SEGMENTATION AND PHYLOGENY OF ARTHROPODA. 481 
evolved from Crustaceans,’ I firmly believe that, in any 
rational system of Arthropod classification, the Merostomata 
and Xiphosura must be regarded as Arachnids. Lankester 
claims that the Trilobita also are Arachnids, while in my 
opinion their possession of feelers and biramous limbs should 
lead us to consider them as Crustaceans. This is, however, 
rather a question of terminology than of principle. Lankester 
admits that the Trilobita had a common ancestry with the 
Crustacea, and I suggest “‘ that the Arachnida arose from the 
base of the Trilobitan branch rather than from the main 
Crustacean stem.” 
The particular question on which Lankester and I are not 
in agreement is the nature of the segmentation of the proto- 
Arachnida. Regarding the Arachnida as descended from 
the Trilobita, and laying stress on the indefinite and often 
rich segmentation exhibited by many members of the latter 
order, he suggests that the definite number of primitive 
somites characterising typical Arachnids has been reached 
by reduction from an originally anomomeristic condition. 
Believing, on the other hand, that the number of segments in 
an Arachnid agrees exactly with the number in a typical 
Crustacean or Insect, I hold to the view that the ancestors of 
all three classes possessed such a definite segmentation. The 
indefinite segmentation of the Trilobita presents no difficulty 
to this view. I have shown in my recent paper (8, p. 333) 
that the average number of trunk segments in the genera of 
Trilobites increases steadily as we trace their history from 
Cambrian to Carboniferous, and that among the most primitive 
members of the group known (Olenellus) were species with 
only sixteen trunk-segments. It is reasonable, therefore, to 
suppose that the richly-segmented Trilobites were developed, 
like the Centipedes and Millipedes, from ancestors with 
moderate and definite segmentation. 
This agreement in the number of body-segments among the 
Crustacea, Insecta and Arachnida, was long ago suggested, 
as mentioned above, by Huxley (15). Lankester, in referring 
to Huxley’s view (19, p. 545), rejects it on the ground that 
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