340 MANUAL OF NATURAL HISTORY. 



ROTIFERS. 



The Rotifers or "Wheel- Animalcules" have been 

 so designated on account of the little, ciliated, circu- 

 lar organs which serve them as means of progression 

 through the water, for, like the Infusory or Poly- 

 gastric Animalcules, they are all aquatic. Like those 

 organisms, also, they are extremely minute, but their 

 structure is much more complex, the intestinal canal 

 having two distinct openings like that of the Polyzoa, 

 to which they somewhat closely approach. In fact, 

 these small and delicately organized Rotifers are co- 

 vered with a clear, transparent skin, which corre- 

 sponds to the hardened shells of the Polyzoa, to 

 which they seem to have the same relation that the 

 Tunicata have to the bivalve acephalous Mollusca. 

 The front part of their thin, coriaceous envelope is 

 open, and frequently toothed round the margins, the 

 hind part is closed, and usually ends in a little pair of 

 forceps, which serves the purpose of anchoring the 

 individuals to fixed or floating bodies. The ciliated 

 wheels which constitute the organs of locomotion of 

 these little animals do not actually rotate, but only 

 appear to do so, on account of the rapid undulations 

 of their marginal cilia. One very remarkable fea- 

 ture in the organization of these microscopic beings, 

 is the presence of a muscular gizzard with distinct- 

 teeth, for the purpose of masticating or grinding the 



