Cuticle 



Cortex 



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 mth the term mammals, some 

 using the term animals in the same sense and others 

 the word beasts. 



437. External Characteristics. — Mammals are dis- 

 tinguished by the possession of hair, in connection 

 with wdiich they have developed sebaceous, or oil, 

 glands. They also possess sweat glands and mam- 

 mary 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. 

 234). 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, mammals possess four feet with five toes 

 on each foot. Both the feet and toes are modified in a 

 variety of ways. 



438. Hair. — Hairs are lifeless epidermal struc- 

 tures arising from a Hving bulb which incloses a 

 dermal pulp (Fig. 225). Since their development 



is initiated by an outgrowing of the dermis, hairs are not strictly homolo- 

 gous with the feathers and scales of birds and reptiles. The latter origi- 

 nate 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 (Fig. 332). 

 On some aquatic mammals the hair covering is reduced to a few bristle- 

 like hairs on the upper lip. 



439. Internal Structure. — The skulls of mammals (Fig. 317) 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 may have united with the 



424 



Medulla 



Cuticular 

 scale 



Piqment 

 qranules 



Fig. 315.— Dia- 

 gram of a longitudinal 

 section of a typical 

 hair, which shows the 

 main parts. {From 

 Hausman, American 

 Naturalist.) 



