128 COMPARATIVE ZOOLOGY. 



which envelopes the body of the Mollusk corresponds to 

 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 

 dennis 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 papillae,, each containing 

 the termination of a nerve; these are the essential agents 

 in the sense of touch. 78 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 



