128 
COMPARATIVE ZOOLOGY. 
which envelopes the body of the Mollusk corresponds tc 
the true skin of higher animals. The border of the man¬ 
tle 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, in¬ 
stead 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 in parts of the body is covered with a vast num¬ 
ber of minute projections, called papilla, each containing 
the termination of a nerve; these are the essential agents 
in the sense of touch. 72 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 moulded 
to all its furrows and ridges, lies the bloodless and nerve¬ 
less 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 un¬ 
usual thickness. The lower portion of the epidermis 
(called rete mucosum) is comparatively soft, and consists 
of nucleated cells containing pigment-granules, on which 
the color of the animal depends. Towards the surface 
the cells become flattened, and finally, on the outside, are 
changed to horny scales (Fig. 2, c). 
These scales, in the higher animals, are constantly wear¬ 
ing off in the form of scurf, and as constantly being 
renewed from below. In Lizards and Serpents, the old 
epidermis is cast entire, being stripped off from the head 
to the tail; in the Toad, it comes off in two pieces; in the 
