1894.] The Classification of the Arthropoda. 225 
Chilognatha are quite obscure. There has been no greater 
stumbling-block for morphologists than the attempt to homo- 
logise the somites of millipeds and centipedes. Attempts to 
bring other organs into harmony are equally futile. The 
three groups under discussion may be contrasted as follows, it 
being of course admitted that we know next to nothing of the 
somites and serial homologies of either Diplopod or Chilopod, 
and that possibly future research will modify some of the state- 
ments below. 
The Diplopod head bears, besides the antenne, but two pairs: 
of appendages—a pair of mandibles and a lower lip, composed 
of a pair of coalesced maxille.” In the Chilopod the condi- 
tions are as in the Hexapod, two pairs of maxille being pres- 
ent. 
In the Chilopods, as in the Hexapods, each somite bears a 
single pair of appendages, while in the Diplopods the majority 
of the segments bear two pairs of appendages, and the re- 
searches of Heathcote (’88) show that each segment is, in 
reality, composed of two coalesced somites, a condition without 
parallel elsewhere in the Arthropoda. In the Chilopods there 
is a wide sternum separating the coxæ of the ambulatory 
appendages; in the Diplopods the coxe are approximate, and 
the sternum is exceeding narrow, or even entirely absent. 
- In the Chilopods the stigmata, a pair to a somite, are lateral 
(dorsal in Scutigera), and are placed above and outside the in- 
sertion of the limbs, exactly asin the Hexapods. The tracheæ 
which arise from them are branched, and the intima is thrown 
into a well-developed spiral thickening as in the six-footed 
insects. In the Diplopoda, on the other hand, the stigmata 
are beneath the body® close to the legs, while the tracheæ (ex- 
_ 7 The attempt made to show that this Jower lip is composed of the two coalesced 
lower jaws, or first and second maxill, of the Chilognaths receives no support from 
the embryology of Julus (Heathcote ’88), where there is but a single somite when the 
hypothesis calls fortwo. Further the innervation of the sense organs of the lower lip 
(fF vom Rath, ’86, PI. XX, Fig. 1) shows that but a single pair of appendages is con- 
cerned in the part. 
. *In former papers I have said that the spiracles might be even im the coxæ. Ire- 
call having seen this statement but recently rather rather extensive reading of Myria- 
pod literature fails to reveal my authority. 
