CHAPTER XVI 

 PARASITIC ROUNDWORMS 



By henry B. ward 



Professor of Zoology in the University of Illinois 



The roundworms or Nemathelminthes constitute a group of 

 convenience into which are put three classes that have Httle in 

 common except general external appearance. But even in this 

 feature differences of a real character appear on closer examination 

 and the study of internal anatomy fails to show any intimate 

 agreement in the fundamentals of structure. The three classes 

 embraced in this phylum are the Nematoda or true roundworms, 

 the Gordiacea or hairworms, and the Acanthocephala or probos- 

 cis roundworms. All agree in the elongated generally cyKndrical 

 form, and in the uniform or monotonous external appearance. 

 The Nematoda show nearly always some taper toward one or both 

 ends, being thus spindle-shaped rather than truly cyKndrical, and 

 possess a smooth, glistening, colorless external surface. The Gor- 

 diacea are larger, more uniformly cyKndrical with blunt rounded 

 ends and an exterior at least faintly colored in whole or in part. 

 The Acanthocephala show a roughened surface sometimes with 

 imperfect rings around the body, and the form usually Kke a 

 carrot is always somewhat irregular. These differences are general 

 and subject to exception but with practice one can usually separate 

 members of the three groups at sight, and the structure is so differ- 

 ent that it is wise to consider each group separately in an inde- 

 pendent section of the chapter. 



Biologically the three classes show certain contrasts. The Nema- 

 toda include many free-Kving forms and many others purely 

 parasitic, but most of the latter have brief free-living stages during 

 which they achieve the transfer to a new host. The Gordiacea are 

 parasitic during early Kfe and spend the adult existence free in 

 water bodies. The Acanthocephala are among the most highly 

 specialized of parasites as they have no free-living stages at all 



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