126 COMPARATIVE ZOOLOGY. 
Endoskeleton. The former is but a modification of the 
skin. 
1. The Skin.—The skin is a very complex tissue, since 
it serves not only for a covering, but also for an organ of 
excretion, absorption, and touch. In the lowest forms, as 
Ameebee and Infusoria, it is an extremely delicate film, or 
membrane, but little more consistent than the body which 
it envelopes. But throughout the animal kingdom, from 
the slimy coat of the Polyp to the thick hide of the Rhi- 
noceros, the skin shows a similar structure—an inner and 
an outer layer; the former called dermis ; the latter, ep7- 
dermis.” 
Except in the low and immature forms, as Worms and 
Caterpillars, the skin of Articulates is hardened into a 
crust. The loose skin, called the mandle, which envelopes 
the body of the Mollusk corresponds to the true skin of 
higher animals. The border of the mantle is surround- 
ed 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, instead of lime, a 
substance resembling vegetable cellulose. 
In Mammals, whose skin is most fully developed, the 
dermis is a sheet of tough elastic tissue, consisting of in- 
terlacing fibres, and containing blood-vessels, lymphatics, 
sweat-glands, and nerves. It is the part converted into 
leather when hides are tanned, and attains the extreme 
thickness of three inches in the Rhinoceros. The upper 
surface is covered with a vast number of minute projec- 
tions, called papille, each the termination of a nerve; 
these are the essential agents in the sense of touch.” 
They are best seen on the tongue of an Ox or Cat, and 
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 
