286 EVENINGS AT THE MICROSCOPE 



ceratophylli). Hitherto we have seen such examples as 

 have the power of freely swimming from place to place at 

 pleasure; but there is a considerable 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 Horn wort 

 (Ceratophyllum demersum)^ that grows commonly in slug- 

 gish streams and pasture-pools, is a favorite resort 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 (Ranunculus 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 sur- 

 face, and which are merely notched. 



You 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 perceive, here and there, little 

 shining objects standing up, or projecting in various direc- 

 tions 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 points 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 

 we will look at one at a time. A long narrow tube. 



