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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 97 



connected by a fibrous commissure. In most of the other errant poly- 

 chaetes the corpora pedunculata are more highly developed, and the 

 cap cells become segregated into two or three distinct globuli (E, F). 

 On the other hand, in the sedentary polychaetes with reduced cephalic 



OpNv 



PlpNv T-^ NCom OpCom 



Fig. i8. — Structure of the brain in Archiannelida and Polychaeta. 



A, outline of head and diagram of simple brain structure in Polygordius (from 

 Hanstrom, 1929). B, horizontal section of brain of Sthenelais picta Verrill 

 (Aphroditidae), diagrammatic (from Hanstrom, 1927). C, brain of Eunice 

 ptmctata Risso (Eunicidae), showing high degree of external differentiation 

 (from Heider, 1925). D, transverse section of brain of Podarke obscnra Ehlers 

 (Hesionidae) through corpora pedunculata (from Hanstrom, 1927). E, hori- 

 zontal section of brain of Nereis virens Sars (Nereidae) showing internal 

 structure (from Hanstrom, 1927). F, diagram of a corpus pedunculatum of 

 Nereis virens (from Hanstrom, 1927). 



a, sensory fibers of palpus nerve; la, 2a, 3a, nerves of lateral and median 

 prostomial tentacles ; b, c, fibers of stomodaeal connectives ; d, axons of glob- 

 uli cells of corpus pedunculatum ; E, eye ; /, central neurocytes in anterior 

 part of brain ; jb, forebrain ; Gb, globulus of corpus pedunculatum ; hb, hindbrain ; 

 mb, midbrain; NL, nuchal lobe; NCom, nuchal commissure; OpCoin, optic 

 commissure ; OpNv, optic nerve ; Ped, pedunculus of corpus pedunculatum ; 

 Pip, palpus ; PlpCom, palpal commissure ; PlpGlm, palpal glomeruli ; PlpNv, 

 palpal nerve ; SCls, sense cells of palpus ; StCon, stomodaeal connective. 



sense organs, the corpora pedunculata are correspondingly reduced 

 or are vestigial, and in the Oligochaeta they are absent. 



The development of the corpora pedunculata in the Polychaeta is 

 clearly correlated with the development of the prostomial sense organs, 

 but the particular relationships of the bodies are with the sensory 

 nerves of the palpi. It is shown by Hanstrom (1927, 1928, 1929) in 



