96 NEMATHELMINTHES 



Filaria equina infests the serous cavities and has been found in the 

 aqueous humor and causes opaque cornea in the horse. 



The female Guinea worm {Dracunculus medinesis) is about thirty- 

 six inches long and as large as packing twine. It produces abscesses 

 under the skin in which the worm is coiled. Embryos must enter 

 water and penetrate the microscopic crustacean Cyclops. The larva 

 reaches man through drinking water containing Cyclops. Guinea 

 worms occur in Asia, Africa, and tropical America. 



Mermithidae. — The hairworm, Mermis subnigrescens, is a 

 parasite of the common grasshopper. The grasshoppers swallow 

 the eggs with their plant food. The hairworm Agamermis decaudata 

 also infests grasshoppers. Cobb and his associates have suggested 

 that control of grasshoppers as pests may be aided by parasitizing 

 them, thus causing sterility and death. They found that a high 

 degree of parasitism caused all developing hairworms to become 

 7nales and a low degree of parasitism resulted in the parasites all 

 being females^ with a gradient (see page 84) between the extremes, 

 corresponding to the degree of parasitism. The Mermithidae 

 constitute a very large group, others of which infest injurious insects, 

 such as mosquitoes, ants, cutworms, and the like. 



Forms Uncertain in Position Formerly Classed with the Nema- 

 thelminthes — Acanthocephala. — In this group of " thorn-headed " 

 worms, we find a protrusible rostrum or proboscis with five or more 

 rows of recurved hooks. An alimentary canal is absent. An un- 

 paired cerebral ganglion is present. 



The larva of Echinorhynchus gigas lives in the larva of the June 

 bug (Melolontha). The adult worm is found in the pig, attached 

 when full grown to the wall of the small intestine. It is dioecious, 

 i.e., the sexes are separate. The ovaries of the female break up into 

 free floating egg groups. The uterus picks up immature and fer- 

 tilized eggs indiscriminately, but only the elongate, shelled ones may 

 pass the canals; immature eggs are led by a ventral opening back to 

 the coelom. In E. gigas, protonephridia open beside the genital 

 opening. The oviducts of the female and the penis of the male are 

 at the posterior end. 



Gordiaceae. — These minute, slender animals are commonly 

 known as " horse-hair worms." They have an esophageal nerve 

 ring, a ventral nerve cord and the female genital opening is at the 

 cloaca. The larvae infest insects and the adults live in water, 

 twining around plants and depositing their eggs. 



