THE FLAT-W0RM8. 49 



the head and tail; many worms havi i . All worms 

 grow from md many have a free swimming embryo 



entirely unlike the pai'ent worm, thus passing through a 

 •■ metamorphosis." 



( 'I. v — i - OF \ I RW 



1. Body flat, often not segmented; oo body-cavity . Platyhelmin 



J. Bod) round, thread like; a body-cavity Yematelminthas. 



.;. Body microscopic, moviug by I wo ciliated flaps. .Rotatoria. 



•1. Body minute, iu cell /' 



.') Bhell worms, attached l>,\ a stalk, with two arms. .Brachiopoda. 



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



1 Body jointed, with feelers, eyes, and gills I tlata. 



Class [. — Platyhelminthes i Flat-worms, Fluke-worms, 



Tape-worms, <■/< . }. 



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

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

 found in any pond on the under side of slicks or sto 

 This flat-worm is called Planaria torva (Fig. 51). It is 

 about J mm. ([ in.) in length, oblong, flat, with two black 

 rye 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- 

 mi ntarybrain, from which pass backwards twoslender ner- 

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

 the earth- wo nn. The digestive canal is also much branched. 

 Besides these organs all the worms of this class have a 50- 

 oalled water-vascular system, somewhat like that of Ecli 

 derms. These systems cons two main tubes which 



branch throughout the body. Many it' not most Plana- 

 nans or free Mat -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 ilat-worms live as parasites in the bodies of other 

 animals. They differ from ordinary rianarians in not be- 



