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Hexopoda, and to regard the Tracheata as composed exclusively of 
the forms that are referred to these last named classes. 
The next question that presents itself for elucidation is naturally 
this: — Is the group, the Tracheata, as thus limited, a natural one? 
Provisionally at all events an affirmative answer may be given to this 
question. Forthe animals known commonly as Millipedes, Centipedes 
and Insects present a number of characters in common, which are 
sufficient to justify the belief that they are tolerably closely related to 
each other. Thus the anterior pair of appendages in the adult are 
preoral in position and, performing the function of feelers, are known 
as the antennae, while behind the mouth not less than two pairs of 
appendages and sometimes as many as four pairs abandon all share in 
locomotion and become modified so as to act as jaws. 
Moreover the dorsal elements of the anterior two or three or per- 
haps even four somites, which bear the antennae and the two or three 
or four pairs ofgnathopods, are represented by asingle plate, frequently 
provided with eyes which is known as the head. The body behind the 
head is composed of a series of segments, varying in number from 
about 10 to over 100, and the tracheae when developed almost always 
communicate with the exterior by means of apertures called stigmata 
situated above the bases of the limbs. There is never more than a 
single stigma for each segment or primitive segment and a study of 
adult forms and of the embryology of certain members of the groups 
points to the conclusion that in the ancestral form each body-segment 
was provided with a single pair of appendages. 
The classification of the Tracheata into Myriopoda and Hexopoda 
is based principally upon the external form. In the Myriopoda the 
region behind the head is not differentiated into separate regions, the 
individual segments being similar to each other, each being provided 
with a single pair of appendages which generally speaking are also 
similar to each other. In the Hexopoda on the contrary the segments 
which succeed the head are not in the adult all alike, the three 
anterior only being provided with appendages. Thus this region is 
divisible into two portions, the anterior of which, bearing the three 
pairs of locomotor appendages, is known as the thorax, while the 
posterior, the apodous region, is called the abdomen. 
Until closely examined it would seem that this classification is 
a perfectly natural one. But when the so-called orders of the Myrio- 
poda are compared with each other on the one hand and with the 
Hexopoda on the other, the conviction that such is not the case is 
irresistibly forced upon one. For it will be found that the so-called 
group of Myriopoda is sharply divisible into two sections upon a 
