i8o AN INTRODUCTION TO ZOOLOGY. 



puzzles, that it will be best merely to name the larger, 

 groups and refer to a few of the more commonly 

 known forms. 



The class Platyhelminthes (worms with flattened 

 body) includes the orders Turbellaria, Tremaioda, 

 Gestoda, and Nemertina. The first includes the Den- 

 drocdla and the BJiahdoccela, mostly fresh-water forms 

 that live by sucking the juices of other animals, 

 Microsiomum forms, by budding, what are apparently 

 segments or metameres, though they often fall apart 

 by transverse fission. It has been suggested that 

 articulated or metameric animals, such as the higher 

 worms (Annelids), and the Arthropods, have originated 

 from worms of the lower types by the formation of a 

 chain of buds like this (Fig. 46). The Ehabdocoela 

 develop direct from the egg, the ciliated embryo being 

 liable to be confused with Infusoria. The Bendrocvela, 

 mostly marine forms, but some of them tei'restrial, are 

 very flat, and receive their name from the fact that 

 the alimentary canal divides into multitudes of minute 

 Taranches, like trees : these are to be noted as includ- 

 ing the first land animals we have yet had to think of. 

 They are only 7ia//-terrestrial, though, for they live in 

 very damp places. Some marine forms have a larval 

 stage. The Trematodes are parasites, and include the 

 "fluke" worm which lives in the bile-duct of the 

 sheep's liver. It requires, however, two " hosts " for 

 its development. The ciliated embryo gets into a 

 water snail {Limnceus truncatulus), and there it en- 

 larges, and develops into a sporocyst. The latter is a 



