460 FRESH-WATER BIOLOGY 



springs and in polar seas. They were thawed out alive from 

 Antarctic ice by members of the Shackleton expedition. An exam- 

 ination of beet seeds imported into the United States disclosed 

 the presence on them of several species of nematodes. The tap 

 water of even well-conducted cities often contains nematodes. 

 Their microscopic eggs and larvae, even more readily than the 

 adults, are transported from place to place by an exceedingly great 

 variety of agencies. They are carried by the wind, by flying 

 birds and running animals, they float in all the waters of the earth, 

 and are shipped from point to point throughout the civilized world 

 in vehicles of traffic. Sometimes the eggs and larvae are so re- 

 sistant to dryness that if converted into dust they revive again 

 when given moisture, even after as long a period as a quarter of 

 a century. There are beneficial nematodes, though knowledge of 

 this phase of the subject is in its infancy. Some nematodes feed 

 exclusively on their injurious brethren. Others devour baneful 

 micro-organisms. Their adaptations in these respects appear to be 

 similar to those of insects. 



The small fraction of the fresh- water nematodes of North America 

 at present known, comprises only about thirty genera, but these 

 present such a variety of form that a thorough knowledge of them 

 insures a fair understanding of all the free-living nematodes.* 



The number of nematode species is enormously greater than 

 commonly supposed. Since most species of vertebrates are in- 

 fested by one or more nematodes, and with comparatively few ex- 

 ceptions a given parasitic nematode infests but one host, it may be 

 estimated that more than 80,000 nematode species infest the forty 

 odd thousand species of vertebrates. Insects, also much infested, 

 will add many thousands of species. The molluscs, crustaceans, 

 and various groups of worms are also infested, and investigation 

 continues from this source also to augment the number of known 

 species of parasitic nematodes. 



Numerous as the parasitic species are, it is certain that 

 the species of nematodes living free in soil and in water far 



* In an attempt to distinguish the parasitic nematodes from the aquatic and soil- 

 inhabiting nematodes, the latter are usually assigned to the group of free-living nema- 

 todes, an arbitrary classification not based on natural relationships. 



