NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 2$ 



numerous agglomerated centers in the larval episphere (fig. 9A, B) 

 a compact cellular and fibrous body of nerve tissue (C, Br), which 

 becomes the brain of the adult worm (D). Hence, as Kleinenberg 

 remarks, the developmental history of the brain in Lopadorhynchus 

 shows how extraordinarily complicated in its origin is the cephalic 

 ganglion even in the annelids. However, that details in the probable 

 phylogenetic history of the nervous system are not necessarily reca- 

 pitulated in ontogeny is shown in many annelids having a direct 

 development, or one in which the trochophoral stage is passed within 

 the tgg, for in such forms the brain is differentiated from the 



Mth 3 2Dsp 



Fig. 10. — Median vertical sections of the anterior end of an embryo of the 

 viviparous polychaete CtenodrUus branchiatus Sokolow (Cirratulidae), show- 

 ing extension of the mesoderm into the prostomium, and the direct development 

 of the brain from the prostomial ectoderm. (From Sokolow, 191 1.) 



A, embryo before appearance of coelom, with mesoderm (Msd) extended into 

 prostomium (PMsd). B, full-grown embryo, with coelom and dissepiments, 

 coelomic cavity of prostomium (PCoel) continuous with coelomic cavity of 

 first postoral somite (metastomium), which in the embryo is separated from 

 second somite by a temporary dissepiment (iDsp). 



Br, brain; Coel, coelom; iDsp, first (temporary) dissepiment, behind first 

 postoral somite; 2Dsp, 3Dsp, second and third (permanent) dissepiments; Ecd, 

 ectoderm ; mcl, muscles ; Msd, mesoderm ; Mth, mouth ; PCoel, prostomial 

 coelom ; PMsd, prostomial mesoderm ; Stom, stomodaeum. 



prostomial ectoderm as a single, compact mass of neural cells 

 (fig. 10, Br). 



The larval innervation of the hyposphere gives way during meta- 

 morphosis to the definitive body nervous system, consisting of the 

 ganglionated ventral aierve cords and their peripheral nerves. The 

 rudiments of this system appear first in the embryo as continuous 

 strands of neurocytes proliferated in the ventral parts of the ecto- 

 dermal somatic plate as the median edges of the latter unite to close 

 the blastopore. The cords later become ganglionated by the segmental 

 aggregation of their cells. The neuroblasts of the somatic nerve cords, 

 Meyer believes, represent the nerve cells of a series of primitive 

 ectodermal sense organs. Though there are no persistent remnants of 



