Chap. 8 



AN AGENT OF EVOLUTION THE BODY COVERING 



127 



General Structure of Skin 



Skin consists of one or more layers of cells which cover the outside of the 

 body and make a sheath over the delicate tissues beneath. Thus the outer 

 layer of protoplasm that covers unicellular protozoans is not related to skin 

 except in function. In all multicellular animals the outermost covering is a 

 layer of epithelial cells, the epidermis. This is the only layer present in the 

 invertebrates, except the starfishes and their near kin (Fig. 8.1). In the 

 vertebrates there is also an underlying connective tissue layer, the dermis, 

 sometimes called leather skin, because when properly prepared it is leather 

 (Fig. 8.2). 



Epidermis. The epidermis is composed of several layers of epithelial cells. 

 The inner ones next to the dermis form a growing zone (malpighian layer) 

 where new cells are constantly being formed and pushed outward by the 

 pressure for space. As this occurs they are gradually flattened and outspread 

 (Fig. 8.2). In fishes and other moist-skinned animals even the outermost 

 cells stay alive for considerable time, but in land animals they become dry 

 and lifeless. Amphibians and reptiles molt the old epidermis in one piece; 

 birds lose their old feathers; and mammals continually shed little fragments 

 of skin. The constant flecking off of the human scalp in dandruff must be 

 familiar to everybody, in advertisements if not otherwise. Epidermal cells 

 become horny by deposits of the protein called keratin (horn). Keratin is 

 prominent in land dwelling vertebrates, in hair and feathers, horns of cattle, 

 footpads of dogs, and hoofs of horses. The "horny hands of toil" are actual 

 facts. 



Many glands originate in the epidermis although they usually enlarge and 





Fig. 8.1. A section of the epidermis and cuticle of an earthworm highly magni- 

 fied. It shows four mucous cells in different stages of secretion, all swollen with 

 the mucus which has pushed the nuclei to the bottom of the cells. It finally pours 

 out through microscopic pores, one at the end of each cell, and spreads over the 

 cuticle (cm). Mucus keeps the surface of the body moist, makes skin respiration 

 possible, lubricates the skin and lines the burrow in which the worm lives. (Cour- 

 tesy, Dahlgren and Kepner: Principles of Histology. New York, The Macmillan 

 Co., 1908.) 



