16 GENERAL HISTORY OF 



a streak or mark, as it dries, left upon the glass, thus 

 giving conclusive evidence of the presence of this organ. 

 Sometimes the mouth is furnished with two of these 

 probosces, or cilia, nearly of equal length with the body, as 

 in the genera Chlorogonium. Other Infusoria have their 

 oral orifices completely encircled with cilia, in which case 

 they are usually shorter than when only one or two are 

 perceptible, rarely exceeding one-fourth of the length of 

 the body. Others, again, have their bodies wholly covered 

 with cilia, which are often arranged in longitudinal rows, as 

 with the Uroleptus. (See Drawing, &c.) 



When these cilia are disposed in clusters, as with some 

 of the larger polygastric animalcules, their structure may 

 be more correctly ascertained. In the family Oxytrichina 

 (see Engraving) J the different modifications of these fila- 

 form organs constitute excellent characteristics of the 

 genera ; as, however, they are not limited to that parti- 

 cular family, I shall make a few general observations 

 respecting them. 



Cilia may be described as hairs seated upon a bulb. 

 They perform a rapid vibratory motion, the point of each 

 describing a comparatively large circle, whilst the base 

 merely turns round upon its articulating surface, or part of 

 the bulb to which it is affixed. Dr. Ehrenberg is of 

 opinion that there are two kinds of cilia, viz. Cilia 

 continua, in which the bulb is a continuation, or merely 

 enlarged termination of the cilium ; and Cilia articulata, 

 in which there is a joint or articulation of the cilium to the 

 bulb. Examples of the former may be observed in the 

 Stylonchia mytillus; and of the latter in the Parame- 

 cium aureli, {fig. 330.) 



