5. NERVOUS SYSTEM AND SENSE ORGANS 



51 



of dorsal longitudinal bundles lying opposite the two cephalic dorso-ventral 

 muscles (Fig. 38) (Ivanov, 1958b). 



The latero-ventral parts of the cerebral nerve ring consist for the most part 

 of transverse nerve fibres; nerve cells are comparatively few here. The 

 posterior part of the brain, situated behind the nerve ring, looks like no more 

 than the very beginning of the dorsal nerve tract. It has a fibrous nature and 



Fig. 36. Diagrams of the nervous system of Polybrachia annulata, Л - in dorsal view, and, В - from 

 the left side, and of Siboglinutn caulleryi, С - in dorsal view, and D - from the left side. 

 с - cephalic lobe ; cer - cerebral ganglion (brain) ; com - transverse commissure ; con - longitudinal 

 fibre bundle; ct - tentacular crown;/- bridle; ms mesosoma; mts - metasoma; na - nerve ring of 

 protosoma ; nam - nerve ring of mesosoma ; nap - nerve ring at boundary between mesosoma and 

 metasoma; nd- dorsal nerve trunk; nl - thickened edge of dorsal nerve plate; nt- tentacular nerve; 

 p - cuticular plaques; pa - papillae; pn - dorsal nerve plate; ps - protosoma; t - tentacle. (After 

 Ivanov, 1958b) 



