EPITIIKLIUM 



79 



the trachea or long as in the epididymis. There is usually a well- 

 delined cuticula from which the cilia appear to spring. According 

 to Apathy, the cilia extend through the cuticula, giving to the latter a 

 striated appearance (Fig. 25). Just beneath the cuticula each cilium 

 shows a swelling — the basal granule. Lenhossek considers these 

 granules centrosomes. The intracellular extensions of the ciHa con- 



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Fig. 25. Fig. 26. 



Fig. 25. — Ciliated Epithelial Cell from Intestine of Mollusk (Engelmann), showing 

 a, cuticula, b, basal granules, and c, intracellular extensions of ciHa. 



Fig. 26. — Tangential Section through Three Cells of Skin of Aurodonta. (Kolacev.) 

 Iron-hsmatoxylin stain, showing relation of cilia to basal granules and cytoreticulum. 



verge toward the nucleus, and are continuous with the reticular or 

 fibrillar structure of the cell body (Fig. 26). The motion of cilia is 

 wave-like, the wave always passing in the same direction. Various 

 explanations of ciliary motion have been given. It has been sug- 

 gested that it is due to the contractile powers of the spongioplasm, 

 also that it is due to changes in surface tension of the film of pro- 

 toplasm which covers each cilium. 



