230 



ZOOLOGY 



Land and 

 fresh-water 



flatworms 



Parasitic 

 flatworms 



rians are free-living flatworms, with ciliated skin, hav- 

 ing the alimentary canal in the form of a blind sac, 



B Drawing by R. Weber 



FIG. 56. A land planarian from Guatemala. A, about twice natural size; B, a 

 cross section of hinder part of body much enlarged, showing the two posterior 

 branches of the intestine. 



that is, with only one opening, which may be 

 simple or variously lobed. On account of the form of 

 the intestine, there is a certain resemblance to the 

 Ccelenterata, but it is superficial, and does not extend 

 to other parts of the anatomy. Turbellarians exist in 

 the sea in great numbers, and are fairly numerous in 

 fresh water. A small, dark species may often be found 

 in mountain springs. There are also many land species, 

 looking something like slugs, living in regions where the 

 climate is moist. A large form has become established 

 in hothouses. Some of the species are quite large and 

 brightly colored ; others, such as the fresh-water Rhab- 

 doccelida, are microscopical and transparent, looking 

 like Protozoa, but easily distinguished by the complexity 

 of their anatomy. 



3. The Trematoda include the parasitic worms known 

 as flukes. They possess an alimentary canal, but have 

 lost the ciliation of the body surface, and have developed 

 suckers or adhesive organs. There is no doubt that 

 they arose from free-living ancestors, and are modified 

 for parasitic life. Throughout the Vermidea we find 

 many instances of such modification, taking place quite 

 independently in the different groups. We can no more 



