144 HIGH SCHOOL ZOOLOGY. 



skeletal and other systems, which, however, it will be easier to 

 understand after we have glanced at the structure of one of the 

 Eutheria, and compared it with that of the foregoing classes. 



3. Any familiar mammal will serve our purpose equally well, 

 but the Cat (Felis domestica) is perhaps as accessible an example 

 of the group as we can study. It belongs to the order of the 

 Carnivores or Beasts of Prey (Carnivora), and is accordingly 

 furnished with claws and teeth which are adapted to its mode 

 of life, and is, indeed, one of the most highly specialised of the 

 order in this respect, so that we must not expect to find it a 

 primitive example of Mammalian structure. 



4. With few exceptions, the mammals are clad with a coat 

 of hair, which like the plumage of the birds enal>les them to 

 preserve their high bodily temperature. The exceptions to 

 this rule are certain aquatic forms where a subcutaneous ac- 

 cumlation of fat, the blubber, furnishes the necessary non-con- 

 ducting envelope, and certain terrestrial forms, where the 

 epidermis is either extraordinarily thick as in the Pachyderms, 

 or has given rise to horny scales or other protective coverings. 

 Even in these cases, however, scattered hairs are present, and 

 in the aquatic forms referred to, bristles occur about the lips of 

 the young. So the hairy covering is as characteristic for the 

 Mammalia as the hoi-ny scales are for the reptiles. Although, 

 like scales and feathers, hairs are epideiTual structures, nourished 

 by a papilla of the corium, yet there is a fundamental difference 

 between them in their development. Both scales and feathei's 

 begin by a thickening of the epidermis which projects beyond 

 the level of the skin, and, in the case of the feather, is only 

 afterwards retracted into the follicle, but the hair begins by a 

 thickening of the epidermis which grows inwards into the cutis, 

 and only afterwards comes to project beyond the level of the 

 skin. (Fig. 100.) 



In the Cat two kinds of haira are present, those which con- 



