392 RADIATA. 



number, but in some there are four. Each wheel 

 consists of a circular disk, surrounded by a fringe 

 of minute hairs, called cilia)* which, standing per- 

 pendicularly to the plane of the circle, give the 

 whole a resemblance to the crown wheel of a watch. 

 To all appearance, these wheels are incessantly spin- 

 ning round in one direction with great rapidity, 

 so as to produce strong currents in the water, form- 

 ing little whirlpools. The use of these rapid revo- 

 lutions appears to be two-fold ; when the animal 

 is stationary, being fixed by the tail-forceps, the 

 currents produced around the mouth bring floating 

 particles of food to be swallowed, and fresh water 

 to be respired ; on the other hand, if disengaged, 

 the wheels, like the paddles of a steam-boat, urge 

 it rapidly through the water with an easy gliding 

 motion. 



It was long supposed that these motions were 

 real revolutions of the wheels ; but it is now clearly 

 established that they are merely an optical illusion, 

 exactly similar to that by which, when the waves 

 are rolling in upon a beach, the particles of ivater 

 appear to the eye to move rapidly forward, while, 

 as is well known, they merely rise and fall perpen- 

 dicularly in constant succession. Dr. A. Farre re- 

 marks, that under high magnifying powers, " the 

 cilia have the appearance of moving in waves, in the 

 production of each of which from a dozen to twenty 

 cilia are concerned, the highest point of each wave 

 being formed by a cilium extended to its full length, 

 * Literally, eye-lashes. 



