CHAPTER XIII 



PHYLUM NEMATHELMINTHES 



This group is known as the unsegmented roundworms or thread- 

 worms. Some of the Nemathelmiuthes (nem a thel min'thez, thread- 

 worms) are free-living in soil, fresh water, and salt water; some are 

 found living in plant tissues ; and others live in animal tissues as 

 parasites. The majority of them are microscopic, but a few are 

 macroscopic in size. These worms are long, slender animals whose 

 bodies are more or less cylindrical but tapering toward each end. 

 The range of length is from i/4 mm. to four feet. They differ from 

 the flatworm not only in shape, but also in that the intestine has 

 two openings, there is a dorsal as well as a ventral nerve cord, they 

 are mostly dioecious, and there is a total absence of cilia. They 

 also lack respiratory and circulatory systems, true coelom, and 

 definite locomotor organs. The group is very widely distributed 

 and is deserving of considerable attention. Some of the better 

 known forms are Ascaris (pigworm or eelworm), "horsehair snake," 

 hookworm, pinworm, Trichinella, Filaria, Guinea worm, whipworm, 

 and eye worm. The former will be discussed in some detail in this 

 chapter, and several others will be considered in the chapter on 

 Animal Parasitism. 



Classification 



Three classes are usually recognized, although some authors prefer 

 to use only two. The three classes are Nematoda, Gordiacea, and 

 Acanthocephala. 



Class Nematoda (nem a to'da — threadworm) is a group occupy- 

 ing almost every possible habitat capable of supporting life. There 

 are many free-living, fresh water, marine, and soil-inhabiting spe- 

 cies, and large numbers of parasitic forms living at the expense of 

 other animals and plants. This is a very important class parasiti- 

 cally. In size they range from %o mm. to more than a meter in 

 length. Locomotor organs are found in a few forms, no segmentation 

 is present, and there is no true coelom. Chemical sense organs called 

 amphids are nearly universal, while eyes and tactile organs are com- 

 mon in the free-living forms. The skeletonlike cuticle, common to 

 all, is shed periodically like the molting of arthropods. The nervous 



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