PHYLUM NEMATHELMINTHES 133 



being introduced into the digestive system of some other mammal, 

 for the larvae have no independent means of ever leaving the 

 body of the host individual which sheltered the mature female 

 in its intestine. The chief source of the adult worms in the 

 human intestine is from the larvae encysted in pork, while 

 the hog in turn receives its intestinal form through eating the 

 viscera of other hogs in slaughter yards or from eating rats 

 which have become infested. 



Blood-inhabiting nematodes such as the Filariae are often 

 carried from one host to another through the bite of blood-sucking 

 insects. Some species which cause elephantiasis through the 

 occlusion of blood and lymph vessels are carried by mosquitoes. 



Free-living Nematodes. — There are numerous species of 

 free-living nematodes representing many genera. In the genus 

 Iota the body is covered with scales which give it a superficial 

 appearance of segmentation. Tylenchus, Dorylaimus, Mermis, 

 and Rhabditis are names of characteristic genera of free-living 

 nematodes, though species in some of these genera may be 

 parasitic. 



Class Gordiacea 



Superficially, the Gordiacea resemble the nematodes, but in 

 finer details of structure they have little in common with them. 

 Some systematists are inclined to disregard these differences 

 and therefore include the Gordiacea as a subclass of nematodes. 

 The adults live free in the water where the females lay strings of 

 eggs which develop into small larvae. These larvae enter the 

 body of some insect where they undergo development to the 

 adult body form. 



The hair snakes, so commonly found in watering troughs 

 and in ponds and streams, are the adults of Gordiacea which 

 have escaped from the bodies of crickets or other insects. They 

 bear no lateral lines, and the nearly cylindrical body with blunt 

 anterior extremity and irregularly roughened cuticula serve to 

 differentiate the Gordiacea from the nematodes. 



Gordius, Paragordius, and Chordodes are genera in this class. 



Class Acanthocephala 



The Acanthocephala are absolutely parasitic in habits. There 

 is no trace of digestive organs in the mature worms and even in 

 the development of the larva there are no specializations of the 



