426 MICROSCOPIC FORMS OF ANIMAL LIFE. 



observable in the higher Animals, that whilst in the latter it is 

 constant, giving the idea of purely mechanical agency, in the 

 former it is so interrupted and renewed, as almost necessarily to 

 suggest to the observer the notion of choice and direction. 



277. 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 resem- 

 blance 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 

 are found in collections of fresh water, and rather in such as are 

 free from actively decomposing matter, than in those which 

 contain organic substances in a putrescent state. Hence when 

 they present themselves in vegetable infusions, it is usually after 

 that offensive condition, which is favorable to the development 

 of many of the Infusoria, has passed away ; and they are conse- 

 quently to be looked for, after the disappearance of many suc- 

 cessions (it may be) of Animalcules of inferior organization. 

 Rotifera are more abundantly developed in liquids which have 

 been long and freely exposed 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. 1 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 ( 216). The wheel- 

 like organs from which the class derives its designation, are 

 most characteristically seen in the common form just mentioned 

 (Fig. 201), where they consist of two disk-like lobes or projec- 

 tions 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. 200, 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 ( 281). The great 

 transparency of the Rotifera permits their general structure to 



1 See a remarkable instance of this in p. 253, note. 



