336 COMPARATIVE ZOOLOGY 



In worms and arthropods, the cellular layer, here 

 called hypodermis, excretes a structureless cuticle, which 

 may become very thick, as in the tail of the horseshoe 

 crab, or may be hardened by deposition of lime salts, 

 as in many Crustacea. The loose skin, called the 

 mantle, which envelops the -body of the mollusk, corre- 

 sponds to the true skin of higher animals. The border 

 of the mantle is surrounded with a delicate fringe, and, 

 moreover, contains minute glands, which secrete the 

 shell and the coloring matter by which it is adorned. 

 The tunicates have a leathery epidermis, remarkable 

 for containing vegetable cellulose instead of lime. 



In mammals, whose skin is most fully developed, the 

 dermis is a sheet of tough elastic tissue, consisting of 

 interlacing fibers, and containing blood, vessels, lym- 

 phatics, sweat glands, and nerves. It is the part con- 

 verted into leather when hides are tanned, and attains 

 the extreme thickness of three inches in the rhinoceros. 

 The upper surface in parts of the body is covered with 

 a vast number of minute projections, called papilla, 

 each containing the termination of a, nerve; these are 

 the essential agents in the sense of touch 126 (Fig. 345). 

 They are best seen on the tongue of an ox or cat, arid 

 on the human fingers, where they are arranged in rows. 



Covering this sensitive layer, and accurately molded 

 to all its furrows and ridges, lies the bloodless and 

 nerveless epidermis. It is that part of the skin which 

 is raised in a blister. It is thickest where there is most 

 pressure or hard usage ; on the back of the camel it 

 attains unusual thickness. The lower portion of the 

 epidermis (called rete mucosnm) is comparatively soft, 

 and consists of nucleated cells containing pigment gran- 

 ules, on which the color of the animal depends. Toward 

 the surface the cells become flattened, and finally, on 

 the outside, are changed to horny scales (Fig. 199, c). 



