SOME STAGES IN THE EVOLUTION OF THE 



NERVOUS SYSTEM AND THE FORE-GUT 



OF THE POLYCHAET 



By frank raw 



University of Birmingham, England 



CONTENTS 



PAGE 



Introduction i 



The brain of Eunice 2 



History of opinion regarding the constitution of the brain in polychaets 3 



Table i. Summary of opinions regarding the significance of the brain 4 



Significance of the divisions of the brain in primitive polychaets 5 



Figures 1-5. Eunice, after Heider 6 



The stomodeum in Eunice and other polychaets 9 



The stomatogastric nervous system in polychaets 10 



Interrelations of the stomodeum, the visceral nervous system, and the brain 



in the ancestry of Eunice 10 



Table 2. Supposed stages in the evolution of the central and visceral systems 



of the polychaet 14 



Completion of the brain 19 



Summary of the author's theory 19 



Limits and significance of the prostomium of the polychaet 22 



Morphological significance of the hind-brain 25 



Morphological significance and relationships of the peristome 26 



Application of the author's theory to polychaets in general 27 



Table 3. Comparison of supposed homologues of the morphological enti- 

 ties of the head region of typical errant polychaets 30 



Conclusion 33 



References 33 



INTRODUCTION 



As polychaets are primitive among the annehds, so are the Errantia 

 among the polychaets; and the Eunicimorpha among the Errantia. 

 Though the Amphinomorpha, since the work of Storch (1913), 

 have by many been placed at the base, the Eunicimorpha are in many 

 respects the most primitive. This is seen in the brain with its ex- 

 tended form and marked subdivisions, and in the stomodeum With 

 its fold form and the very small amount of its protrusibility. This 

 paper, at its first inception in 1926 ( !), was suggested by the char- 

 acters of Eunice {Leodice) as given by Heider (1925, see p. 61). 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. Ill, NO. 8 



