FREE-LIVING FRESH-WATER NEMATODES 37 



tically only in the vermiform appendix of man; another has its 

 adult form only, in the seeds of wheat. A third form occurs in 

 the felt mats on which the Germans are accustomed to set their 

 mugs of beer, and has been found in no other habitat. On the feet 

 of birds and insects the eggs, larvae, and adults of certain nema- 

 todes are carried to the tops of the tallest trees. The sour sap is- 

 suing from the wounds of a tree, often many feet above the ground, 

 not infrequently contains nematodes that are specific to the wounds 

 of that particular kind of tree. The tap water of even well-con- 

 ducted cities often contains nematodes. 



Nematodes are inconceivably abundant. A thimbleful of mud 

 from the bottom of the ocean may contain hundreds of specimens. 

 The number of nematodes in the top six inches of an acre of ordi- 

 nary arable soil amounts to thousands of millions. Statistical cal- 

 culations relative to the number of nematodes in a single acre of 

 soil near San Antonio, Texas, U. S. A., disclosed that if they could 

 start in a procession for Washington, D. C., two thousand miles 

 away, each close on the tail of the one in front, the head of the 

 procession would reach Washington before the rear had left San 

 Antonio. As nematodes are usually very prolific, a single female 

 sometimes producing thousands of eggs, the number of eggs vastly 

 exceeds that of the adults". 



We must therefore conceive of nematodes and their eggs as 

 being carried by the wind, and by flying birds and running animals ; 

 as floating from place to place in nearly all the waters of the earth ; 

 and as shipped from point to point throughout the civilized world 

 in vehicles of traffic. 



There are beneficial nematodes, though knowledge of this phase 

 of the subject is in its earliest infancy. Some nematodes feed ex- 

 clusively on their injurious brethren. Others devour baneful micro- 

 organisms. Their adaptations and relationships appear to be sim- 

 ilar to those of insects. 



NORTH AMERICAN FRESH-WATER SPECIES. 



The nematodes here described are the main portion of those 

 found during a rather casual search for aquatic species that would 

 serve as the basis of a special chapter in a zoological textbook de- 

 voted to fresh water organisms. The object of the textbook was 



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