18 STUDIES IN ANIMAL LIFE. 



eters or upward, a very obvious agitation will be 

 perceived on the edge of the fold, and this appear- 

 ance is caused by the moving cilia with which the 

 surface of the membrane is covered. Being set 

 close together, and moving simultaneously or in 

 quick succession, the cilia, when in brisk action, 

 give rise to the appearance of a bright transparent 

 fringe along the fold of the membrane, agitated by 

 such a rapid and incessant motion that the single 

 threads which compose it can not be perceived. 

 The motion here meant is that of the cilia them- 

 selves; but they also set in motion the adjoining 

 fluid, driving it along the ciliated surface, as is in- 

 dicated by the agitation of any little particles that 

 may accidentally float in it. The fact of the con- 

 veyance of fluids and other matters along the cili- 

 ated surface, as well as the direction in which they 

 are impelled, may also be made manifest by im- 

 mersing the membrane in fluid, and dropping on it 

 some finely -pulverized substance (such as charcoal 

 in fine powder), which will be slowly but steadily 

 carried along in a constant and determinate direc- 

 tion."* 



It is an interesting fact, that while the direction 

 in which the cilia propel fluids and particles is gen- 

 erally toward the interior of the organism, it is 

 sometimes reversed, and, instead of beating the par- 



* Quoin's Anatomy. By SHAEPEY and ELLIS. Sixth edition. 

 I., p. Ixxiii. See also SHARPEY'S article Cilia, in the Cyclopaedia 

 of Anatomy and Physiology. 



