7 6 TENACITY OF LIFE. 



upon the head. These so-called wheels are two disk-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 dis- 

 covered. 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 transpa- 

 rent, 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 growth, 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 noth- 

 ing is 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 Rotifer, 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 back- 

 ward 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 com- 

 posed of double lines drawn over its surface. Even in the very youug and undeveloped 

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

 spicuous portions of their structure, and enable them to be recognized long before 

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



