454 



INFUSOEIA ! — CILIARY MOTION. 



body, as well as around the oral-aperture (Fig. 238 a, b), or are 

 limited to some one part of it, which is always in the immediate 

 vicinity of the mouth (Fig. 239), supplies the means by far the 

 most frequently employed by the beings of this class, both for pro- 

 gression through the water and for drawing alimentary particles 

 into the interior of their bodies. In some their vibration is con- 

 stant, whilst in others it is only occasional, thus conveying the 

 impression that the Animalcule has a voluntary control over them ; 

 but there is strong reason for questioning the existence of any such 

 self -directing power. These cilia, like those of the zoospores of Pro- 

 tophytes, can usually be distinctly seen only when their movement 

 is very much slackened in its rate, or when it has entirely ceased. 

 Sometimes, however, instead of a multitude of short cilia, we find 



a small number of 

 Fig. 239. long slender filaments 



usually proceeding 

 from the anterior part 

 of the body (that 

 nearest the mouth), 

 and strongly resem- 

 bling the elongated 

 cilia of Protococcus 

 (Plate viii., fig. 2, h) 

 or of Volvox (Plate ix. , 

 figs. 9, 10, 11). But 

 in other cases, the fila- 

 ments are compara- 

 tively short and have 

 a bristle- like firmness ; 

 and instead of being 

 kept in vibration, they 

 are moved (like the 

 spines of Echini) by 

 the contraction of the 

 substance to which 

 their bases are at- 

 tached, in such a 

 manner that the Ani- 

 malcule crawls by 

 their means over a 

 solid surface, as we 

 see especially in Try- 

 choda lynceus (Fig. 

 241, P, q). In Chilo- 

 don and Nassula, the 

 mouth is provided 

 with a circlet of these bristles, which have received the designa- 

 tion of 'teeth ;' their function, however, is rather that of laying 

 hold of alimentary particles by their expansion and subsequent 



Group of Vorticella nebulifera, showing, 

 A, the ordinary form ; B, the same with the 

 stalk contracted; c, the same with the bell 

 closed ; d, e, f, successive stages of fissiparous 

 multiplication. 



