SEGMENTATION AND PHYLOGENY OF ARTHROPODA. 485 
limited number of segments, and if these ancestors were 
essentially Crustacean in nature, we are precluded from con- 
sidering Apus or any similar form as the representative of 
such an ancestral stock. And this suggests the last, and 
perhaps the most important question on which my recently- 
published views are in disagreement with Lankester’s: What 
were the earliest Crustacea like, and what was their relation- 
ship to the Annelida? 
On the assumption that Apus and its allies represent the 
most primitive of living Crustaceans, the foliaceous appen- 
dages of those animals are derived directly from Annelidan 
parapodia, and the ancestry of the Arthropoda is traced 
directly to richly-segmented Cheetopods. The typical bira- 
mous Crustacean limb is, on this view, to be understood as a 
specialization of the Branchiopodan limb owing to the sup- 
pression of four of the endites, and the elongation of the 
remaining two, as explained by Lankester (19, pp. 551-9). 
But there is much reason for regarding the Branchiopoda as 
somewhat specialized animals. The extreme reduction of 
both pairs of feelers, the absence of a palp on either 
mandible or first maxilla (maxillula) and its vestigial con- 
dition on the second maxilla, the modification of certain 
endites of the first trunk limb as tactile organs—all these are 
undoubtedly specializations, and it is not unreasonable to 
suppose that the foliaceous condition of the other trunk limb 
may be regarded as a specialization. Such foliaceous deve- 
lopment is most nearly matched, among other Crustacea, in 
the maxille and anterior maxillipeds of many genera belong- 
ing to different orders. It is a modification that 
accompanies excessive crowding of the appendages, 
and in Apus we may readily conceive that the secondary 
development of very numerous pairs of limbs has led to their 
crowding together and to the correlated development of the 
leaf-like endites and exites. 
For limbs like those of Apus are quite exceptional among 
the Crustacea; while the typical biramous limb, with or with- 
out an exite (epipodite) occurs in members of every crustacean 
