soil nematodes parasitic in plants grew very slowly. 

 Beginning in the 1 940's, however, Western countries 

 have been greatly stepping up their efforts at nema- 

 tode research, having suddenly begun to appreciate 

 the ways in which modern farming methods have in- 

 tensified the competition between man and the nem- 

 atodes for large agricultural crops. Parasites are usu- 

 ally highly specialized, able to live on only one or a 

 few closely related species of host, so that originally 

 soil nematodes found their wild plant host sparse and 

 widely scattered. Then man discovered agriculture 

 — how to grow edible plants in such dense concen- 

 trations as to make it worth while for the big human 

 animal to feed on even the smallest grains. 



Nematode head with hooks (top) 

 and whole woim 



The nematode threat must have grown slowly 

 through the centuries, until recent methods for plant- 

 ing vast acreages to the same crop, and the explosive 

 expansion of the human species, created unlimited 

 horizons for nematode hangers-on. Though round- 

 worms parasitic in man share only a small part of his 

 food after he digests it, the soil nematodes parasitic 

 in plants take more than a tenth of the crops grown 

 by American farmers, for example, even before the 

 harvest. The damage in the United States is estimated 

 at $500,000,000 each year; in Great Britain the an- 

 nual loss of potatoes alone is judged to cost about 

 £2,000,000. Since the worms increase with the years 

 and with crop size, the best remedy is crop rotation 

 to deny the nematodes access to their host. Much of 

 what was in the past attributed to soil exhaustion, to 

 be cured by crop rotation, was in reality nematode 

 damage, especially by those nematodes that pierce 

 plant roots and suck the juices. The plant symptoms 

 are wilting, stunted growth, leaf discoloration, and 

 root swellings or galls — none of these specific to nem- 

 atode infestation or always easy to tell from losses 

 due to drought or a lack of soil nutrients. Our con- 

 centration on methods for treating these last prob- 



lems has delayed appreciation of the role of parasitic 

 worms. 



Lists of animal numbers usually credit the nema- 

 todes with about ten thousand known species. The 

 true number of existing species is estimated to be 

 about five hundred thousand — second only to the in- 

 sects. The discrepancy is easily explained. The larger 

 number takes into account all the as yet unexamined 

 but highly specialized parasites of many thousands 

 of vertebrates, invertebrates, and plants, plus all the 

 free-living forms, judged to outnumber the parasites. 

 Nematodes are typically minute, cylindrical, tapered 

 at both ends, covered with a tough cuticle that is 

 transparent or translucent; they usually thrash about 

 in a way that immediately identifies them to the eye, 

 and they look so much alike, even under the micro- 

 scope, that the job of distinguishing and naming so 

 many superficially similar species makes even the 

 experts stand back. 



Nematodes used to be studied either as parasites 

 or as free-living worms, and students of one group 

 often paid scant attention to the other. Since the hab- 

 its of nematodes, like those of most animals, do not 

 necessarily correspond to the evolutionary relation- 

 ships on which classification must rest, the grouping 

 of these worms has had recently to undergo exten- 

 sive repairs in order to combine the two kinds of 

 worms. For details one may refer to Volume III of 

 the treatise by Hyman, to the specialized volumes 

 on nematodes by the Chitwoods, or to Chandler's 

 very readable text on parasitology. Here we shall 

 merely present some points about roundworms in 

 general and then go on to discuss a few kinds of 

 worms of special interest. 



Nematodes occur in two general forms. The really 

 long, threadlike ones, that have hardly any taper, 

 are greatly outnumbered by the shorter spindle- 

 shaped forms, which taper markedly to blunt or slen- 

 der tips, the rear end often the more tapering and 

 pointed. Especially in the minute forms, the animal 

 is colorless and the cuticle is transparent, putting the 

 internal organs on full display. Or the cuticle is trans- 

 lucent and lends a whitish or yellowish cast. There 

 are no cilia, outside or in, and in parasitic forms the 

 cuticle is often very smooth; but it may be finely 

 striated, or bear bristles, spines, ridges, or other 

 markings and expansions. The mouth is at the front 

 tip, surrounded by little sensory lips, which may also 

 be muscular and used in sucking. Just inside the 

 mouth there may be cutting ridges, teeth, or piercing 

 stylets for puncturing plant or animal prey. Beyond 

 these there is usually a short muscular pharynx that 

 sucks food into the intestine. The sexes are almost al- 

 ways separate; and the smaller male bears special 

 equipment at his slightly curved rear end. The stitT 

 cuticle and the lack of any but lengthwise muscles 

 permit only serpentine undulations for swimming or 



134 



