INTRODUCTION ix 



Gordiacea, the Acanthocephala, and possibly some other small 

 groups of very uncertain affinities. Without entering into 

 the vexed question of the " naturalness " of this arrangement, 

 we propose here to deal only with the Nematoda proper, to 

 the exclusion of all the other groups. 



The primary divisions of the Class Nematoda are treated 

 as Orders, corresponding broadly to some of the " Super- 

 families " originated by Railliet and others, and now accepted 

 by the majority of helminth ologists. The classification we 

 have adopted starts with the assumption, which seems to us 

 to be well-founded, that the forms with three lips, or obvious 

 modifications of three lips, are to be regarded as primitive. 

 The majority of these have been grouped together in one 

 large Order, Ascaroidea.* 



The other Orders are regarded as collateral branches that 

 have originated independently of each other from the original 

 stock now represented by the Ascaroidea. Thus the Strongy 

 loidea still retain evidences of the three-lipped arrangement, 

 although they have become considerably modified in other 

 directions ; they have a true bursa in the male, and a definite 

 system of bursal rays which is not paralleled among the 

 Ascaroidea. There is a striking resemblance between the 

 free-living larval stages of Strongyloidea generally, and 

 particularly of the forms with a buccal capsule, and the adults 

 of many of the free-living terricolous forms here included in 

 the Ascaroidea. A similar resemblance exists between the 

 larvae of the Mermithidae and the adults of the Anguillulinidae. 



The Filarioidea have departed much more widely from the 

 primitive type, having lost all traces of the triradial sjmimetry 

 of the lips, except in retaining the usual number (six) of 

 cephalic papillae. This divergence may possibly be connected 

 with the introduction of a complex life-cycle involving an 

 intermediate host. The two remaining Orders (Dioctophy- 

 moidea and Trichinelloidea) are small, very highly-speciaUzed 

 groups whose relationships cannot easily be determined. 



A survey of the whole of the Nematoda shows that there 

 has been too great a tendency on the part of workers with 

 comparatively restricted interests to raise the rank of the 

 small groups with which they were dealing. For instance, 

 Cobb, in a recent publication, divides the free-living Nematodes 

 alone into two subphyla with classes, subclasses and orders. 

 One of his subphyla, the Alaimia, corresponds to the family 



* The use of the name Ascaroidea for this group should not be taken 

 to imply that the Ascaridae are regarded as its most primitive members. 

 These are undoubtedly to be found among the free-living genera, and 

 the Ascaridae, apart from the primitive arrangement of the lips, are 

 very highly specialized in accordance with their strictly parasitic mode 

 of life. 



