CHAPTER XV 

 FREE-LIVING NEMATODES 



BY N. A. COBB 



U. S. Department of Azrkulture 



PRACTICALLY any collection of sand, mud, debris or aquatic vege- 

 tation, from standing or running water, in any part of the coun- 

 try, will yield, on examination with a hand lens, minute slender 

 organisms which whip themselves about by means of more or 

 less rapid contortions of the whole body. This type of 

 movement identifies them as nematodes; it differs from that 

 of other small organisms in that, though often vigorous and con- 

 spicuous, it is in one plane only, the dorso-ventral plane of the 

 body, and in that the length and proportions of the body mean- 

 while remain unchanged. In pure water, moreover, this thrash- 

 ing about seems to produce no locomotion; the animal remains 

 in the same spot unless among vegetation, debris or particles 

 of soil. When quieted by stupefying or killing, these fresh- 

 water nematodes (" threadworms " or " roundworms ") are seen 

 to be more or less cylindroid unsegmented, without locomotor 

 appendages, varying in length up to a centimeter or more. 

 They belong to a group in the animal kingdom comparable in 

 number and importance with the insects; nematodes of other 

 sorts live free in the soil, and in the sea, and infest as parasites 

 an immense variety of plants and other animals. They are in- 

 conceivably abundant. 'A tablespoonfui of ooze from the bottom 

 of the ocean may contain thousands of specimens. The number 

 of nematodes in the top six inches of an acre of ordinary arable soil 

 is shown by statistical calculations to reach thousands of millions. 

 The number of eggs vastly exceeds even that of adults; for they 

 are usually very prolific, a single female sometimes producing 

 hundreds of thousands of eggs. 



Even the free-living soil and water nematodes have become 

 adapted to an astounding variety of habitats; they occur in arid 

 deserts, at the bottom of lakes and rivers, in the waters of hot 



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