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LIFE. IN ITS INTERMEDIATE FORMS. 



and their comparatively complex structure, constitute a 

 peculiar class of themselves, under the title of Rotifera. 



The relation which this group bears to other recognised 

 classes of animals, has been a matter of some debate. It 

 seems generally admitted that they come into the Annu- 

 lose Division ; and we are inclined to place them above 

 the Annelida, having relations both with Insecta and 

 Crustacea, and forming a link by which the lowest forms 

 of these highly-organised creatures are linked with the 

 highest Infusoria. 



The earliest known species is one of the most abundant ; 

 and though it is not so typical of the class as many 

 others, being one of the lowest forms in organisation, yet 

 as it exhibits, with peculiar clearness, the characteristic 

 movement which has given a name both to itself and its 

 class, it is popularly considered as the representative of 

 the whole. We allude to the common Wheel-animalcule 

 (Rotifer vulgaris), which is found in most collections of 

 fresh water, and in vegetable infusions that have stood 

 exposed for a few days. 



The form of this tiny creature, which does not exceed 

 3oth of an inch in length, is spindle-shaped or barrel- 

 shaped, the lower part tapering to a slender foot, of many 

 joints, capable of being sheathed, telescope-fashion, one 

 within another, and the upper part expanding into two 

 circular disks set with vibrating cilia. It is the motions 

 of these cilia which are so peculiar and characteristic of 

 the whole class ; they immediately arrest the attention of 

 the observer, who is struck with wonder to behold what 

 appear to be the dark teeth of two cogged- wheels, set side 

 by side horizontally, and running rapidly round with the 



