INFUSORIAL ANTMALCULES. '71 



fringing the head of the Rotatoria. Ftiraday explains it by sup- 

 posing the distinct cilia to become visible by slowly returning to 

 an erect state, after having been suddenly bent pre vioiisly. Ehrenberg 

 again assumed the existence of four muscles at the base of each 

 cilium, each acting upon it in its own direction, and so producing a 

 revolution around the fixed point of attachment or base of the 

 cilium. In this way each cilium would be alternately nearer to, or 

 more remote from the eye, and, consequently, more or less visible. 



Another explanation has been offered by Dujardin. He says, 

 " The vibratile cilia being arranged parallel, and at equal distances, 

 will equally refract or intercept the light, and none will be more 

 visible than the rest, but if, by a movement propagated along the 

 row of cilia, some, momentarily inclined, are brought into juxta- 

 position with adjoining cUia, the light will be more intercepted, and 

 a band, more or less dark, will be the result. It can be imagined, 

 therefore, that if the cilia come to be inclined one after another, a 

 series of juxta-positions, or of apparent intersections, will be pro- 

 duced, and this, in the direction of the general movement. Further, 

 if each of the intersections preserves the same form as if produced 

 by a number of equal lines, and are equally inclined to each other, 

 an appearance of a solid body of a definite form, like the tooth of a 

 saw or the spokes of a wheel, moving uniformly, presents itself to 

 the eye." 



The situation, number, and disposition of the rotatory organs, are 

 employed in the grouping of the Rotatoria into families and genera. 



The rotary apparatus is single, double, or made up of several 

 portions. Its wheel-like motion is most evident where the rotary 

 apparatus forms an unbroken circle, as in Conochilus, (figs. 365-370) 

 Philodina, (figs. 487-490) and Actinurus, (figs. 481-484.) Where 

 the apparatus is interrupted by a notch, and is made up of two or 

 more smaller distinct wheels, the delusion of complete revolutions is 

 removed, as in Hydatina, (fig. 394) Notommata, (figs. 418-420) 

 Diglena, (figs. 403-405), &c. Exceptional forms are exhibited in 

 Floscularia and Stephanoceros. 



The action of the rotary apparatus may be arrested at the will of 

 the animal, or the entire organ be retracted within the body. 



In addition to the rotary organ, the head is sometimes crowned by 



