NEREIS AND ITS ALLIES 



153 



T) 



pin-worm of man. Others still penetrate into the mus- 

 cles, and cause great pain and often death. Such is the 

 pork-worm, Trichina, 1 which gets into man through un- 

 cooked pork, multiplies in the 

 food canal, migrates in great 

 numbers into the muscles and 

 encysts itself there (Fig. 141). 

 Another group which is 

 largely parasitic is that of the 

 flatworms. Some flatworms 

 live free in ponds. They will 

 be found abundantly among 

 plants taken from small ponds 

 in the summer and autumn, and 

 are commonly known as Pla- 

 naria 2 (Fig. 142). They may 

 be recognized not only by their 

 flattened form, but also by a 

 curious proboscis which pro- 

 trudes from the middle of the 

 under side of the body and bears 



/ 



mouth at its tip. These 



a 



FIG. 142. Species of fresh-water 

 Plaiiaria. 1, Dendrocoelum 

 lacteum, cream color; -, Pla- 

 naria maculata ; 3, head end of 

 same to show light streak. 

 After Woodworth. 



creatures have a marvellous 



power of regeneration, so that 



every piece into which a worm 



is cut will reproduce an entire 



one (Fig. 143). If the worm 



is mutilated but not wholly cut in two, bizarre forms may 



result by a modified tendency to regenerate (Fig. 144). 



Other flatworms are parasitic, such as the liver-fluke 

 of the sheep (Fig. 145). This destructive parasite has a 



hair. 2 planus, flat. 



