NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I43 



the venter of the body segment (H, C.v), carrying mesad of the base 

 of each telopodite a small stylus (Sty) and an eversible vesicle (Vs). 

 The end segments of the legs are reduced to small dactyls (K, dac), 

 but each has an accessory claw (un) arising from its base. The first 

 legs are usually reduced in size and lack tibiae. The last body segment 

 bears a pair of cerci (L, Cer), which presumably are homologues of 

 the legs or possibly of the styli of the preceding segments. Com- 

 pound eyes are absent. The mandibles preserve the movable laciniae 

 (E, Lc) ; the maxillae have both laciniae and galeae (B), but the 

 palpi are small or vestigial; the labium (C) is a simple flap without 

 palpi. Lateral lobes of the hypopharynx (superlinguae) are present 

 at least in Scutigerella, as shown by Hansen (1930), and a pair of 

 slender apodemal arms extend into the head from the hypopharyngeal 

 base. The single median genital aperture is situated on the anterior 

 part of the venter of the fourth body segment, but since the paired 

 gonopores of Pauropoda and Diplopoda are on the third body seg- 

 ment, the median genital outlet of the symphylids might be supposed 

 to have migrated secondarily into the fourth segment. 



20. — The Diplopoda are a specialized branch of the early Symphyla, 

 in which the somites back of the fourth postcephalic somite are united 

 in pairs to form double segments. The mandibles are well developed 

 and have strong movable lacinial lobes, but there is only one post- 

 mandibular appendage of the head, the gnathochilarium (fig. 52 G), 

 the morphology of which is uncertain, though the organ is probably 

 either a combination of the maxillae with the labium, or the labium 

 alone. The legs of the first body segment are absent, and there are 

 no cerci on the last somite. Body segments are numerous in most 

 forms, and all but the first few are generated teloblastically in pairs 

 during postembryonic development. The paired gonopores are on 

 the third postcephalic somite at the bases of the second pair of legs. 

 The Pauropoda are probably an early branch of the Diplopoda, in 

 which a union of the somites in pairs had already taken place, and 

 the first legs had been much reduced but not yet obliterated. Special 

 characters of the pauropods are the lack of movable laciniae on the 

 mandibles, a weak development of the gnathochilarium, and a branch- 

 ing of the antennae beyond the fourth segments. 



21. — The Hexapoda resemble more closely the Symphyla than any 

 other of the modern arthropods, a fact recognized by several of the 

 earlier writers, and Packard (1898) first formulated a definite theory 

 of the origin of insects from symphylid ancestors. Recently the 

 evidence in favor of this theory has been more thoroughly reviewed 

 in the light of present-day knowledge of the apterygote hexapods by 



