SEGMENTATION AND PHYLOGENY OF ARTHROPODA. 475 
(labium) in the Insects. Together with these correspondences 
we find (seeT'able, p. 472) an exact numericalagreement between 
the trunk-segments of an Insect and those of a Malacostracan. 
Heymons has shown (9) that the cercopods of an insect belong 
to the embryonic eleventh abdominal segment, which, during 
development, fuses with the tenth—the evident cercopod- 
bearing segment in the adult. That an exactly similar fusion 
of the hindmost trunk-segments has taken place in the Mala- 
costraca is shown by the presence of an additional somite in 
the Leptostraca, and by the structure of the uropod-bearing 
segment—evidently composed of two fused somites—in certain 
Schizopods of the genus Gnathophausia described and figured 
by Sars (26). 
I rejoice to find myself in such close agreement with 
Lankester’s main position, for if the kinship of Insects to 
Crustaceans be generally admitted, belief in the polyphyletic 
origin of Arthropods must be rapidly given up. He, indeed, 
considers the relationship between the two classes closer than 
I do, for he writes: “It seems probable that in the case of 
the Hexapoda, at any rate, the point of departure [from the 
Crustacean main stem] was subsequent to the attainment of 
the nomomeristic character presented by the higher grade of 
Crustacea.” My view is that the most primitive Crustacea 
were nomomeristic, and that the ancestors of the Insects and 
their allies must be sought far down the Crustacean stem. 
This difference of opinion in details depends on the differences 
between our respective estimates of the relationship of Insects 
to the various classes of “ Myriapods,” and between our views 
as to the nature of the most primitive Crustaceans. 
Relationships between Insecta, Chilopoda, and 
Diplopoda. 
Turning, then, to the discussion of these details, I take first 
the question of the relationship between Insects and the 
other classes of tracheate Arthropods. It is satisfactory to 
* Lam indebted to my friend Dr. W. T. Calman for kindly bringing this 
fact to my notice. 
