VERTEBRATES — CLARK 295 



dugongs, and the cetaceans have the hair covering greatly reduced, 

 in the last two vestigial or absent in the adults. In many rodents and 

 in the echidnas the hairs may be transformed into spines. In the 

 pangolins they are grouped into broad scalelike structures. Some 

 mammals have bony plates in the skin which in a few, as the arma- 

 dillos, may form a complete bony dorsal covering. The skin is 

 more or less moist because of the occurrence of sweat glands. 



In some shrews a pair of salivary glands is transformed into a 

 pair of poison glands from which ducts lead to the lower incisors, 

 recalling the poison apparatus of the venomous snakes. 



Most mammals have two pairs of functional limbs terminating in 

 separate digits. The limbs may be subequal, or either pair may be 

 enlarged or reduced. In the seals the limbs are modified into paddles 

 without separate external digits, and in the cetaceans and manatees 

 the forelimbs are paddle-shaped, the hind limbs absent. 



The bats have the forelimbs developed as wings and all of them 

 are capable of sustained and long-continued flight. Most of the bats 

 are rather small and insectivorous, but some, including the largest, 

 eat fruit, and a very few feed on other bats, birds, fish, or the blood 

 of mammals and birds. A number of mammals in widely different 

 groups, as the flying squirrels, flying phalangers, and Galeopithecus, 

 have the skin of the body greatly extended laterally forming a para- 

 chute by means of which they are able to make long glides through 

 the air, like the gliding lizards and gliding snakes. 



In most mammals the body temperature is constant and high, 

 commonly well above that of the surrounding air, at least at night. 

 But the temperature of the monotremes fluctuates in response to that 

 of the surroundings. Some mammals, as some rodents, some bats, 

 and some northern bears, hibernate with reduced body temperature, 

 or at least become torpid, in very cold weather. Some mammals 

 estivate in dry hot weather. 



A few bats migrate south in autumn, and a few large mammals such 

 as the bison withdraw southward in winter, at least in some areas. 

 In winter many mammals wander widely in search of food. 



In the mammals all the senses — sight, hearing, smell, and touch — 

 are highly developed, though very unequally in the different groups. 



Mammals are not dependent on bright sunlight, which most of 

 them avoid. Some live in caves, holes, or burrows, but feed in the 

 open. A few, largely blind, are subterranean. 



All mammals except the egg-laying platypus and echidnas are 

 viviparous. They are divided into the placentals, in which a pla- 

 centa is present and the young at birth are at least fairly well de- 

 veloped, and the marsupials, in which there is usually no placenta 

 and the young are born in an extremely undeveloped condition; in 



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