LEE BARKER WALTON 169 



of the typical insect is illustrated in many forms. Thus 

 in Eulepis splendida Treadwell a marine form described 

 from Porto Rico by Treadwell (1901) the neuropodium 

 is much larger than the notopodium, a functional develop- 

 ment which as already noted appears to be correlated with 

 the swimming and creeping mode of progression. The 

 dorsal cirrus is prominent and has a position about mid- 

 way between the basal and distal part of the parapodium. 

 In Anthostoma latacapitata Treadwell, another Poly- 

 chaete from the same region, a similarly relatively large 

 neuropodium is to be found although the dorsal cirrus is 

 less prominent and relatively more attenuate. Treadwell 

 does not figure the dorsal and ventral aciculus, although 

 their position is to be inferred from the form of the para- 

 podium. 



In reference to existing insects a few representative 

 forms taken from different orders will serve to indicate 

 the structural relationship with the typical segment pre- 

 sented in the preceding part of the paper and with the 

 parapodium as illustrated in fossil and in existing Poly- 

 chaetes. 



The Mecoptera, more familiarly known as "scorpion 

 flies," form a small but an extremely interesting phyletic 

 group of insects, as has been pointed out by Tillyard 

 (1920). The mesothorax of Panorpa communis, with 

 its anterior episternum-coxon-trochanter-femur area and 

 its posterior epimeron-meron area, presents a structural 

 arrangement easily to be derived from the elongate bira- 

 mose parapodium of a form not far removed from the 

 Middle Cambrian Worthenella ca^nhria Walcott. The 

 general transformation so far as the external integument 

 is concerned is completed through the shortening of the 

 notopodium, the fusion of the base of the notopodium 

 and neuropodium with the lateral part of the segment, 

 and the development of the segmentation of the para- 

 podium, as indicated, form the corresponding segmenta- 

 tion in Panorpa. The position of the wings is in accord 

 with their possible derivation from basal cirri, as earlier 

 suggested. Basal cirri are present in many existing Poly- 

 chaetes, and the small "oval scale-like objects" to which 



