122 



Fig. 113. 

 ris, a large 

 Nematode para- 

 sitic in the intes- 

 tine of the horse; 

 natural 

 (Drawn from 

 specimen.) 



SYSTEMATIC ZOOLOGY 



troublesome in greenhouses among such plants 

 as the tomato, cucumber, violet, and rose, and in 

 Europe has caused much damage to beets, cab- 

 bages, and various grains, such as barley and wheat. 

 Another closely related genus does much damage 

 to wheat by eating the growing ears. Of the ani- 

 mal parasites, several species of the genus Ascaris 

 (Fig. 1 13) occur in man and various domestic ani- 

 mals in the intestine, probably entering as eggs in 

 the water which the animal drinks. In man thev 

 are from ten to thirty-five centimeters long, — in the 

 ] iig somewhat shorter, in the horse and ox sometimes 

 over forty-five centimeters. Other roundworms, 

 from one to five centimeters long, are common 

 in the alimentary canal of man, particularly in 

 children. The genus Filaria is one of the most 

 dangerous to man in the tropics; it occurs in the 

 blood and lymphatic systems, and is believed to be 

 connected with the condition known as elephan- 

 tiasis, in which parts of the body increase enor- 

 mously in size. The Guinea worm belongs to this 

 genus and lives in the subcutaneous tissue, where 

 it forms abscesses which allow the escape of the 

 embryos. 



In Europe and North America the most danger- 

 ous of the Nematoda to man is the little Trichina 

 spiralis, of which the male is about one millimeter 

 long and the female about three millimeters (F 

 1 14). The parasite enters man, embedded in the 

 muscle fibers of pork, and if this is raw or insuf- 

 ficiently cooked, the worms rapidly mature in the 

 intestine, and in about a week the young are born 

 alive, each female being capable of producing 

 about a thousand embryos. These make their 

 way through the lining of the intestine and eventu- 

 ally into the various muscles of the bodv, where* 

 they coil up within the muscle fibers and there 

 remain inclosed in a cyst. The disease produced 

 by these worms is called trichinosis, and it is fre- 





