SEGMENTATION AND PHYLOGENY OF ARTHROPODA. 487 
the class, we may be tempted to regard the small number of 
trunk-seements in a typical Copepod as indicating a stage 
between the naupliar condition and the segmentation of the 
common ancestors of Arachnida, Crustacea, and Insecta with 
their fifteen limb-bearing trunk-segments. But when we 
consider the shortening that has undoubtedly taken place in 
many Crustaceans—e. g. the Cladocera, the Ostracoda, the 
Cirripedia—we must admit the great probability that the 
Copepoda have also undergone reduction in their segmenta- 
tion, and that the Leptostraca represent in this respect the 
primitive Arthropodous condition. 
Arthropoda and Annelida. 
The ancestral standing of the nauplius suggested nearly 
forty years ago by Miiller (23) has not been abandoned by 
all modern students of the Crustacea, although the tendency 
of most present-day zoologists is to derive Arthropods directly 
from elongate Annelid worms. No student can deny some 
relationship between Arthropods and Annelids, but we must 
ask whether the Annelida were the direct ancestors of the 
Arthropoda or whether both phyla must be traced back to an 
immensely remote common ancestry ? 
The opinion that the Apodide are the most primitive of 
Crustacea is held by those zoologists who believe in the 
homology between the parts of a polychetan parapodium 
and those of a branchiopodan limb, and in the direct deriva- 
tion of Arthropods from Annelids. This position, first 
suggested, I believe, by Hatschek (8), is virtually accepted 
by Lankester, who considers that the most primitive Arthro- 
poda “arose by modification of parapodiate annulate worms 
not very unlike some of the existing Cheetopods” (19, p. 527). 
I have always regarded with suspicion this modern hypo- 
thesis, because its acceptance has led to the present wide- 
spread disbelief in the monophyletic origin of the Arthropoda. 
And it seems that many recent morphological advances have 
tended to sharpen the distinction between all Annelids and all 
