ROTIFERA, OR WHEEL-ANIMALCULES. 317 



Worms ; but is found in the areolar issue, and in the substance 

 of the various membranes. It is not uncommon in Man ; but 

 frequently multiplies to a great extent in the Pig. 



CLASS OF ROTIFERA. 



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

 allies, is probably 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. XX V.), there can be 

 no doubt whatever. 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 exceed 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 generally succeed animal- 

 cules of inferior organisation. A few, however, can live in 

 moist earth. 



856. 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. 537, #) 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. 537, a). The body is covered by a double envelope, both 



