BRANCH NEMATHELMIN'THES 



Round- or Thread-worms.— The worms of this branch are 

 elongated and cyUndric and have a coelom or body cavity. 

 The vinegar-eel affords a good example. They differ from 

 annelids in that they are not divided into segments or rings. 



CLASS I. NEMATODA 



The members of class Nemato'da are best known as para- 

 sites, but there are many fresh-water and marine forms. 



The tough body wall encloses a body cavity which surrounds 

 a straight alimentary tube having a terminal mouth and a 

 ventral anal opening. An excretory system is usually present. 

 The nervous system consists of an esophageal nerve ring which 

 sends out six nerves anteriorly and six posteriorly. The only 

 sense organs are sensory papillae on the lips. The sexes are 

 usually separate. Many of the aquatic forms are free. Some 

 of the parasites infect plants, as Tylen'chus trit'ici, which does 

 great damage to wheat, and Heterode'ra schach'tii, to turnips in 

 Europe. 



One form, Ascaris nigrovenosa,^ living a parasitic life in the lungs of 

 frogs and toads, is hermaphroditic. The embryos reach the alimentary 

 canal and pass out with the waste material. In water they develop into 

 a stage in whieh the sexes are separate. The eggs dcveloji in the body of 

 the female and devour the entire substance of the tissue of the mother, 

 leaving only the cuticle. When set free they live in the mud until they 

 are taken into the mouth of a frog, when they pass into the lungs and 

 develop into the hermaphroditic stage. Here, again, is a peculiar alterna- 

 tion of generations (heterogeny), the alternation of an hermaphroditic with 

 a dioecious form. 



Trichinel'Ia spiralis (Fig. 31) is another member of this class. In the 

 adult stage it lives in the alimentary canal of man or of other mammals. 

 The length of the adult male is about jV inch, and that of the female about 

 5 inch. The sexes are separate. The young, at least one thousand, are 

 born alive. The young worms (Figs. 32, 33) pass through the intestinal 

 wall and make their way to the voluntary muscles, where they penetrate 

 the sarcolemma and become encysted. 



1 Parker and Haswell, vol. i., p. 286. McMurrich, p. 176. 



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