THE SKIN AND SKELETON. 
129 
Pig. 94.—Section of Skin from Horse’s Nostril: E, epidermis; D, dermis; 1, horny 
layer of epidermis2, rete mucosum; 3, papillary layer of dermis; 4, excretory 
duct 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 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 
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