CHAPTER XXI. 



Rotifera ( Wheel-bearers). 



Nearly two centuries ago, a great impetus was given to 

 physical science by the application of the newly-invented 

 Microscope to substances so minute, that their forms, or 

 at least their structure, could scarcely be appreciated by 

 the unassisted sense. The waters were found to be teem- 

 ing with living creatures of multitudinous kinds, consti- 

 tuting what might be called a new world of life. The 

 excessive minuteness of these creatures being their most 

 obvious character, and the imperfection of the instruments 

 as yet in use permitting no more than a vague perception 

 of their exterior figure, it was natural that they should 

 be associated in one group, under the term Animalcules. 



More precise observation has, however, determined 

 that, among these minute forms, are comprised animals 

 of very various grades of organisation. Some are the 

 simplest creatures known, as the Monads, of which we 

 treated in the first chapter of this volume ; some are 

 not animals at all, but plants of a very low grade, endowed 

 with animal-like powers of locomotion ; some are the larv£e 

 of Insects ; some are minute Annelida and Crustacea ; 

 and a considerable number, of high interest from their 

 beauty, their sprightly motions, their perfect transparency, 



