THE SKIN AND SKELETON. 



129 



E'iG. 94 — Section or Sfein from Horse's Nostril : E, epidermis ; D, dermic ; 1, horay 

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

 dnct of a sudoriparous, or sweat, gland ; 5, glomerule, 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 i)f 

 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 



