CHAPTER LIX 



CLASS MAMMALIA 



The last and highest class of vertebrates is Mammalia. The mammals 

 present without any question the dominant forms of animal life on the 

 earth today, being supreme over land and sea. Not even man, however, 

 has yet successfully questioned the dominance of birds in the air. Few 

 people are familiar with the term mammals, some using the term animals 

 in the same sense and others the word beasts. 



435. External Characteristics. — Mammals are distinguished by the 

 possession of hair, in connection with which they have developed seba- 

 ceous, or oil, glands. They also possess sweat glands and mammary 

 glands and in some cases scent glands. Lips and cheeks are found in 

 all except the whales, and there is a fleshy and cartilaginous lobe about 

 the external opening of the ear known as a -pinna (Fig. 221). The eyes 

 are protected by lids, the upper of which is the movable one, in contrast 

 with the birds, in which the lower lid is movable. The facial portion of 

 the skull usually projects to form a muzzle, or snout. Typically, mam- 

 mals possess four feet with five toes on each foot. Both the feet and 

 toes are modified in a variety of ways. 



436. Hair.— Hairs are lifeless epidermal structures arising from 

 a Hving bulb which incloses a dermal pulp (Fig. 212). Since their 

 development is initiated by an outgrowing of the dermis, hairs are not 

 strictly homologous with the feathers and scales of birds and reptiles. 

 The latter originate in a thickening of the epidermis. Sometimes hairs 

 are replaced by overlapping dermal scales, while in the case of the 

 armadillo the body is invested by an armor or carapace of bony dermal 

 plates. On some aquatic mammals the hair covering is reduced to a 

 few bristle-like hairs on the upper lip. 



437. Internal Structure.— The skulls of mammals (Fig. 277) are 

 compact and have fewer bones than those of the reptiles, although the 

 various parts are not fused so completely as in the birds. Of the bones 

 which in reptiles made up the lower jaw, part have united to form a 

 single bone which articulates directly with the skull, and others have 

 passed into the service of the ear (page 410). The lower jaws of the two 

 sides are sometimes fused to form one mandible. Cartilaginous discs 

 separate the bodies of the successive vertebrae. The ribs articulate both 

 with the vertebrae and with the sternum and thus the expansion and 

 contraction of the thoracic cavity are made possible. 



406 



