SEGMENTATION AND PHYLOGENY OF ARTHROPODA. 477 
Now, the head of a Diplopod cannot be regarded as mono- 
prosthomerous. ‘The ocular segment must be accounted for, 
and its presence makes the antennal segment deutocerebral 
as in Insects. As yet we have no embryological evidence of 
the tritocerebral segment of any millipede, but if the appen- 
dages of the diplopod head can be shown by comparative 
morphology to correspond with those of the insectan head, 
we may expect with confidence that the existence of the 
tritocerebral segment in an embryo Millipede will one day be 
demonstrated. 
‘The Symphyla are regarded by Lankester (19, p. 567) as 
belonging to the Diplopoda. The forward position of their 
genital aperture shows, indeed, that they have affinity with 
that class, but it is impossible to accept Schmidt’s contention 
(28) that they are specialised millipedes. 'The aspect of the 
head and feelers of Scolopendrella has struck all observers 
as distinctively thysanuran. It is certain that there are two 
evident pairs of maxille behind the mandibles, as shown by 
Hansen (7) and Grassi (5) ; and it occurred to me that if the 
characteristic maxillule of the Collembola and Thysanura 
could be demonstrated in Scolopendrella, its aflinity with those 
lower insects could no longer be regarded as doubtful. I 
found the maxillule, and figured them in my recent paper 
(3, fig. 3); and while that paper was in the press a brief 
account of them, with figures, was published by Hansen (7). 
Scolopendrella, then, has three pairs of mouth-appendages 
behind the mandibles ; and when we find that in the number 
of its trunk-segments it agrees exactly with a typical insect, 
and when we remember that in many Thysanura nearly all 
the abdominal segments carry short appendages, we cannot 
deny the near relationship of the Symphyla to the Aptera. 
And admitting such an affinity, without denying the affinity 
of the Symphyla to the Diplopoda, we bring all the tracheate ! 
classes near to one another, for a rather close kinship 
between the Insecta and the Chilopoda is admitted on all 
hands, 
1 I do not include the Arachnida among the ‘‘ tracheate ” classes. 
