22 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 97 



while other superficial cells assumed the receptive function and trans- 

 mitted secondarily the impulses from external stimuli to the first set 

 of cells, which now became purely motor neurones. Finally, still other 

 neurocytes gave rise to a subepidermal plexus of fibrous tracts that 

 formed lines of intercommunication between the scattered motor and 

 sensory elements, and thus unified and coordinated the entire nervous 

 system. Then the nerve cells of the prostomial region became aggre- 

 gated into a number of ganglionic centers, principally associated with 

 groups of receptive cells in primitive sensory organs, and the nerve 



rno3^^ 



rnu 



~Stom 



Fig. 8. — The nervous and muscular elements of a young trochophore larva of 

 Lopadorhynchus, nerve tissue represented in white, muscle tissue in black. 

 (From E. Meyer, 1901.) 



A, aboral surface of larva. B, oral surface. 



da, rudiments of so-called dorsal antennae ; dn, median dorsal nerve of hypo- 

 sphere; gSo, ganglion of left ciliary organ; Ks, apical plate; mcl, muscle fibers; 

 Msd, mesoderm; n, longitudinal nerves (seven pairs in episphere) ; NCls, nerve 

 cells; Rn, circular nerve of prototroch ; riio^, ruo^, rno^, circular nerves of 

 episphere; rnu, circular nerve of hyposphere; So, left ciliary organ; so, rudimen- 

 tary right ciliary organ ; Stom, stomodaeum ; Vdn, larval stomodaeal nerve. 



tracts assumed definite courses. Thus was evolved the primary 

 nervous system of the polychaete larva. The prostomial ganglia of 

 this system later coalesce to form the definitive brain. The somatic 

 nervous system, subsequently developed in correlation with meta- 

 merism, took its origin from restricted ventrolateral tracts of the 

 somatic ectoderm, became connected with the brain, and finally re- 

 placed the primary system in the body region. 



The most primitive nerve center of the annelids probably is repre- 

 sented by the apical ganglion of the polychaete trochophore (fig. 4 A, 

 ApGng) situated beneath the ectodermal apical plate (ApPl), which 



