442 ZOOLOGY 



Compare the anterior with the posterior appendages in a selected series of 

 mammals, keeping in mind the following points: size, length, strength, uses; the 

 number and character of the digits. Compare similarly the corresponding (i.e., 

 anterior with anterior) appendages in another series. Keep in mind, throughout, 

 the adaptations of structure to the conditions of life, method of locomotion, etc. 



451. Integtxment. — The skin, as in forms already described, 

 consists of two portions, — an ectodermal portion, the epidermis, 

 and the dermis or true skin which is derived from the mesoderm 

 (Fig. 233). Hair is found in the young of all mammals, though 

 it may be wanting in the adult (as in whales), or may occur only 

 sparsely. Hair is produced by the epidermis, but is nourished 

 by a papilla of dermal tissue (Fig. 233, /). Each hair consists 

 of a central part, or pith, surrounded by a denser cornified por- 

 tion, the cortex. Hair differs much in color and in structure, — 

 from the soft fur of the seal to the quills of the hedgehog and 

 porcupine. Hair is considered, as morphologically similar to the 

 feathers of birds and the scales of reptiles. 



To be considered in the same connection with hair are the 

 nails, claws, and hoofs, the scales on the tail of the rat or beaver 

 and on the body of scaly ant-eaters, and the horny material of 

 horns of cattle and of rhinoceros. The whalebone of certain 

 whales is also an epidermal product in the roof of the mouth. 



452. Supplementary Studies for Field and Library. — What is the economic 

 value of the skins of mammals? How are they prepared for the uses to which they 

 are put? What animals are prized for their hairy products (fur, wool, etc.)? 

 What special qualities must the hair have to be useful in making cloth? 



What instances can you adduce of advantageous coloring in the hair of mam- 

 mals? What variations of color may be found within a single species? What 

 changes of color are possible to a single individual? How are these changes 

 brought about? What peculiar qualities have the quills of the porcupine? 



453. Integumentary Glands, derived from the epidermis, 

 are common in mammals. Associated with the hairs are the 

 oil glands. Over various parts of the body are long tubular 

 sweat glands buried in the dermis. There are also integumen- 

 tary glands producing characteristic odorous substances. These 

 may be for recognition within the species, or for protection from 

 enemies. The tear glands of the eye are modified cutaneous 

 glands. 



The mammary glands, which are characteristic of the group, 

 are specially developed skin glands, apparently more allied to 



