NEREIS 



191 



and structure. Such is the case with some of the round- 

 worms. Some of these are thread-lili;e, live in springs 

 or pools, and are regarded by the uninitiated as ani- 

 mated horsehairs. Others are spindle-shaped, as, for example, 

 the " vinegar eel " and the 

 roundworms that are common 

 in stagnant water. Others 

 live in the food canal, as, for 

 example, the stomach-worm 

 (As'caris) of the horse and the 

 pinworm of man . Others still 

 penetrate into the muscles, 

 and cause great pain and often 

 death. Such is the pork- 

 worm (Trichi'na^), which gets 

 into man by means of un- 

 cooked pork, multiplies in the 

 food canal, migrates in great 

 numbers into the muscles 

 and encysts itself there (Fig. 

 186). 



The second group that is 

 partly 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 commonl}^ known as Plana'ria ■■ 

 (Fig. 187). They may be recognized not onlj' by their flat- 

 tened form, but also by a curious jiroboscis which protrudes 

 from the middle of the under side of the body and bears a 

 mouth at its tip. These creatures have a marvellous power 



' thrix, hair. ^ planus, flat. 



Fig. 18.5. — Ccrebratulus, a cream-col- 

 ored iiemertean. Head end at upper 

 part of figure ; mouth turned toward 

 observer, proboscis retracted. In- 

 stantaneous photograph of U\'ing 

 worm by W. H. C. P. 



