2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 89 



Probably the most convenient systematic arrangement would be 

 one which treated the Aschelminthes as a phylum or subphylum, if 

 one is inclined to retain the useful group Vermes as a phylum. The 

 Nematoda would then be a class. But if one considers the Nematoda 

 as a phylum, it seems impossible as yet to distinguish classes within 

 the group. In the animal kingdom a class is a unit of very high 

 morphologic differentiation. Such differences do not seem to exist 

 among the various groups of Nematoda, their organization being 

 very uniform when compared with the classes of a well-established 

 phylum, such as the Mollusca or Vertebrata. If it is a phylum, it is 

 a phylum with only one class. 



The class Nematoda could be defined as having, in addition to 

 the chief characters shared with the other Aschelminthes, the fol- 

 lowing characters: 



Small worms, consisting of comparatively few cells ; body 

 elongated, bilateral in structure, although without a physiological 

 differentiation between dorsal and ventral sides ; with a hard external 

 cuticula ; with a single layer of longitudinal muscle cells, divided by 

 two large lateral epidermal thickenings, the lateral cords ; a muscular 

 esophagus, usually with a triangular lumen; separate sexes; double 

 genital organs, the female genital opening being situated in the ven- 

 tral line, as a rule, at varying points from anterior to posterior; in 

 the male the terminal genital tract and the posterior intestine unite 

 and have a common aperture ; with a very feeble regenerating power 

 and an absence of cilia. 



There are several exceptions to this definition, but they are 

 manifestly secondary. The purpose here is neither to discuss these 

 exceptions nor to advance theories as to how they arose. The 

 feature which should be expressly pointed out is the importance of 

 the cuticula in the nematode history. It could be considered as 

 differentiating the Nematoda as a separate group, because their 

 simple musculature, and, in this connection, the peculiar mode of 

 motion, the absence of power of regeneration, and the peculiar biol- 

 ogy of many of them, are possible only by virtue of the presence of 

 this cuticular armor. 



The classification of the Nematoda is in a rather bad state. Much 

 has been gained in the last 15 years in arranging the parasitic forms, 

 and several systems have been proposed for the free-living ones. 

 But inasmuch as the systems for the two groups are widely different, 

 an attempt to combine the two in one whole was made only by 

 Baylis and Daubney, but it must be regarded as wholly inadequate 

 where it deals with the free-living forms. 



