NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I5 



body region between tbe mouth and the pygidium (E). On the same 

 day, however, the mesodermal bands also become divided into seg- 

 mental sections. At first the ectodermal and mesodermal somites of 

 Capitella do not entirely correspond, there being several super- 

 numerary mesodermal divisions in the mouth region, but by the 

 twelfth or thirteenth day the larva has 13 somites with coincident 

 limits in both the ectoderm and the mesoderm. 



The segmentation of the mesoderm bands as described by E. Meyer 

 (1901) in Psygmobranchns, Polygordius, and Lopadorhynchus ap- 

 pears to be determined by elements of the mesenchymatic primary 

 mesoblast in the form of spindle-shaped cells that penetrate into the 

 mesoderm bands at the intersegmental lines and cut the bands into a 

 series of segmental sections. From the penetrating mesenchyme cells 

 are later formed, according to Meyer, the muscles of the interseg- 

 mental dissepiments. Similarly in the Serpulidae and Spionidae the 

 larval segmentation is said by Iwanoff (1928) to be secondarily im- 

 posed upon the mesoderm bands by metamerism in other parts of the 

 body, as by the ectodermal segmentation, the ingrowth of the chaetal 

 sacs (fig. 7 B), the penetration into the mesoderm of mesenchymatous 

 muscle elements, or by the segmental formation of blood lacunae in 

 the general mesoderm mass. 



The primary larval segments are seldom as fully developed in the 

 adult worm as are the teloblastic segments, and both the segment 

 limits and the differentiation of ganglia on the nerve cords may remain 

 obscure. In the Spionidae, Iwanoff (1928) says, the trochophoral 

 mesoderm is very weakly developed, the dissepiments are only imper- 

 fectly formed, often absent, and in some species a segmentation of 

 the mesoderm in the primary segments is absent even in the adult. 

 Chlorogogen cells are not developed in the coelomic walls of the larval 

 segments, and in these segments germ cells are never present. 



As a result of body metamerism, the mesoderm bands are divided 

 each into a series of segmental sections, and these sections, as the 

 bands themselves, are at first solid blocks of cells (fig. 6 G). Later the 

 coelomic cavities appear as cleavage spaces within the cell blocks (H). 

 Hence, just as there is no evidence that the primary mesoderm bands 

 represent primitive sacs, so there is no evidence from ontogeny that 

 the coelomic cavities of the annelids took their origin as a series of 

 separate mesodermal pouches. The facts of development suggest only 

 that the primitive mesoderm bands were continuous tracts of cells, 

 and that the formation of cavities within them was a secondary 

 process, subsequent to segmentation. 



With the formation of the coelomic cavities in the mesoderm, the 

 young annelid becomes a coelomate animal. Before the appearance 



