SEGMENTATION AND PHYLOGENY OF ARTHROPODA. 489 
a Rotifer like Pedalion, and a Trocophore helps us to conceive 
the hypothetical ancestral stock of Annelids and Arthropods. 
But the lines of evolution must, on that assumption, converge 
in a past so immensely remote that we are hardly justified in 
including both Annelids and Arthropods within the limits of 
a single phylum. Good reason might indeed be shown for 
deriving the Mollusca from the same common ancestry. We 
conclude, then, that the divergences between Annelids and 
Arthropods are profound, while their correspondences are 
comparatively superficial. The Arthropoda must stand, there- 
fore, in our zoological systems, not only as a natural monophy- 
letic group, but also sharply marked off from other phyla as 
a great primary division of the Animal Kingdom. 
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8. Hatscuex, B.—‘ Studien zur Entwicklungsgeschichte der Anneliden,”’ 
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