5. NERVOUS SYSTEM AND SENSE ORGANS 



53 



is very poor in nerve cells. It is comparatively thin. Two stouter bundles 

 often run amongst the numerous longitudinal fibres in this region, and seem 

 to represent the longitudinal bundles running along the line of the dorso- 

 ventral muscles of the protosoma. These bundles are much more prominent 

 in the Athecanephria, in connexion with the stronger development of these 

 muscles (Fig. 52). 



The dorsal nerve tract consists of densely crowded bundles of longitudinal 

 nerve fibres. It is narrow in the region of the mesosoma, but in the front part 

 of the metasoma it expands to form the thin nerve plate of the dorsal ciliated 

 band (Figs. 33, 36, 68). This consists of small fibre bundles which are more 

 or less separated from each other by the basal ends of the ciliated epidermal 

 cells. Large nerve cell bodies are attached to the distal sides of these bundles. 



Fig. 38. Transverse section of Siboglinum caulleryi at the level of the attachment of the tentacle. 

 cer - brain; eg - gland cell; сое - coelomic canal of tentacle; mdv - dorso-ventral strand of muscle 

 fibres; ml - longitudinal muscle fibres; my - cell body (myocyte) of longitudinal muscle fibre; 

 na - nerve ring of protosoma; «/-tentacular nerve; /-base of tentacle; ton - tonofibrillae; 

 vl - lateral cephalic blood vessel; vm - median cephalic blood vessel. (After Ivanov, 1958b.) 



