NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I47 



The hexapod structure, with the locomotor function centered in 

 the thorax, apparently gave little if any advantage over the polypod 

 structure for ordinary terrestrial life, but it furnished a condition 

 particularly fitted for the development of wings. Hence, with the 

 appearance of alar lobes on the thorax, the evolution of these lobes 

 into organs of flight was readily accomplished, and the pterygote 

 insects quickly achieved a great superiority over the other arthropods. 

 While there is much to suggest that the winged insects are most 

 closely related to the apterygote thysanurans, their direct origin from 

 the latter is questionable. It is difficult to explain, for example, how 

 it comes about that the pterygote Ephemeroptera and Dermaptera 

 have paired genital openings while secondary median ducts are already 

 established in the Thysanura, with openings on the same segments as 

 in the higher Pterygota. 



2/^.. — The Chilopoda are the conservatives among the arthropods ; 

 they are the least-modified descendants of the Protomyriapoda, and 

 in certain phases of their embryogeny they still follow the course of 

 development in the Onychophora. The gnathal appendages are prob- 

 ably more generalized than in any other of the Mandibulata; though 

 the bases of the mandibles are deeply sunken into pouches of the head 

 wall, they have strongly musculated lacinial lobes (fig. 53 E, F), and 

 the two maxillary appendages (C) are but little modified except by 

 reduction of the telopodites and a partial union of the coxopodites. 

 The suspensorial sclerites of the hypopharynx maintain connections 

 with the cranial margins, and bear the apodemes on which the ventral 

 muscles of the gnathal appendages are attached. The characteristic 

 specialization of the chilopods is the conversion of the first legs into 

 a pair of poison claws (B). Most of the other body appendages 

 retain the structure of simple 7-segmented legs, though at the base 

 of each is an extensive subcoxal sclerotization suggestive of that in 

 the insect thorax. The last two pairs of legs are reduced and modified 

 to serve as genital accessories, and consequently there are no terminal 

 cerci. Styli and eversible vesicles are absent. The genital opening is 

 always on the last somite before the telson, but since the total number 

 of somites is variable, the genital segment may be a quite different 

 somite in different chilopod groups. Anamorphic postembryonic 

 development persists in some forms, while in others segmentation 

 is complete at hatching. 



2^. — Evolution may be accepted as a fact, but the true history of 

 phylogeny can never be demonstrated. Though the main branches of 

 the genealogic tree of any major group of animals are fairly evident, 

 an endeavor to follow in detail the phylogenetic connections between 

 more closely related forms invariably leads into a maze of difficulties. 



