PARASITIC ROUNDWORMS 521 



2 (3) Free-living in moist earth or water. Many embryonic and larval 



stages of parasitic nematodes. 



Not distinguishable from free-living nematodes except by exact data concerning specific 

 forms which are available only in a few cases. Such are the larvae of the human hookworms 

 (Ancylostoma duodenak or Necator americanus) , of Strongyloides stercoralis, known to be present 

 generally in infected areas. They depend for their development upon the opportunity of 

 entering a new human host. 



3 (2) Encysted in the viscera or flesh of various fishes. 



Agamonema capsularia (Rudolphi) 1802. 



The name covers what is probably a wide variety of different species from different sources. 

 Thus worms under this name are listed from migratory fishes, and these are very likely to 

 represent encysted larvae of marine adults; and also from fresh- water fishes in which case 

 they are doubtless of fresh-water origin. The descriptions of these forms are brief, general, 

 and inadequate to differentiate larval forms of different genera. 



Among the other species recorded from North America are: 



Agamonema papittigerum, a single specimen of which was found by Leidy in Philadelphia, 

 in the body cavity of a pike. Later regarded by him as young Filaria solitaria. 



Agamonema piscium from the white fish, listed by Stiles and Hassall in the collection of the 

 Army Medical Museum. 



Such forms may be found in other hosts than fishes like the embryos recorded by Leidy as: 



Nematoideum integumenti lumbriculi limosi, encysted in the skin of a mud-inhabiting annelid. 



4 (i) Mature. Sexual organs developed; worms active, not encysted. . . 5 



Most adults are easily recognized as the eggs can be seen in the female and the sperm mass 

 in the male. The open sexual pore in the female and the spicules in the male when exserted 

 aid in reaching a diagnosis. 



5 (6) Small transparent nematodes; in general appearance identical with 



free-living forms. Few eggs in uterus. 



Not a very satisfactory means of separating this group from certain species in the subse- 

 quent divisions which approach rather closely to the brief description of the key line above. 

 In case of doubt regarding a specimen the student should try also the latter alternative, 6 (5) 

 of the key. 



These forms are all minute (less than 5 to 6 mm. long). Furthermore they are simple in 

 structure and not easy to differentiate from, free-living species. They possess a double esoph- 

 ageal bulb and ventral glands often in lieu of lateral excretory canals. The male has two 

 similar spicules and in some cases a bursa. The female sexual pore is found in the posterior 

 half of the body and the uterus contains only a few thin-shelled eggs. 



One family, the Anguillulidae, includes the vinegar eel, the paste eel, various plant para- 

 sites of some economic importance, and many free-living* forms. These do not show any alter- 

 nation of generations in the life history. 



Only group containing animal parasites. 



Family ANGIOSTOMJDAE Braun 1895. 



Characterized by heterogony. Otherwise very much like the Anguillulidae, and united 

 to them by many authors. Parasitic generation contains no males. 



Only genus recorded for North America. . Angiostoma Dujardin 1845. 



Representative species in North America. 



Angiostoma nigrovenosum (Goeze) 1800. 



In Bufo kntiginosus; lung. District of Columbia. Listed by Stiles and Hassall under the 

 name Rhabdonema nigrovenosum as in the Bureau of Animal Industry Collection. 



6 (5) Nematodes larger than free-living species; almost always distinctly 



less transparent and often even opaque. Uterus contains 

 many eggs 7 



The unsatisfactory character of the key at this point has already been noted. The nema- 

 todes which follow are usually well differentiated parasites, recognizable by one or another 

 typical structure not present in the previous group. They are, however, distinguishable from 

 the latter only in general aspect and the key is open to doubt in a few cases. 



