196 LIFE, IN ITS INTERMEDIATE FORMS. 
and their comparatively complex structure, constitute a 
peculiar class of themselves, under the title of Botifera. 
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 tho 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. 
Tho form of this tinv creature, which does not exceed 
Mh 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 
tho 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 
