120 SYSTEMATIC ZOOLOGY 



SUBTYPE III. NEMATHELMINTHES 



The Nemathelminthes (Gr. vr/fxa, thread, and e\fxiv<;, worm), 

 or threadworms, are readily distinguished from the two preced- 

 ing types by their cylindrical bodies, which usually are very 

 long in proportion to their diameter and tapering at the ends. 

 Internally the chief difference lies in the well-marked body- 

 cavity. The sexes are usually separate, and in the few species 

 which are hermaphroditic the individuals are first male, produc- 

 ing spermatozoa only, and in later life female, producing ova 

 only. Most of the Nemathelminthes are parasitic, chiefly on 

 animals, but in some cases on plants, though many are free- 

 living, occurring in fresh and salt water and in moist earth. 

 Two classes are generally included under this group, but they 

 differ from one another in several points of considerable im- 

 portance, so that some zoologists are inclined to place them in 

 separate subtypes. 



CLASS I. NEMATODA 



The Nematoda (Gr. v>]/xaTa)8i]<;, threadlike) are long, thread- 

 like worms, varying from practically microscopic individuals to 

 the Guinea worm, which may be from thirty centimeters to two 

 meters long. They are provided with a mouth at the anterior 

 end of the bod}', leading into an alimentary canal, which ter- 

 minates at the anus on the ventral side near the posterior end. 

 The surface of the body is smooth, and eyes occur in the free- 

 living species only. The excretory organs are small tubes, and 

 the reproductive organs are likewise very simple, consisting in 

 both sexes of elongated tubules lying in the body-cavity, which 

 open in the female near the middle of the body on the ventral 

 side, and in the male at the posterior end, usually near the anus. 

 The males are generally much smaller than the females. 



Most of the free-living Nematoda are small. The vinegar eel. 

 which sometimes occurs in vinegar and paste, is only one or two 

 millimeters long. As an exception, there is one genus, Gordius, 

 usually classified with the Nematoda, which may attain a length 

 of fifteen centimeters; it lives in fresh water, while the imma- 

 ture stages of its development are passed in the bodies of various 

 water animals which serve as its host. Several of the parasitic 





