90 



CHORDATE ANATOMY 



superficial covering of non-living, mechanically protective substance; 

 (2) production of special secretions such as mucus; (3) reception of external 

 stimuH; (4) provision for motile activity. Two or more of these functions 

 may be carried on by one "simple" epithelium or by a stratified epi- 



FiG. 82. — Developing scales of dogfish, Squalus; sections perpendicular to surface 

 of skin; much enlarged, c, upper layers of epidermis; d, dentine of scale, deposited by 

 dermal cells beneath it; ee, enamel-forming organ of scale — a specialized region of the 

 germinative layer (w) of the epidermis; p, "pulp", the dentine-forming organ. (From 

 Kingsley.) 



thelium. Within the epithelium, then, cells will exhibit differentiation 

 of as many types as there are functions. 



(i) Most epithelia produce a protective covering at the free surface. 

 A cuticula is a dense, tough or hard nitrogenous material deposited 



on the exposed surface of an epi- 

 thelium. The cells which produce 

 it and underlie it remain alive. 



Keratin is a nitrogenous organic 

 substance which is formed within 

 some epithelial cells. It is the 

 basis of the homy structures of the 

 vertebrate skin. 



Tha "horny layer" (stratum 

 comeum; Fig. 81) developed on the 

 skin of vertebrates other than fishes, 

 consists of one or more of the outer 

 strata of the epidermis, the cells more or less filled with keratin and 

 strongly adherent to one another so that the whole layer acquires a 

 high degree of mechanical resistance. Completely keratinized cells are 

 dead. Hair, feathers, reptilian scales, claws, nails and hoofs are horny 

 structures. 



Calcareous material may be deposited by an epithelium, either 

 at its outer surface {e.g., shell of a mollusk) or, exceptionally, at its inner 

 surface {e.g., enamel of teeth; Figs. 82, 129). 



Fig. 83. — Sensory cells. A, cell from 

 the sense organ (crista acustica) of an 

 ampulla of the ear; B, rod cell from the 

 retina; C, cell from the olfactory epithelium. 

 (From Kingsley, After Furbringer.) 



