NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA — SNODGRASS 1 3 



sion that a relatively large number of somites are formed all at once, 

 but Sokolow (1911) says that in Ctcnodrilns the intermediate somites 

 or the more anterior ones are first differentiated and the series then 

 completed anteriorly and posteriorly. Segmentation may be delayed 

 until the beginning of metamorphosis, as in Polygordius, or it may 

 take place while the larva is still in the swimming trochophore stage. 



-MtK 



Fig. 6. — Transformation of the annelid blastopore, primary segmentation of 

 the body, growth and segmentation of the mesoderm bands, and formation of 

 the coelomic sacs, diagrammatic. 



A, blastopore and mesodermal teloblasts at posterior pole of embryo. B, 

 blastopore elongating forward on ventral surface ; rudiments of mesoderm gen- 

 erated from teloblasts. C, blastopore still more elongate, closing posteriorly ; 

 mesoderm growing forward. D, blastopore closed posteriorly ; mesoderm bands 

 extended to prostomium. E, blastopore obliterated except for mouth opening 

 at anterior end ; anus formed secondarily at posterior end ; mesoderm segmented 

 following metamerism of body, and extended into prostomium. F, polychaete 

 trochophore before segmentation. G, same after segmentation, mesoderm cut 

 into solid segmental blocks. H, same, mesoderm blocks excavated by coelomic 

 cavities. 



AlCnl, alimentary canal; An, anus; Bpr, blastopore; Cod, coelomic cavity; 

 Epsp, episphere ; Msd, mesoderm ; MsT, mesodermal teloblast ; Mth, mouth ; 

 Prst, prostomium ; Pyg, pygidium ; ZG, zone of growth. 



In Polynoe, as described by Hacker (1895), seven somites are first 

 marked out in the body of the trochophore, which is transformed 

 while still active into a swimming "nectochaete" larva with seven 

 segments and corresponding chaeta-bearing parapodia. The number 

 of larval somites is always small, three or four being usual (fig. 7 A, 

 B, C), the maximum not more than 13. Completion of larval meta- 

 merism is followed by a pause in development. 



