.520 FRESH-WATER BIOLOGY 



of North American forms makes it impossible to do more than 

 group the few records available into an arbitrary key. A natural 

 classification lies far in the future and collecting in any region will 

 surely result in extending greatly the list of species included in the 

 subjoined key. Parasites from terrestrial hosts have not been 

 included in the synopsis; doubtless many of them depend upon 

 water for their transfer during a free-living stage from one host to 

 another and some of them may even utilize aquatic species as in- 

 termediate hosts, just as the guinea worm larva occurs in a fresh- 

 water copepod and reaches the human host in drinking water. 

 The nematode parasites of fishes, amphibians, and aquatic species 

 among reptiles, birds, and mammals may safely be assigned to the 

 fresh-water fauna. They are included here so far as described from 

 North America. 



Undoubtedly the larvae of the hookworms (Ancylostoma duode- 

 nale and Necator americanus of man; Uncinaria stenocephala of 

 the dog), of the parasite of Cochin China diarrhoea (Strongyloides 

 stercoralis) , and of many other parasites which occur in North 

 America are aquatic organisms and live for considerable periods 

 in pools of water or in moist earth, awaiting an opportunity to 

 gain entrance into a suitable host. Yet as immature forms they 

 can be differentiated with great difficulty if at all, and do not show 

 the structural features that characterize the adults to which they 

 belong. Hence they are only noted collectively in the subjoined 

 key. The adults which parasitize land animals are not included 

 in the list. 



KEY TO NORTH AMERICAN PARASITIC NEMATODA 



i (4) Immature. Sexual organs only partly developed, if at all. 



Agamonema Diesing 1851 . . 2 



A collective name for the group of imperfect, larval nematodes not yet developed so that 

 the worms can be definitely classified. Many such forms occur encysted in fish, and the group 

 was originally proposed to hold fish parasites. Now it is used to include all agamic nema- 

 todes that cannot be referred to a more definite group. The rudiment of the sexual organs 

 can usually be seen as a large cell or a discrete mass of a few cells, lying near the center of the 

 worm. In older individuals this sexual rudiment has begun to grow out into a long cord of 

 cells which marks the place of the future reproductive system. In these forms the lips, papil- 

 lae, and other features of special adult structure are wanting or only generally and indefinitely 

 laid down. Sometimes distinct characters, such as the three lips of the Ascaridae which are, 

 nowever, easily confused with similar conditions in other groups, may enable one to assign 

 these immature forms to a definite family, subfamily, or genus, and other collective names 

 are then applied to such forms, e.g., Agamomermis. These larval forms are very similar and 

 are apt to be confused because of their general resemblances. 



