PLATYHELM1NTHES 231 



put the parasitic worms together, because of their 

 parasitism, than we could unite in one group of plants 

 the parasitic rust fungi and the mistletoe. The de- The liver 

 structive liver fluke of the sheep may be taken as an 

 example of this group. It lives in its early stages in a 

 small fresh-water snail (Lymneza), common in Europe 

 and America. The young flukes, known as cercaricz, 

 eventually leave the snails and attach themselves to the 

 grass at the edge of the pond, where grass and worms are 

 eaten together by the sheep. In the body of the sheep 

 they seek the liver, where they develop to full size. 

 Eggs are produced, which become scattered over the 

 pastures, and when they hatch, the snails, if present, be- 

 come infested. It used to be estimated that a million 

 sheep died annually in the British Islands from the at- 

 tacks of the liver fluke, but now that the life history is 

 known, it is comparatively easy to guard against infes- 

 tation. The European liver fluke is not native in 

 America, but has been introduced unintentionally by 

 man. There is, however, a large native American 

 species. 



4. The cestodes, or tapeworms, represent the most Tapeworms 

 extreme specialization for parasitic life among flat- 

 worms. They are flat and white, resembling tape, but 

 usually segmented. The alimentary canal is wholly 

 absent, even in the early stages. The unsegmented 

 tapeworms are rarely observed, and we may take the 

 common segmented forms, such as Tcenia, as typical of 

 the group. In these the adult worm possesses a so- 

 called head, which produces no eggs, but carries the 

 organs which fix the animal to the intestine of the host. 

 Following the head are numerous segments or pro- 

 glottids, which are egg-producing, and usually drop off 

 from time to time when mature. The segmentation is 



