588 WHEEL ANIMALCULES. 
ROTIFERA. 
ALTHOUGH the Rotifera, or Wheel Animalcules, are generally placed 
among the Infusoria, on account of their minute dimensions and aquatic 
habits, it is evident, from many peculiarities of their formation, that they 
deserve a much higher place, and in all probability constitute a class by 
themselves. 
They are called Wheel Animalcules on account of a curious structure 
which is found upon many of their members, and which looks very like a pair 
of revolving wheels set upon the head. These so-called wheels are two disc- 
like lobes, the edges of which are fringed with cilia, which when in movement 
Rotifer c'trinus. 
give to the creature an appearance as if it wore wheels on its head, like those 
of the fairy knight of ballad poetry. These wheels can be drawn into the 
body at will, or protruded to some little extent, and their object is evidently 
to procure food by causing currents of water to flow across the mouth. All, 
however, do not possess these appendages, but have a row of cilia, mostly 
broken into lobes, extending all round the upper portion of the body. 
These remarkable beings are mostly found in water that has become 
stagnant but is partially purified by the presence of the Infusorians, which 
always swarm in such localities. 
The typical genus of this class is known by the naine of ROTIFER, an 
example of which is seen in the illustration. In all the members of this 
genus the body is rather elongated, and furnished at the hinder end with a 
kind of telescopic tail, by means of which they can attach themselves at will 
to any object, and release themselves whenever they please. Sometimes they 
move their bodies gently about, while still grasping by the extremity of the 
tail; sometimes they are nearly motionless, while they frequently rock them- 
selves backwards and forwards so violently that they seem almost to be 
testing the strength of their hold. 
These creatures can both swim and crawl, the former act of locomotion 
being achieved by the movement of the cilia, and the latter by creeping along 
after the fashion of the leech, the head and tail taking alternate hold of the 
object on which they are crawling. The masticating apparatus is always 
conspicuous, whether the animal has the wheel protruded or withdrawn. It 
is situated behind the bases of the wheel-lobes, and looks, when the animal 
is at rest, something like a circular buckler with a cross composed of doubie 
lines drawn over its surface. 
RHIZOPODA. 
_ THE whole arrangement of the beings which we are now about to examine 
is still very obscure, and the best zoologists of the present time have 
