. Comparative anatomy. Anatomy, Comparative. 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 th
. Comparative anatomy. Anatomy, Comparative. 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
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