84 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



Branch V. NEMATHELMINTHES 



The round, or thread, worms include free forms, as 

 the vinegar eel ; parasitic forms, as the pin worm 

 (Ascaris) and trichina ; and forms free when adult, and 

 parasitic when young, as the hair worm ( Gordius). The 

 body is usually elongated and cylindrical in shape, 

 whence the name. In most forms there are 

 plainly marked digestive and nervous systems. 

 The trichina is usually derived by man from 

 11 the flesh of the pig. It 



exists in the muscles, 

 inclosed in micro- 

 scopic cases or cysts, 

 composed of calcareous 

 matter. If the meat be 

 eaten uncooked or par- 

 tially cooked, the cases 

 are dissolved, and the 

 trichinae become sexu- 

 ally mature in the in- 

 testines. The young 



FIG. &.- Trichina spiralis (much enlarged): are pro( Juced and bur- 

 I, male; a, mouth; c, intestine; II, capsules, 

 with trichinae in muscle. TOW their Way into the 



muscles, usually of the back and limbs, where they be- 

 come encysted in the muscle fibers. In burrowing they 

 cause great pain and fever, and sometimes death. The 

 adult trichina is about -^ of an inch long. 



The " jiorse-hair snake," a hair worm {Gordius), 

 passes the early part of its existence in larval or adult 

 insects, e.g., the cricket. When mature the worms leave 

 the body of the insect and lay their eggs in damp places. 

 The eggs or the immature worms are then taken into 

 the bodies of other insects in which the parasites later 

 reach their full development. 



