318 ROTIFERA, OR WHEEL-ANIMALCULES. 



CLASS OF ROTIFERA. 



927. The class of ROTIFERA, or Wheel- Animalcules and their 

 allies, is certainly to be regarded as belonging to the Articulated 

 division of the animal kingdom ; although the characters of that 

 group are by no means distinctly marked in it. That these 

 beings possess a structure much more complex than that of the 

 Infusoria, to be hereafter considered (Chap. XXVI.), there can 

 be no doubt whatever, and indeed some authors have even pro- 

 posed to arrange them with the Crustacea. That this complexity 

 of structure was for a long time overlooked, was owing to the 

 minute size of the animals in question ; scarcely any of them ex- 

 ceed a line in length, and many are less than -^^ of an inch. 

 Nearly all the species of this class are aquatic in their habits, 

 some living only in salt water, others frequenting stagnant ponds, 

 and others appearing in vegetable infusions, where they gener- 

 ally succeed animalcules of inferior organisation. 



928. The great transparency of the bodies of the ROTIFERA 

 permits their general structure to be easily recognised. They 

 have usually an elongated form, similar on the two sides. At 

 the anterior extremity, we observe one or more rows of vibratile 

 cilia, usually arranged in a circular manner. When these are in 

 motion, an appearance as of revolving wheels is produced, from 

 which the class derives its appellation ; and this is particularly 

 evident in one of its commonest forms, usually known as the 

 Wheel- Animalcule, which possesses a circular row of cilia on 

 each side, (Fig. 637, b). In many species we find a prolonga- 

 tion of the body in front, extending beyond the ciliary apparatus ; 

 this, which sometimes bears one or more red spots, that are 

 believed by Ehrenberg to be eyes, may be regarded as a head 

 (Fig. 637, a). The body is covered by a double envelope, both 

 layers of which are extremely thin and flexible in some species ; 

 whilst in others the outer one seems to possess a horny consist- 

 ence, and may even contain silicious or flinty matter, like the 

 sheaths of the lower Infusoria. In the former case, the whole 



