MAMMALIA 319 



for warning, or for protection by mimicking the odors of a more 

 formidable foe; as the odor of the musk deer may suggest that of 

 a crocodile. 



It is readily seen that the nervous system is the highest and 

 most complicated of any chordate's, thus giving mammals 

 highest rank in the scale of intelligence. 



Order I. Monotrem'ata. — The animals of this order are 

 primitive mammals, but that they are mammals is proved by 

 the fact that they are covered with hair and nourish their young 

 with milk. The heart has an incomplete auriculoventricular 

 valve. The temperature is lower and more variable than in 

 the higher mammals. ' The brain has no corpus callosum. Like 

 birds and reptiles, they are oviparous, and the intestines open 

 into a cloaca. These animals are characterized by a temporary 



9 



Fig. 261. — A spiny ant-eater. (From Claus.) 



" mammary pouch," in which they are hatched or to which they 

 are transferred after hatching, and into which open the ducts of 

 the mammary gland. 



The spiny ant-eater {Echid'na aculea'ta) (Fig. 261) is a small nocturnal 

 animal about the size of a duck-bill. It is covered with spines mingled 

 with hairs. \Yhen danger threatens it curls up like a hedge-hog. Its 

 legs are short and stout, and its feet are armed with strong claws for tearing 

 open ant-hills. Its tail is vestigial. It has a long, pointed, toothless snout 

 and a long, extensible tongue for licking up ants, other insects, and worms. 

 The salivary glands are very highly developed, and when the tongue, cover- 

 ered with sticky saliva, is thrust into an ant-hill, it is soon covered with the 

 insects. The tongue is then drawn back into the mouth and the adhering 

 insects swallowed. It seems that the mother places the egg in the mammary 

 pouch with her mouth. When the young is hatched it is nourished with 

 milk. When it attains sufficient growth she remo\'es it from the pouch, re- 

 placing it from time to time for nourishment. She shows further intelli- 

 gence by digging a burrow and concealing her young in it when she goes out 



1 Beddard, p. 112. 



