WHEEL-BEAEEKS. 289 



such examples as have the power of freely swimming 

 from place to place at pleasure ; but there is a consid- 

 erable group, of which this is a member, which are 

 permanently stationary, being fixed for life to the leaves 

 or stems of the vegetation that grows under water. 

 The stiff and spinous whorls of the Homwort {Oera- 

 tophyllum demers^mi), that grows commonly in sluggish 

 streams and pasture-pools, is a favourite resoi't of the 

 species, but it is not confined to any one plant. Here, 

 for instance, it has chosen as the site of its residence 

 the much-cleft leaves of the "Water Crowfoot {JRa/ivwn- 

 culus aquatilis) ; those leaves, I mean, which, growing 

 wholly under the water, are divided into a multitude 

 of slender finger-like filaments, so different from those 

 which float on the surface, and which are merely 

 notched. 



Tou can readily find the Tube-wheels by the aid of 

 a pocket lens, and even with the naked eye when you 

 have seen one or two. By holding up this phial, in 

 which a little plant of the Crowfoot is growing, and 

 searching, with the lens, the window being in front of 

 you, the filaments one by one, you will readily per- 

 ceive, here and there, little shining objects standing 

 up, or projecting in various directions from the surface 

 of the leaves. The colony is rather numerous in this 

 case, and we shall have no difficulty in selecting our 



specimens. 



On this filament, which I have seized with the joints 



of a pair of pliers, I can see at least half a dozen of the 

 little parasites. This, then, I will nip off from the plant, 

 and put it with its tiny population into the live-box. 

 Here it is, ready for examination. 



Several of the animals are in the field of view ; but 

 13 



