CILIARY MOVEMENT. 441 



and in the stomach and its appendages in the Asterias, as well 

 as in many other situations. 



995. The presence of cilia, when they are moving with 

 rapidity, can frequently be inferred only from the eddies which 

 they produce in the neighbouring fluid. Sometimes the return- 

 stroke, which is made more slowly, can be seen when the direct- 

 stroke is too rapid to be followed. This is particularly the case 

 with the wheels of the Rotifera ( 537), which appear to revolve 

 continuously in one direction, from the observer being only able 

 to trace one set of the vibratory movements of the rings of cilia 

 which compose them. In general, however, the cilia may be 

 best seen when their motion slackens; and their shape, size, 

 arrangement, and manner of moving, may then be distinguished 

 with tolerable accuracy. Their figure is that of slender filaments, 

 sometimes a little flattened, tapering gradually from the base to 

 the point. Their size is extremely variable, the largest being 

 about 1 -500th of an inch long, and the smallest being stated at 

 1 -13000th. They are generally arranged in regular order, some- 

 times in straight rows, sometimes spirally or in circles ; and they 

 are usually set pretty close together, so as even occasionally to 

 resemble continuous fan-like membranes. When in motion, each 

 cilium appears to bend from its root to its point, returning again 

 to its original state, like the stalks of corn when depressed by the 

 wind ; and when a number are affected in succession with this 

 motion, the appearance of progressive waves following one 

 another is produced, as when a corn-field is agitated by frequent 

 gusts. Mr. Lister has described the action of the cilia, which 

 surround the respiratory apertures in the branchial sac of a com- 

 pound Ascidia ( 978), as resembling that of delicately-toothed 

 oval wheels revolving continually in the same direction ; the 

 movement ascending on one side, and descending on the other. 

 But the cilia are very much closer than the apparent teeth ; and 

 the illusion seems to be caused by the fanning motion which is 

 given to them in regular and quick succession. This will pro- 

 duce a series of short waves, of which every one corresponds to 

 an apparent tooth. The movement of the cilia is sometimes, 

 however, of a different kind. The point of each describes a circle, 



