous submaxillary salivary glands. The claws of the front 

 legs are very large and used for defence as well as for digging. Taman- 

 dua and Cyclopes differ from Myrmecophaga in other features besides 

 the length of snout. They are arboreal and the tail is prehensile. 



The sloths (Bradypodidae) (Fig. 379) are fully adapted for arboreal 

 life and can hardly walk on the ground. They show, as do the bats, 

 how the mammalian skeleton can be used with surprisingly little 

 change to support weight by hanging, the limbs being used as tension 

 members rather than as pillars. In marked contrast to the ant-eaters 

 the face is short and the head rounded, with large frontal air sinuses. 

 The neck is peculiar for the presence of nine or ten cervical vertebrae 

 in the three-toed sloth, Bradypus. This might be supposed to provide 

 a flexible neck for an animal that must often face backward, were it not 

 that in the two-toed sloth Choloepus there are but six cervical vertebrae. 



