138 COMPARATIVE ANATOMY 



contact. In the adult state, then, they are epithelioid, but in origin and 

 essential nature they are truly epithehal. 



Most receptors of external stimuli — that is, the exteroceptors of the 

 body — are more or less specialized sensory epithelia. 



(4) Cilia are extremely delicate motile filaments borne by the free end 

 of an epithelial cell (Fig. 93). A single cell may carry from one (called 

 flagellum if especially long) to over a hundred cilia. 



The vibratile beating of a cilium is caused by a motor mechanism which 

 may be either at the base of the cilium or within the filament itself. As in 



Fig. 100. — Retina of a mammal. The upper part of the figure is a simpHfied repre- 

 sentation of a section through the retina; much enlarged. Below are shown the relations 

 of individual elements of the several layers. The cavity of the eye-ball is toward the 

 left, c, cone; cc, cone cell; g, nerve cells; ig, inner "granular" layer, the granular 

 appearance being due to numerous small nerve cells; im, inner "molecular" layer 

 consisting of the processes of the nerve cells of adjoining layers; in, basal membrane; 

 nf, nerve fibers of optic nerve; og, outer "granular" layer, the "granules" being the 

 deeper nucleated portions of the rod cells and cone cells; om, outer "molecular" layer; 

 r, rod; re, rod cell. (From Kingsley.) 



rowing a boat, the stroke of a cilium is effective in one direction only. The 

 beat of all cilia in a region is not simultaneous, but progressive waves of 

 ciliary motion pass along the epithelial surface. 



Beating of cilia may cause motion of the body on which the cilia are 

 carried, or of an external fluid medium within which the cilia beat, or it 

 may transport small solid bodies along the cihated surface. The gastrula 

 of Amphioxus is propelled through the sea water by beating of ectodermal 

 ciHa. The ciliated nephrostomes of the mesonephros (page 98) cause a 

 flow of coelomic fluid toward the mesonephric duct. Certain regions of 

 kidney tubules are ciliated. The cilia on the lining of the trachea and 

 bronchi propel inhaled foreign particles upward and outward. Oviducts, 

 efferent sperm ducts and many other hollow organs are lined by ciliated 

 epithelium. 



