NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 29 



somite. From this point the germ cells must have been distributed to 

 the secondary somites when the latter began to be developed during 

 the course of evolution. Hence, primarily, the entire series of telo- 

 blastic somites would appear to have been genital segments. 



Fig. 12. — Examples of the growth of larval Archiannelida and Polychaeta 

 by proliferation in a subterminal zone of growth of teloblastic segments added 

 to the primary larval body. 



A, larva of Eupomntus uncinatus with series of teloblastic, or "postlarval," 

 segments (TScgs) interpolated between the three primary larval somites (I, 

 II, III, see fig. 7 A, B) and the terminal pygidium (simplified from Iwanoff, 

 1928). B, larva of Polygordius ncapoUlanus Fraipont during metamorphosis, 

 with series of teloblastic segments added to the trochophoral body, which is 

 itself unsegmented and contains no primary mesoderm (from Woltereck, 1905). 



C, half-grown young of Ncrilla antcnnata Schmidt (from Schlieper, 1925). 



D, larva of Lopadorhynchus brevis Grube with series of teloblastic segments 

 (from Kleinenberg, 1886). E-G, growth stages of "nereidogen" larva of Platy- 

 nercis dumcrilii Aud. & Milne-Edw. (from Hempelmann, 191 1, see also fig. 7 C, 

 first stage larva). 



AnCir, anal cirrus ; Cirl , Cirll , tentacular cirri of first two somites, united 

 in peristomium; I-XI, somites; Papd, parapodium ; Perst, peristomium ; Pip, 

 palpus; Prst, prostomium; Pyg, pygidium; Tl, prostomial tentacle; TSegs, 

 teloblastic segments. 



A condition similar at least to that which we should expect to find 

 in the primitive annelids is seen in the archiannelid Dinophihis (fig. 

 13 A). The body of Dinophihis consists of six or seven somites 

 clearly defined externally between the prostomium and the pygidium, 

 but there are no coelomic cavities in the diffuse mesoderm of the 



