THE ROTIFERS. 581 
year old the roots are veritable flying buttresses. Remark how well adapted this plan is 
to finish the growth of the island, to bring it up toa safe height, when other elements shall 
be utilized. These flying buttresses catch all débris of the ocean, and hold it until a soil is 
formed. Now, birds come to roost here; they bring seeds, which are deposited in the excre- 
ment. Among these seeds are several kinds of great convolvuli, morning-glory plants, whose 
habits are to run on the ground like a pumpkin vine. 
These great vines take root at intervals—many of them form resting-places for moving 
rubbish. Sand begins to collect. Innumerable agencies conspire to bring this low island to a 
greater height above water, when the land becomes dry; hence Dry Tortugas, in contra- 
distinction to Wet Tortugas, or wet land that has not yet reached the point of being above 
water. Once the surface has become somewhat permanently dry, other seeds germinate, and 
grasses appear—the beach-grasses, whose rootlets catch and hold the sands. Eventually a 
considerable soil is formed. The visitation of sea birds brings guano, shrubs appear, and then 
great trees. Some of the older keys are heavily wooded with a variety of trees. By these 
processes it is supposed the larger portion of the State of Florida has been built up. 

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LTHOUGH the Rotifera, or Wheel Animalcules, are generally placed among the 
7 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 revolv- 
ing 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, 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 them- 
selves 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 
Cy 
