No. 634] PHYLOGENY OF THE ARTHROPOD A 403 



proponent of this association, did not consider either sub- 

 class derivable from the other. Walcott has more re- 

 cently stated that the trilobites were derived from the 

 branchiopods and in this has been followed by Crampton. 

 The points of relationship are: in both subclasses the 

 number of segments is not fixed, and in both there are 

 some species which have large numbers of them; both 

 have a well-developed labrum (hypostoma) ; both have 

 functional gnathobases along the body; the change in 

 metamorphosis of the branchiopod is comparatively 

 small, although Apus is by means a " grown up nauplius," 

 as Bernard put it. 



So far as these similarities are important, they do show 

 a close relationship of the two groups, but none of them 

 indicates that either is more primitive than the other. 

 "When a closer comparison is made, it at once becomes 

 evident that the trilobites are much more primitive than 

 the branchiopods. For example : trilobites have no cara- 

 pace ; some branchiopods do ; trilobites have serially sim- 

 ilar appendages on all segments, branchiopods have very 

 different appendages on the head from those on the 

 thorax, and some of the abdominal segments lack them 

 entirely; trilobites have antennas like the other cephalic 

 and trunk appendages, branchiopods have the antenna? 

 highly modified, degenerate, or absent. In other words, 

 branchiopods are in all these respects much more spe- 

 cialized than the trilobites. Finally, the limbs may be 

 considered. Lankester has shown that the schizopodal 

 limb of the higher Crustacea may be explained as derived 

 from one like that of the thorax of Apus, and most stu- 

 dents of the Crustacea have followed him in considering 

 the phyllopodous limb the most primitive among the 

 Crustacea. This theory has now been completely upset, 

 for Walcott has found several undoubted braik'hiop.xls 

 with appendages in the Middle Cambrian, and the best 

 preserved of them (Burgessia) show that the limbs were 

 not phyllopodan, but like those of trilobites. The. ancient 

 branchiopods having had simple trilobite-like limbs, it 



