12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



It is claimed by both Kleinenberg (1886) and E. Meyer (1901), for 

 example, that in the larva of Lopadorhynchiis the mesoderm arises 

 from the ectoderm, and in Capitella, according to Eisig (1899), the 

 coelomic mesoblast is produced from blastomeres other than 46. 

 Furthermore, the mesoderm of the postlarval somites is said by 

 Iwanoff (1928) to be formed in many polychaetes directly from the 

 ectoderm, and the same is probably true in cases of regeneration. The 

 mesoderm of certain other coelomate invertebrates also may have no 

 genetic relation to the endoderm, as in the gastropod Paludina, in 

 which the embryonic mesoblast that gives rise to the usual mesodermal 

 organs is generated directly from cells of the ventral ectoderm (see 

 Dautert, 1929). 



During larval life, or at the transformation of the larva to the 

 worm, the annelid mesodermal teloblasts, however formed, proliferate 

 within the haemocoele two masses of mesoderm cells (fig. 5 E, F, G, 

 Msd), which eventually take the form of ventrolateral bands extend- 

 ing forward at least as far as the sides of the mouth (fig. 6 F). These 

 primary mesoderm bands are solid cell masses; they are never ob- 

 served at this early stage to contain cavities, and there is no evidence 

 from annelid embryology to suggest that they represent phylogeneti- 

 cally a pair of open pouches. Later, with body segmentation, the 

 bands are broken up into solid segmental blocks (G), and finally the 

 blocks are excavated by coelomic cavities (H). The nature of the 

 mesoderm and the primitive function of the coelomic cavities can be 

 better discussed after we have examined the known facts concerning 

 the beginning of metamerism, but it should be noted here that the 

 formation of the mesoderm bands precedes body segmentation. 



Metamerism in the polychaete larva becomes first evident as a 

 subdivision of the body region between the mouth and the pygidium 

 into a small number of somites (fig. 7 A, I, II, III). There is ample 

 reason to believe, as Iwanoff (1928) claims, that the formation of 

 these primary somites, or larval segments of ontogeny, represents the 

 beginning of metamerism in phylogeny, and, as we shall see, the same 

 phenomenon of direct segmentation in the body of the embryo or 

 young larva recurs in various arthropods. The primary somites are 

 thus to be distinguished from the secondary somites later added by 

 teloblastic growth in a subterminal generative zone, and which will 

 constitute the major part of the adult animal. The larval somites of 

 the Polychaeta, Iwanoff shows, are formed approximately simul- 

 taneously in contrast with the successive, individual generation of 

 the teloblastic somites. E. Meyer (1901) observes that in Lopa- 

 dorhynchus metamerism takes place so rapidly as to give the impres- 



