760 TENACITY OF LIFE. 



the head. These so-called wheels are two disc-like lobes, the edges of which are fringed 

 with cilia, which, when in movement, 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. 



They have a well-defined muscular system, while their jaws are nearly, if not quite, as 

 complicated as those of the echinus. Most of them can swim, some are able to attach 

 themselves at will to any fixed objects, while others are fixed to one spot from which they 

 do not stir. 



Distinct sexes have been discovered in several genera of Rotifers ; and in those cases 

 where the male has not been found, it is generally thought that the very small size and 

 eccentric shape of the opposite sex may be the reason why it has not been discovered. In 

 those instances where his existence has been indubitably ascertained, he is always a 

 strange being, very unlike the female, very small, and, what is even more strange, 

 possessing neither jaws, throat, stomach, nor intestines. His life must therefore be very 

 short, as is known to be the case with the male sex in many insects. It has been well 

 suggested, that perhaps the males are only produced at certain times of the year, and are 

 not therefore found so plentifully as their mates. 



Fortunately for observers, the integuments of these animals are extremely transparent, 

 so that it is possible to watch the whole of the vital processes, and to see the various 

 functions carried on with as much ease as if the skin were of crystal. Their development 

 is wonderfully rapid ; for although but a few eggs are produced at one time, they are so 

 quickly hatched, and the animal is so rapid in its gr. v /th, that Professor Ehrenberg 

 calculated that in the genus Hydatica, although only three or four eggs are produced at 

 a time, a single individual will be the progenitrix of nearly seventeen million descendants 

 within the space of twenty-four days. 



In this class the arrangement is very perplexing to systematic naturalists, and nothing 

 ; s as yet settled about it. 



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. There is, however, one very strange residence of the common Eotifer, namely, 

 within the leaf-cells of the common bog- moss (Sphagnum). These cells are very large in 

 proportion to the size of the leaf, are kept open by spiral threads coiled in their interior, 

 and their walls are pierced with large apertures, so as to form a general communication 

 throughout the whole mass of cells. Within these curious chambers the Rotifer is 

 found, and is able to pass freely from one cell to another. They probably gain their 

 admission in the egg state, and find sufficient moisture in the cells for their seeds. 



The typical genus of this class is known by the name 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 

 tail ; sometimes they are nearly motionless, while they frequently rock themselves backward 

 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 have 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 double lines drawn over its surface. Even in the very young and undeveloped 

 animals which are seen within the body of the parent, these jaws form the most 

 conspicuous portions of their structure, and enable them to be recognised long before they 

 are able to go out into their watery world and shift for themselves. 



