WHEEL-BEARERS. 
195 
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 now world of life. The 
excessive minuteness of these creatures being thoir 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 larvae 
of Insects ; some are minute Annelida aud Crustacea ; 
and a considerable number, of high interest from their 
beauty, their sprightly motions, their perfect transparency, 
