290 Characteristics of Rotifers. 



cilia of the pharynx. The aperture of the ruouth, therefore, 



lies between the upper and lower ciliary bands." A simple 



set of wheel-cilia, always acting in the same way, would 



throw their whirlpool currents in one direction, but the 



addition of other cilia may readily enable the direction of the 



currents formed by the wheel-cilia to be changed, so as to suit 



the varying wants of the creature. The precise modes in which 



the wheel-cilia and the subordinate cilia are arranged vary 



considerably, but in every case in which I have been able to 



examine the ciliary apparatus under favourable circumstances, 



and with powers of from 600 or 700 to 1000 or 1500, I have 



found the quantity of cilia much larger than lower powers 



indicated, and the functions of the cilia, or of some of them, 



more varied than is commonly described. In examining a 



tube-dwelling species last year — the OEcistes, which Mr. Davis 



brought before the Royal Microscopical Society, with Beck's 



one-twentieth object-glass and powers of from about 900 to 



1500 — the wheel-cilia appeared as dense tufts, and many cilia 



were noticed in a position to assist in directing the chief 



currents, and which were not concerned in producing them. 



This creature repeatedly bent its head on one side, and, if I 



may use such a phrase, ' e licked " the matter adhering to a 



thread of conferva, with long cilia, apparently belonging to the 



wreath or to one of its rows. This licking action sometimes 



went on simultaneously with the rotatory movement. 



Placing a single Conocliilus . under a compressorium, and 

 viewing it with a power of 1000, au inner row of cilia was 

 seen to be engaged in conveying particles down the buccal canal. 

 In the common rotifer, when the creature is extended and 

 its wheel organs retracted, it is common to see a few cilia 

 moving about its mouth. It can alter the form of its anterior 

 extremity to a considerable extent, and appears to have full 

 control of the cilia it occasionally throws out. 



With respect to the cause of ciliary movement, nothing 

 positive is known, but some cilia appears to be always in 

 motion, while others belong to a voluntary or quasi-voluntary 

 system. The wheel appearance results from the rythmical 

 motion of various cilia moving in succession. When the move- 

 ments are not too fast, and a sufficiently high power is dis- 

 played, it appears that each cilium is thrown into progressing 

 and retreating curves, just as a long thin wand of whalebone 

 may be thrown into wave motions by the wrist. When a breeze 

 moves over a field of corn the aspect of advancing or retreat- 

 ing waves is produced by the alternate bendings and straigh- 

 tcnings of the corn stalks, and a somewhat similar set of 

 movements progressing round a ring, gives rise to the appear- 

 ance of rotation so strikingly shown by the common rotifer and 

 its allies. 



