SEGMENTATION OF THEIR ORGANS 



119 



One recalls that a number of observers have noted the presence 

 of longitudinal series of repetitive organs in the lateral fields of nem- 



sct Cfh 



set cph subin 



Fig. 4. Scaptrella cincta n.g. 

 n. sp. 1 and 2 illustrate re- 

 spectively the inflexed and ex- 

 tended attitudes of one of the 

 jointed mandibles. Scaptrella 

 has the general appearance of 

 Axonolaimus de Man '89, but 

 has jointed mandibles, circular 

 amphids, a single ovary, and 

 spicula with only faint accesso- 

 ries. No supplementary organs. 

 Characters of S. cincta, type 

 species, given in formulae and 

 illustrations. Formula explain- 

 ed on p. 7. 



atodes, though attention has never been called to the fact that these 

 organs on opposite sides of the body may be symmetrical to each 

 other. Sometimes they are exactly so. 



How is the prominent trilateral 

 symmetry of the nematode head to 

 be expressed in terms of bilateral 

 symmetry? Selachinema and Cheir- 

 onchus assist in answering this ques- 

 tion. A second species of Selachi- 

 nema, not yet described, has little or 

 no trace of the vestigial dorsal jaw 

 present in S. vorax (p. 113), so that 

 the two projecting mandibles become 

 practically lateral, and are bilaterally 

 symmetrical. An even more com- 

 plete transformation occurs in Cheir- 

 onchus (Fig. 5), where by a complete 

 disappearance of the dorsal sector of 

 the pharynx the submedian ones 

 have become truly lateral. In both 

 these genera the resulting transfor- 

 mation to a two-jawed animal gives 

 rise to symmetrical mandibles, acting w 

 from side to side. 



It WaS, Of COUrSe, Conceivable that 



any such transformation might take 



the form of a union of the submedian SS'J formX! pecie8 ' * lven 



Mi/til. 



Fig. 5. Cheironchus vorax n.g., n.sp., a nema 

 hose bilaterally symmetrical mandibles have 

 Arisen from the ventrally sub'median sectors 

 of the head at the same time that the dorsal 

 elements of the pharynx have disappeared. 

 Cheironchus has the general appearance of Selach- 



