NORTH AMERICAN 



FREE-LIVING FRESH-WATER 

 NEMATODES 



CONTRIBUTIONS TO A SCIENCE OF NEMATOLOGY 



II 



BY N. A. COBB 

 (With eighty illustrations) 



INTRODUCTION. 



The little creatures described in these pages belong to a very 

 important but as yet comparatively little known class of animal 

 organisms, the nematodes. 



Something more than a mere reconnaissance leads to the con- 

 clusion that over nine-tenths of the nematode species still remain 

 unknown, a greater disproportion between the known and the un- 

 known than exists in almost any other class of organisms. 



Nematodes are distributed far and wide in inconceivable num- 

 bers, and without doubt constitute a group in the animal kingdom 

 comparable with insects both in number of species and economic 

 importance. They vary in length from one two-hundredth of an inch 

 to several feet, though the great majority are less than half an inch 

 long. They differ from most other slender, wormlike forms in 

 their lack of locomotive appendages, the outer surface, except for 

 the presence of setae, is usually quite smooth. Being more or less 

 cylindrical in form, they are often called "round-worms." They 

 live free in the soil, in fresh and salt water, and are found parasitic 

 in a great variety of animals and plants. 



NEMATODES AS CAUSES OF DISEASE 



The parasitic species often cause fatal diseases of plants and 

 of animals, including mankind. The dreaded hookworm is a nema- 

 tode. So is that scourge of the tropics, the guinea worm. Trichina,* 



"Trichinella spiralis. 



Reprinted from Trans. American Microscopical Society. Vol. XXXIII, April, 1914. 



