510 FRESH-WATER BIOLOGY 



reference to the longer discussion in the opening paragraph of 

 this chapter. 



A (B) With anterior, protrusible proboscis covered with rows of recurved 



hooks Class Acantbocephala (page 542) 



B (A) Without proboscis at anterior end C 



C (D) Adult free-living, aquatic, long, cylindrical, with posterior end bifid 

 or bluntly rounded Class Gordiacea* (page 535) 



The family of the Mermithidae (page 534) agrees in some of 

 these particulars with the Gordiacea, although the structure shows 

 that these species are true Nematoda and not Gordiacea; they are 

 readily distinguished by the acutely pointed posterior end and 

 terrestrial habit. 



D (C) Adult usually spindle-shaped, tapering rather than cylindrical. Pos- 

 terior end never bifid or bluntly rounded, usually acutely 

 pointed, occasionally peculiarly modified in form. 



Class Nematoda . . E 

 E (F) Free-living during entire life cycle. Adults small, transparent. 



Free-living Nematoda (page 459) 



F (E) Parasitic during most or all of the life cycle. Larvae small, transpar- 

 ent; adults variable in size, often more or less opaque. 



Parasitic Nematoda 

 PARASITIC NEMATODA 



The nematodes are easily recognized by their appearance, which 

 has given them the common name of round- or threadworms. 

 Most of them are small, measuring only a few millimeters in length 

 and a fraction of a millimeter in diameter, and resemble a fragment 

 of a violin string. A few of the larger sorts reach a length of 

 several centimeters or even a meter. The external surface is usu- 

 ally smooth and glistening and the body is not divided into joints 

 or segments. In some cases a fine surface striation is present 

 which appears under a lens as delicate circular grooves; the 

 exterior may also bear irregular beaded tubercles or fine scales, 

 spines, or hairs. When present these are usually confined to 

 certain regions and the remainder of the surface has the typical 

 nematode appearance. 



The body tapers slightly towards one or both ends and only 

 very rarely can one find marked differences in diameter or dis- 



* Some authors designate the class Nematomorpha and rank the Gordiacea as an 

 order under it. 



