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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 97 



also two cell groups of unknown significance situated on the dorsal 

 surface of the episphere (fig. 8 A, da). From some of the ganglion 

 cells nerve processes go to the muscles, and from others fibers pene- 

 trate centrally into the nerve tracts. 



Before the beginning of larval metamorphosis, Meyer says, the 

 production of myocytes ceases in the larval neuromuscular centers, 

 and during metamorphosis a large part of the larval musculature is 



Fig. 9. — -Theoretical evolution of the annelid nervous system, diagrammati- 

 cally following Kleinenberg's and Meyer's accounts of the development of the 

 nervous system in the larva of Lopadorhynchiis. 



A, early trochophore with diffuse nerve cells (NCls) along the radial and 

 circular nerve tracts. B, nerve cells of episphere concentrated in incipient 

 ganglia connected with ectodermal sensory organs along lateral radial nerves ; 

 ventral nerve cords (VNC) developed from ectoderm of hyposphere. C, scat- 

 tered ganglia of episphere condensed into a cerebral mass (Br) ; cerebral con- 

 nectives united with ventral nerve cords. D, generalized adult nervous system ; 

 podial ganglia developed at bases of body appendages. E, nervous system of 

 adult polychaete, lateral view. 



AlCiil, alimentary canal; A71, anus; Br, brain; E, eye; Mth, mouth; NCls, 

 nerve cells; NO nuchal organ; PdGng, podial ganglion; Pip, palpus; Prst, 

 prostomium ; Tl, tentacle ; VNC, ventral nerve cord ; ZG, zone of growth. 



lost. Many of the primary muscles, however, remain, including those 

 of the persistent tentacles and nuchal organs, and certain other muscles 

 of the prostomium. The neural cells of the various ganglionic centers 

 of the larval episphere, on the other hand, increase in number until 

 they become so crowded that details of their further development 

 cannot be followed. The cells thus generated, however, are massed 

 upon the large lateral nerve trunks of the episphere (fig. 8 B, n^) 

 and their anterior commissure. In this manner there is formed from 



