136 



COMPARATIVE ANATOMY 



the basal surface and in coordination with the building of the dentine of 

 the scale or tooth by the adjacent dermal or mesenchymal layer (Fig. 96). 

 (2) A glandular epithelium is one in which certain of the cells are 

 specialized for secretion. Single glandular cells may be scattered more or 

 less abundantly throughout an epithelium, either simple or stratified. In 



Fig. 96. — 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 (m) of the epidermis; -p, "pulp", the dentine-forming organ. (From 

 Kingsley.) 



stratified epithelium such cells are at or near the surface (Fig. 94, u) and 

 open upon it, usually a prolonged and narrow extension of the cell serving 

 as a ductule. The mucus which coats the surface of an earthworm or fish 



is produced by such unicellular 

 epidermal glands. Many internal 

 epithelia are mucous. 



(3) A sensory (or neuro-) epi- 

 thelium is one in which certain cells 

 are specialized for reception of 

 stimulation by some agency in the 

 cell's environment. Such a cell is 

 usually elongated and slender and 

 characterized by one or more ex- 

 ceedingly delicate "hairs" or 

 "bristles" at its distal end (Fig. 

 97). The hair, which is evidently 

 an important part of the receptor mechanism, extends through the 

 cuticula (if present) or is otherwise disposed so as to be readily accessible 

 to the appropriate stimulating agency. 



Sensory cells may occur in either simple or stratified epithelia. They 

 may be scattered singly throughout an epithelium as in the epidermis of 

 various invertebrates. In the olfactory epithelium of the nasal cavity of 

 mammals the non-nervous cells are of very elongated form and interspersed 

 among them are the even more attenuated olfactory sensory cells (Fig. 



Fig. 97. — 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 Fiirbringer.) 



