NO. II LEGS OF PRIMITIVE ARTHROPODS — EWING 2/ 



duced. The first legs of proturans have more of a lateral than ventral 

 position and are held over the body, not under it. Consequently the 

 muscles and the sclerites have become so shifted that their homologies 

 cannot with definiteness be vv^orked out. 



It may l>e also that the first postcephalic segment of Symphyla and 

 Pauropoda becomes incorporated -with the head in Insecta ; possibly 

 becoming or in fact being the labial segment. If so the head of Pauro- 

 poda and Symphyla should have one less segment than an insect head, 

 which appears contrary to known facts. 



ABDOMINAL SEGMENTATION 



In this paper, abdominal segmentation will only be touched upon. 

 Of special significance among primitive land arthropods is the occur- 

 rence of double segments which characterize particularly the Diplo- 

 poda. The opinion has been frequently expressed that such a con- 

 dition is brought about by the pairing or coupling of two contiguous 

 segments. 



THE DOUBLING OF BODY SEGMENTS 



In pauropods there is some variation in regard to the individual 

 dorsal and ventral segmentation. The genus Eurypauropus shows 

 what Kenyon (1895) regarded as an indubitable diplopod condition, 

 but the same authority held that in Pmiropus (pi. 8, fig. 25) about 

 half of the legs came in between the dorsal plates. Kenyon even de- 

 scribed and figured intersegmental pleural areas bounded by con- 

 spicuous folds over the so-regarded intersegmental legs. Although 

 the diplopod condition is not sO' evident in Pauropus and does not 

 hold for the first two body segments, the present writer has failed 

 to substantiate Kenyon's contention that pronounced intersegmental 

 pleural regions exist. It does appear, however, that some of the legs 

 arise almost directly below the dorsal segmental sutures. 



In Symphyla the diplopod condition does not exist, although there 

 is evidence, as in SymphyJella, that small legless segments have united 

 with legbearing segments. According to Muir and Kershaw (1909) 

 the young of Scutigerella have seven pairs of legs, twelve tergites 

 and nine sternites when newly hatched. When adult they have twelve 

 pairs of legs, sixteen tergites and fourteen sternites. It is important 

 to note that Scutigerella has the same number of body segments when 

 hatched that proturans have, but, according to Muir and Kershaw, 

 must add one more than proturans do during postembryonic de- 

 velopment. 



