WHEEL-ANIMALCULES (KOTATORIA)’. 
WE return from the consideration of different animals whose 
bodies amongst the Invertebrates may be styled large to that world, 
invisible 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 
perceptible 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 4— 5 millimeter. LEnu- 
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?. 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 Jnfusoria of 
MUuELLER, EHRENBERG and Dugarpin. Also may be compared O, Scumipt Versuch 
einer Darstellung der Organisation der Réderthierchen, in ERIcHSON’S Archiv f. Natur- 
geschichte, 1846, s. 67—81, Taf. 111. 
2 Send-brieven, 1718, vit. Brief. bl. 67. DutRocuet 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 papillz, whence a circular motion appears 
to arise. Ann. du Muséum, xx. 1813, pp. 469—473. 
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