THE FLAT- WORMS. 39 



the head and tail; many worms have ears. All worms 

 grow from eggs, and many have a free swimming embryo 

 entirely unlike the parent worm, thus j^assing through a 

 " metamorphosis." 



Classes op Vermes. 



1. Body flat, often not segmented; no body-cavity. . Plaiyhelminthes. 



2. Body round, thread-like; a body-cavity Nematdiniathes. 



3. Body microscopic, moving by two ciliated flaps. .Rotatoria. 



4. Body minute, in a solid cell Pulyzoa. 



5. Shell-worms, attaclied by a stalk, with two arms. . Bvachiopoda. 



6. Body rounded, or riljhon-like, with a proboscis... .Nemertina. 



7. Body jointed, with feelers, eyes, and gills Annulata. 



Class I.— Platyhelminthes {Flat-worms, Fluke-worms, 

 Tape-xvorms, etc. ). 



General Characters of Flat-worms. — The commonest ex- 

 ample of this class is a small dark flat-worm which may be 

 found in any pond on the under side of sticks or stones. 

 This flat-worm is called Planaria torva (Fig. 39). It is 

 about 7 mm. {^ in.) in length, oblong, flat, with two black 

 eye-spots, each with an oblong whitish space in front. Its 

 body is covered with microscojiic hairs (cilia), enabling it 

 to move easily in the water. These worms have a rudi- 

 mentary brain, from which pass backwards two slender ner- 

 vous-threads, which do not have nervous swellings, as in 

 the earth-worm. The digestive canal is also much branched. 

 Besides these organs all the worms of this class have a so- 

 called water- vascular system, somewhat like that of Echino- 

 dernis. These systems consist of two main tubes which 

 branch throughout the body. Many if not most Plana- 

 rians or free flat-worms have nettling organs somewhat like 

 the lasso-cells of jelly-fishes, except that the rods are short 

 and stiff, and are not known to be bai'bed. 



Many flat- worms live as parasites m the bodies of other 

 animals. They differ from ordinary Planarians in not be- 



