J. D. TothiU—The Ancestry of Insects. 383 



others and suggest the insect cerci. The problem of head 

 segmentation has been studied by Heytnons for Scolopendra 

 and the head is composed of six segments as in insects. I find 

 the same condition in Scolopendra, Linotamia and Cryptops. 

 In all these forms the mandibles and two pairs of maxillae cor- 

 respond to those in the Pterygogenea and the maxillae are bira- 

 mous. The whole group exhibits the condition of serial 

 polypody so characteristic in insect embryos. Several kinds of 

 eyes are also found in the group (although reduced by reason 

 of cave habits) and from these it would seem an easy transition 

 to the eyes of insects. Embryological development is also 

 remarkably similar in the two groups but that of Chilopods is 

 in several important respects less specialized. 



In short the Chilopods are unquestionably very intimately 

 related to the Pterygogenea, as Kingsley, Korscheldt and 

 Heider, Heymons and others have pointed out. An ancient 

 Chilopod showing no specialization of the maxilliped would 

 have looked suspiciously like fig. 3 and would have fulfilled 

 the specifications better and more closely than any trilobite 

 known at the present time. 



The Ancestry of the Opisthogoneata. 



If, as seems probable, the Pterygogenea arose from an 

 ancient Chilopod stock it may be interesting to speculate con- 

 cerning the ancestry of Chilopods. The primitively biramous 

 condition of appendages is suggested by the embryonic con- 

 dition of the two pairs of maxillae and of the maxillipedes, also 

 by the slight thickenings at the base of all the embryonic 

 trunk appendages in Scolopendra, Cryptops and Linotcenia. 

 The spiracles develop early but perhaps not quite as early as in 

 insects — the point needs reinvestigation. In embryos of the 

 same forms also, as pointed out by Heymons for Scolopendra, 

 there are rudiments of two pairs of antennae. 



The extremely interesting and anomalous Onychophora 

 invite at least inspection in this connection. Many investiga- 

 tors have supposed that this group, represented by the single 

 genus Peripatus, is related to the tracheate arthropods because 

 of the paired appendages and tracheae. As Handlirsch points 

 out, however, the tracheae are in position and histological 

 structure utterly unlike those of Chilopods and Hexapods. It 

 may be recalled that tracheae also occur in many terrestrial 

 Arachnoidea and in some half dozen or so genera of Isopod 

 crustaceans. In these latter cases the tracheae have clearly 

 arisen independently and there seems good reason for suppos- 

 ing an independent derivation in the case of Peripatus. The 

 peculiar fleshy legs and the nervous system are also very 



