APPENDIX B 



639 



these live within the bodies of other animals, and many of them cause diseases 

 of man and domestic animals. As in the instance of other parasitic animals, they 

 show great specialization related to their peculiar mode of life, such as enormously 

 increased powers of reproduction and extremely complicated life cycles that in 

 certain cases involve three or four different larval forms, each requiring a different 

 host. 



Approximately 5,000 species of the phylum are known. 





Fig. B.8. Roundworms (Nernathelminthes, class Nematoda). Left, the vinegar eel, Turba- 

 trix, a free-living form. Right, the larvae of Trichinella spiralis, the trichina worm, encysted 

 in muscle. If meat containing encysted larvae is eaten by a susceptible mammal (including 

 man) the larvae are freed by digestion of the meat, grow to maturity and mate in the 

 intestine. The female worm produces as many as 1,500 minute larvae. These enter the 

 lymphatics in the intestinal wall, pass to the blood, and are carried to various parts of the 

 body of the host, where they burrow into the tissues and encyst. Heavy infestation causes 

 the sometimes fatal disease trichinosis. (Courtesy General Biological Supply House, Inc.) 





Phylum VI. NEMATHELMINTHES (nem' a thel min' thez; Greek, 



nematos, "thread," and helminthos, "worm"). 



The unsegmented roundworms or threadworms. Intermediate Metazoa, 

 exhibiting for the most part the following characters: 



Triploblastic ; bilateral symmetry; no metamerism. Digestive tract 

 complete, both mouth and anus being present. Body cavity or coelom 

 present, though incomplete or atypical in not being lined throughout 

 with mesoderm. Body elongated, cylindrical, usually pointed at both 

 ends. Body organization of the most primitive tube-within-tube type. 



