308 



Basic Structure of Vertebrates 



Fig. 248. 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 special- 

 ized region of the germinative layer (m) of the epidermis; (p) "pulp," the dentine- 

 forming organ. (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," 

 Philadelphia, The Rlakiston Company.) 



of it are constantly being sloughed off. The material thus lost is re- 

 placed by perpetual growth in the deep layer of epidermis. 



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. 35, 248). 



2. A glandular epithelium is one in which secreting cells, either 

 singly or organized into multicellular glands, are distributed more or 

 less abundantly throughout the layer (Fig. 246). 



3. In a sensory (or neuro-) epithelium certain cells are special- 

 ized for reception of stimulation by some agency in the cell's environ- 

 ment (Fig. 249). They are usually elongated slender cells possessing one 

 or more delicate "hairs" or "bristles" at the end exposed to the stimu- 

 lating agency. The opposite end is related to a nerve-fiber. Epithelial 

 sensory cells may be grouped in clusters to form sense-organs (Fig. 250). 

 An epithelium may be rendered sensory by free nerve-termination, 

 that is, the terminal twigs of a nerve-fiber ramifying among the epi- 

 thelial cells (Fig. 251). These nerve-fibers, however, are not produced 

 by the epithelium itself but invade it from adjacent tissue. 



4. Cilia are extremely delicate motile filaments borne by the free 

 ends of epithelial cells. A single cell may carry from one to over a 

 hundred. A ciliated epithelium is one in which some or all of the 

 cells carry cilia (Figs. 244i?, 245). 



The stroke of a cilium is effective in one direction only — as in row- 

 ing a boat. Beating of cilia may produce motion of the body which 

 carries them; e.g., the gastrula of Amphioxus is propelled through the 

 sea water by the cilia on the ectoderm. Or cilia may cause movement 

 of fluid or of solid bodies along the ciliated surface, as when cilia on 



