CLASS VI. 

 WHEEL-ANIMALCULES (ROTATORIA} 1 . 



WE return from the consideration of different animals whose 

 Dodies amongst the Invertebrates may be styled large to that world, 

 .nvisible to the naked eye, with which in the class of the Infusories 

 we began to treat of the animal kingdom. And in the classes 

 that follow, however some species may be found that are scarcely 

 Derceptible to the unassisted eye, no one of them consists entirely 

 of creatures so small as Infusories and Wheel-animalcules. Wheel- 

 animalcules, as a whole, surpass Infusories in size ; still they are 

 very minute animal forms, mostly between J ^5 millimeter. LEEU- 

 WENHOECK, who discovered the Infusories, was also the first who 

 observed some species of Wheel-animalcules. 



The name of Wheel-animalcules is borrowed from the vibratile 

 cilia which at the anterior extremity of the body are set upon the 

 margin of a disc capable of eversion and inversion. In species, 

 where that margin is not divided or indented, an optical illusion is 

 caused by the motion of the cilia, as though a toothed wheel were 

 revolving with great velocity in a circle, and so LEEUWENHOECK 

 thought such was really the case, who compared the rotatory organ 

 with the wheel of a watch-work 2 . Every one who has observed 

 the phenomenon of vibrating cilia is aware that the deceptive 

 appearance of a rapid motion or current in a given direction is pro- 

 duced : if, then, vibrating cilia be met with on the smooth margin 

 of a circular structure, the appearance of a rotating wheel will 

 follow of course. It is to be remarked, however, that the motion is 



1 See on this class the works referred to (p. 37) at the class of Infusoria of 

 MUELLER, EHRENBERG and DUJARDIN. Also may be compared O. SCHMIDT Versuch 

 einer Darstellung der Organisation der RdderihiercJien, in ERICHSON'S Arcliiv f. Natur- 

 geschichte, 1846, s. 67 81, Taf. in. 



2 Send-brieven, 1718, vn. Brief, bl. 67. DUTROCHET has attempted to explain the 

 phenomenon by muscular motion ; according to him the wheel is merely a circular, 

 muscular string, which by its contraction causes other parts of the gelatinous substance 

 to project alternately in the form of conical papillae, whence a circular motion appears 

 to arise. Ann. du Museum, XX. 1813, pp. 469 473. 



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