viii INTRODUCTION 



tion. Accordingly, although we have frequently made 

 reference to literature published since 1923, where it has been 

 important for our purpose, we have not included the new 

 genera proposed, unless they appeared to be synonyms of 

 earlier genera, or were merely new names proposed to replace 

 older ones which were preoccupied. 



We consider that the prevailing custom of separating the 

 Nematodes into free-living and parasitic groups leads to an 

 entirely unnatural scheme of classification. All degrees of 

 parasitism, permanent and temporary, are found among the 

 Nematodes, and any such arbitrary division must be made in 

 total disregard of obvious morphological similarities. All the 

 Nematodes resemble each other, in general appearance and 

 structure, far more closely than do, for example, Insects of 

 different orders. Yet within a given order of Insects it is well 

 known that there may be both free-Uving and parasitic forms, 

 and a preliminary division of the Insects as a whole into 

 free-living and parasitic groups would not be regarded seriously 

 by any modern entomologist. In the Nematodes the custom 

 referred to was doubtless adopted by sjiecialists for their own 

 convenience, since it was almost impossible for one worker to 

 be conversant with the literature of the whole group. 



The evidence available points, in our view, to the derivation 

 of all parasitic Nematodes from free-Uving ancestors, and there 

 are indications that the habit of parasitism has been developed 

 not once only, but at various times in the evolutionary history 

 of the group. It is accordingly impossible to point to any one 

 group of free-living forms as the origin of all the parasitic 

 forms. Certain parasitic forms, not closely related to each 

 other, still retain evident traces of primitive structure, and 

 their affinities to free-living genera are clear. For example, 

 a close comiection appears to exist between the semi-parasitic 

 Mermithidae and the Anguillulinidae, and between the parasitic 

 Oxyuridae and Atractidae and the maLnly free-Uving 

 Rhabditidae. 



We have, therefore, attempted to unite the whole of the 

 Nematoda in a common system of classification. It is not 

 suggested that the proposed system is in any way perfect or 

 permanent. It may, however, point the way to a more 

 natural classification in the future, especially if our knowledge 

 of the imperfectly- diagnosed forms becomes more complete. 

 It seems to us convenient, and not in any way unscientific or 

 illogical, to regard the Nematoda as a self-contained Class of 

 the animal kingdom. We do not feel called upon, in a work 

 of this character, to enter into a discussion of the relationships 

 of this Class to other groups. The Nematoda have been 

 generally treated as forming part of a Phylum Nemathel- 

 minthes, which is taken to include also the Nematomorpha or 



