172 SECRETS OF EARTH AND SEA 



ment. It is an important fact that in the possession of a 

 toothed gizzard, in the hard body-case or cuirass of some 

 kinds, and in Pedalion's rapidly-moving legs or paddles, 

 fringed with plumose hairs and moved by that peculiar 

 variety of muscular tissue which is called "striped 

 muscular tissue," the wheel animalcules give evidence of 

 relationship to the Crustacea — that is to say, it appears 

 to be probable that they were derived from the common 

 ancestor of marine worms and Crustacea before those two 

 lines of descent had diverged. 



Rotifera or wheel animalcules are found all over the 

 world, in the tropics, the temperate zones, the Arctic and 

 Antarctic, and many sp'ecies have a world-wide distri- 

 bution. They occur in fresh waters and in the sea, in 

 great lakes, in gutters which dry up, in pools in the polar 

 regions and on high mountains which are solid ice for the 

 greater part of the year. A few are parasitic, some living 

 on the legs of minute Crustacea. One which I discovered 

 in 1868 in the Channel Islands lives in crowds on the skin 

 of a remarkable sea-worm (Synapta), which burrows in 

 the sand, exposed at low tide. It holds on (as I found 

 and figured) by a true sucker, which replaces the forked 

 tail of other commoner Rotifers. It was named " Dis- 

 copus " by Zelinka, who searched for it in consequence of 

 my description, and gave a very detailed account of it. 

 Others are parasitic inside earthworms, and one is found 

 inside the globe animalcule Volvox ! Another causes the 

 growth of warts or "galls" in a curious kind of Alga 

 called Vaucheria. 



