132 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



Hey mens and by Wheeler in Dermaptera, and, according to Heymons 

 (1901), the definitive ducts of Scolopendra are formed from two 

 united pairs of coelomic sacs. In such cases we have, perhaps, 

 examples of the supplanting of one pair of exit sacs by another pair. 

 The heterogoneate condition of modern arthropods, therefore, must 

 be the result of mutations that occurred among ancestral forms. The 

 faculty of mutation afifecting the position of the genital ducts was 

 carried over into the entomostracan branch of the Crustacea, and was 

 not entirely extinct in the early forms of the Hexapoda. Moreover, 

 in the Chilopoda, as in the Onychophora, there still exists a variability 

 as to the segment of the genital ducts, for, though the genital outlet 

 is always on the subterminal segment in Chilopoda and on the ante- 

 penultimate segment in Onychophora, the genital segment is not 

 morphologically the same somite in all cases, since the number of 

 somites preceding it may be quite different in different genera. In 

 the Geophilomorpha, furthermore, the number of pregenital somites 

 is said to vary even among individuals of the same species. 



VII. PHYLOGENETIC CONCLUSIONS 



I. — A planulalike creature with an open posterior blastopore was 

 probably the ancestor of the Metazoa. A creeping form adapted to 

 feeding on a subsurface by the forward elongation of the blastopore 

 on the under side of the body might readily have evolved into a 

 worm by the partial closure of the blastopore, producing thus an 

 alimentary canal wdth a ventral subapical mouth and a terminal anus. 

 The subapical position of the mouth differentiated the animal into an 

 acronal sensory region, or prostomium, and a postoral visceral and 

 motor region, the body, or sojiia in a restricted sense. 



2. — The unsegmented progenitors of the annelids were probably 

 small, creeping, wormlike creatures having a simple alimentary canal, 

 a mouth on the anterior part of the under surface of the body, and a 

 terminal anus. Locomotion on solid surfaces was effected by a ventral 

 clothing of cilia, and body movements were produced by a system of 

 muscle fibers on the inner surface of the body wall, derived from 

 the ectoderm. The body cavity was a blastocoelic haemocoele, and 

 was largely occupied by lateral bands of a mesoblastic parenchyma 

 proliferated in the gastrula stage from endodermal or ectodermal 

 teloblastomeres. The nervous system consisted of longitudinal and 

 circular nerve tracts centering in ganglionic cell groups of the pro- 

 stomium, which latter eventually united to form a "brain." Sensory 

 organs may have included tactile tentacles and photoreceptive "eye 

 spots" located on the prostomium. 



