150 ROTATORIA. 



enable the animals to cling to objects. The most 

 characteristic organ, however, is a kind of rounded or 

 oval disk, placed at the anterior end of the body, and 

 furnished with cilia. When these are in active mo- 

 tion, the organ appears as a revolving wheel, whence 

 the name of Wheel-animalcules, by which they are 

 sometimes designated. The wheel-organ enables the 

 animals to swim through the water, and also brings 

 their food to the mouth by the currents which it 

 produces. It is usually cleft into two or more lobes, 

 and can be retracted, as is commonly the case when 

 the animals are disturbed. 



In many of these animals the body is more or less 

 covered by a horny shell or carapace ; and in some it 

 is fixed at the bottom of a tube, within which it can 

 be withdrawn. On the anterior part of the body are 

 frequently seen two or more red spots, which re- 

 present eyes. The alimentary canal is mostly dis- 

 tinct, being indicated by the colour of its contents, 

 and it is lined with cilia. Towards its front portion is 

 a gizzard (PL XI. fig. 2 a) containing teeth, which 

 are sometimes attached to a jointed jaw-like frame- 

 work ; these are usually in active motion. No heart 

 or blood-vessels have been observed in the Rotatoria ; 

 but on each side of the body in many of them is a 

 long wavy tube, containing at intervals minute cili- 

 ated bodies, the cilia propelling the water through 

 the tubes, and so exerting an aerating or respiratory 

 function. The reproduction of the Rotatoria takes 

 place by the formation of ova, which may often be 

 distinguished within the body of the parent. 



Rot'ifer vulgdris (PL XI. fig. 2) is a common species. 

 It has a spindle-shaped body, which is capable of con- 

 traction almost into a ball. The front or head end 

 is sometimes protruded (fig. 2), at others retracted 

 and obscured by the exserted disk (fig. 2*) ; and be- 

 neath it is a tentacle-like organ, supposed to repre- 

 sent an antenna (fig. 2 b] . The position of the jaws 



