THE FLAT -WORMS, 
49 
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 passing through a 
metamorphosis." 
Classes of Vermes. 
1. Body flat, often not segmented; no body-cavity. . Platyhelmintlies. 
2. Body round, thread-like; a body-cavity Nematelminthes. 
3. Body microscopic, moving by two ciliated flaps. .Rotatoria. 
4. Body minute, in a solid cell Polyzoa. 
5. Shell-worms, attached by a stalk, with two arms. . Brachiopoda. 
6. Body rounded, or ribbon-like, with a proboscis — Nemevtina. 
7. Body jointed, with feelers, eyes, and gills Annulata. 
Class I.^Platyhelminthes {Flat-iuormSy Fluhe-worms, 
Tape-worw,s, 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. 51).^ 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 microscopic 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 bmnched. 
Besides these organs all the worms of this class have a so- 
called water- vascular system, somewhat like that of Echino- 
derms. 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 barbed. 
Many flat- worms live as parasites in the bodies of other 
animals. They differ froni ordinary Planari^ns iij not be- 
