488 GEORGE H. CARPENTER. 
Arthropods. The hemoccelic body cavity and reduced ceelom 
of Arthropods offer the most marked contrast to the closed 
vascular system and extensive ccelom of Annelids. True 
nephridia, so characteristic of the Annelids, are apparently 
unknown among the Arthropoda. The only characters 
common to the two groups are the metameric segmentation, 
the presence of paired hollow limbs, and paired ccelomoducts 
arranged segmentally, and the general structure of the 
nervous system. This last-named character—on which especial 
stress has been laid as indicative of affinity—is a necessary 
accompaniment of metameric segmentation in animals whose 
brain—whether archicerebrum or syncerebrum—is connected, 
by a nerve-ring surrounding the gullet, with paired lateral or 
ventral cords, so soon as the nerve-cells become aggregated 
segmentally into ganglia. And that such aggregation has 
arisen independently among Annelids and among Arthropods 
is strongly suggested by the condition of the nervous system 
in the Malacopoda (Onychophora), which, though essentially 
Arthropoda, possess laterally-situated nerve-cords without 
distinct ganglia. Thus the most primitive of living Arthro- 
pods exhibit a condition of the nervous system more primitive 
than is to be found among those Annelids from which the 
Arthropoda are believed by most zoologists to have been 
evolved. The structure of the eye is, indeed, the only un- 
equivocal annelidan character exhibited by a Peripatid. 
The more probable conclusion therefore seems to be not 
that Arthropods and Polychete Annelids stand to each other 
in the relation of descendants to ancestors, but that the two 
groups represent specialised collateral branches from a com- 
mon stock. My own view is that these common ancestors 
were microscopic animals, unsegmented, or with compara- 
tively few segments between a broad head-lobe and a narrow 
tail-somite. The occurrence of the nauplius larva in some 
members of all the great Crustacean groups justifies the 
phylogenetic importance attached to that form by Miller, 
whose views will probably, in the near future, again dominate 
zoological opinion on the subject. Comparison of a Nauplius, 
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