1S2 



SYSTEMATIC ZOOLOGY 



the dog his from cattle and rabbits, the sharks from other 

 fish, etc. 



CLASS II. NEMATHELMINTHES. 



In these roundworms the body is long and cylindrical, 

 and is covered with a firm, tough cuticle. Usually both 

 mouth and vent are present, and the alimentary canal 

 runs through a large body-cavity (coelom), being con- 

 nected with the body-walls only at the two ends. There 

 is never any division of the body into segments. Some 

 roundworms live freely in the water, some are parasitic 

 in plants, and some infest animals. Among them are to 

 be mentioned the vinegar and paste 'eels/ which are 

 occasionally found in these substances. Here, too, 



belong the 'horsehair- worms,' which are 

 frequently believed to be horsehairs con- 

 verted into worms by soaking in water. 

 These hairworms are at one period of their 

 lives parasitic in insects, especially in 

 grasshoppers. Some of the roundworms 

 occur as parasites in man. The stomach- 

 worms and pinworms of children belong 

 to the roundworms, and these obtain 

 entrance to the human system only as 

 the exceedingly minute eggs are taken 

 into the stomach by way of the mouth. 



Worst of all the parasitic Nernathel- 

 minthes is the Trichina (fig. 27), which 

 when adult is scarcely an eighth of an 



FIG. 27. Trichina, 



encysted in human mc } 1 ' m length, and yet which not inf re- 

 muscle. After J 



Leuckart. quently causes death. Man is usually in- 



fected with them by eating raw or partially cooked pork. 

 In the pig they first appear in the alimentary canal, where 



