ROTIFERA OR WHEEL-ANIMALCULES. 469 



as almost necessarily to suggest to the observer the notion of choice 

 and direction. 



353. Rotifera, or Wheel-Animalcules. — We now come to 

 that higher group of Animalcules, which, in point of complexity of 

 organization, is as far removed from the preceding, as Mosses are 

 from the simplest Protophytes ; the only point of real resemblance 

 between the two groups, in fact, being the minuteness of size 

 which is common to both, and which was long the obstacle to the 

 recognition of the comparatively elevated character of the Rotifera, 

 as it still is to the precise determination of certain points of their 

 structure. Some of the Wheel -Animalcules are inhabitants of 

 Salt-water only, but by far the larger proportion arej found in col- 

 lections of Fresh-water, and rather in such as are free from actively 

 decomposing matter, than in those which contain organic sub- 

 stances in a putrescent state. Hence when they present them- 

 selves in Vegetable infusions, it is usually after that offensive 

 condition which is favourable to the development of many of the 

 Infusoria has passed-away ; and they are consequently to be looked- 

 for after the disappearance of many successions (it may be) of 

 Animalcules of inferior organization. Rotifera are more abun- 

 dantly developed in liquids which have been long and freely ex- 

 posed to the open air, than in such as have been kept under 

 shelter; certain kinds, for example, are to be met-with in the 

 little pools left after rain in the hollows of the lead with which the 

 tops of houses are partly covered; and they are occasionally 

 found in enormous numbers in cisterns which are not beneath roofs 

 or otherwise covered-over.* They are not, however, absolutely 

 confined to collections of liquid ; for there are a few species which 

 can maintain their existence in damp earth ; and the common 

 Rotifer is occasionally found in the interior of the leaf-cells of 

 Sphagnum (§ 275). 



354. The Wheel-like organs from which the class derives its 

 designation, are most characteristically seen in the common form 

 just mentioned (Fig. 244), where they consist of two disk-like 

 lobes or projections of the body, whose margins are fringed with 

 long cilia ; and it is the uninterrupted succession of strokes given 

 by these cilia, each row of which nearly returns (as it were) into 

 itself, that gives rise by an optical illusion to the notion of 

 ' wheels.' This arrangement, however, is by no means universal; 

 in fact, it obtains in only a small proportion of the group ; and by 

 far the more general plan is that seen in Fig. 243, in which the 

 cilia form one continuous line across the body, being disposed upon 

 the sinuous edges of certain lobes or projections which are borne 

 upon its anterior portion. Some of the chief departures from this 

 plan will be noticed hereafter (§ 363). 



355. The great transparency of the Rotifera permits their 



* See a remarkable instance of this in p. 246, note. 



