CLASS PLATYHELM1NTHES. 25 



into a branched stomach. The peculiar gliding motion 

 is the result of the combined action of invisible cilia. 



Though at first glance, the structure of this animal 

 may appear quite new to us, by comparing it with the 

 Anemone, we find several resemblances. If the latter 

 animal be inverted, its tentacles retracted, and its body 

 made capable of movement by being provided with cilia, 

 we should have a. very fair Planarian. The worm also 

 shows ccelenterate features, in that it is armed with lasso- 

 like cells, which it throws out on being irritated ; and in 

 sometimes multiplying by self-division. Though many 

 .Planarians of the warmer countries are terrestrial, all of 

 our forms, with a single exception, are aquatic. 



The remaining Flat Worms are mostly parasitic ; that 

 is, they attach themselves to some larger animal, their 

 host, from which they draw their nourishment. When 

 animals thus become parasitic, they are apt to lose, 

 through disuse, many organs, which a free life would 

 render quite necessary. Nature is thus found to be 

 economical, never supporting useless organs. 



ORDER TREMATODA (trem a to'da). 



The Liver Fluke, nearly an inch in 

 length, broad and flat, is one of the most 

 common parasites. It is found in the 

 bile-duct and other organs of herbivorous 

 animals. Subsisting there on the fluid 

 in which it floats, it has no need of eyes 

 nor of special means of protection, hence 

 these are not present. It is, however, pro- 



vided with two suckers, ventrally placed, surrounded by a suck- 

 ing disk. .4. Alimen- 



With Which it holds tO its host. tary tract. S, Ventral 



rm T-e i j J.T i sucking disk. (Eu- 



The life history of this animal is of 



