THE SKIN AND SKELETON. 



129 



Fia. 94. Section of Skin from Horse's Nostril: E, epidermis; D, denni*; 1, horny 

 layer of epidermis; 2, rete mucosum ; 3, papillary layer of dermis ; 4, excretory 

 duct of a sudoriparous, or sweat, gland ; 5, grloinerule, or convoluted tube of the 

 same ; 6, hair follicle ; 7, sebaceous gland ; 8, internal sheath of the hair follicle ; 

 9, bulb of the hair ; 10, mass of adipose tissue. 



Frog, in shreds ; in Fishes and some Mollusks, in the form 

 of slime. However modified the epidermis, or whatever 

 its appendages, the like process of removal goes on. Mam- 

 mals shed their hair; Birds, their feathers; and Crabs, 

 their shells. When the loss is periodical, it is termed 

 moulting. 



2. The Skeletons. ( l > The Exoskeleton is developed 

 by the hardening of the skin, and, with very few excep- 

 tions, is the only kind of skeleton possessed by inverte- 

 brate animals. The usual forms are coral, shells, crusts, 

 scales, plates, hairs, and feathers. It is horny or calca- 

 reous; while the endoskeleton is generally a deposit of 

 earthy material within the body, and is nearly confined 

 to the Vertebrates. The exoskeleton may be of two kinds 

 dermal and epidermal. 



Some of the Protozoa, as Polycistina and Foramini- 

 fera, possess siliceous and calcareous shells of the most 

 beautiful patterns. The Toilet Sponge has a skeleton 



9 



