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 to a safe height, when other elements shall 

 be utilized. These flying buttresses catch all debris of the ocean, and hold it until a soil is 

 formed. Now, birds come to roost here ; they bring seeds, which aie 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. 



ROTIFERA. 



LTHOUGH 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 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 



