CHAPTER XVII 

 PHYLUM NEMATOIDEA 



The thread- worms, or round- worms, are slender and 

 cylindrical in form. They have a tubular digestive system 

 beginning with a mouth and opening at the opposite end 

 through an anus. There is a body-cavity between the 

 digestive tube and the body wall (Fig. 76). Different 

 species of round-worms vary greatly in habits : some are free 

 living, others are scavengers, and many live as parasites in 

 plants or animals. There are three classes: 



m 



-. ' 



fi 

 ft n 



A 

 FIG. 76. Cross-sections of a flatworm (A) and a round-worm (B) compared. 



Class 1. Nematoda, round-worms with a well-developed digestive 

 system and a rather simple life history. 



Class 2. Gordiacea, horsehair worms. Parasitic Nematoidea with 

 the digestive system often partly degenerate; life history very complex 

 and requiring two or three hosts for its completion. 



Class 3. Acanthocephala, hook-headed worms. Parasitic worms 

 without a digestive system and with a proboscis at the anterior end which 

 is armed with spines; in the life cycle there is an alternation between 

 two hosts. 



CLASS 1. NEMATODA 



Most nematodes are. very resistant to variations in the 

 conditions of life, hence it is easy for them to live in all sorts 



183 



