Fig. 40. Dorsal nerve trunk of Polybrachia annulata in the middle of the metasoma, in transverse 



section. 



ep - epidermis ; int - intima of dorsal blood vessel ; mb - basement membrane ; mc - circular 



muscle fibre ; ml - longitudinal muscle fibre ; my - muscle cell-body (myocyte) ; nc - neurocord 



(giant nerve fibre); nd - dorsal nerve trunk; per - peritoneum; vd - dorsal blood vessel. (After 



Ivanov, 1958b.) 



belong, however, seem to lie in the region of the brain and it is not unlikely 

 that they are the large unipolar cells mentioned above. 



Since the giant nerve fibres of other invertebrates (nemertines, annelids, 

 Phoronidea, Enteropneusta, [Crustacea and cephalopods]) are axons of motor 

 nerve cells, it is not improbable that the development of the neurocords in 

 pogonophores is connected with the necessity for instantaneous contraction 

 of the longitudinal musculature at the moment of "panic withdrawal" into 

 the protection of the tube (Ivanov, 1952). Such rapid contraction must occur 

 only in those parts of the body in front of the girdles of toothed bristles, with 

 which the animal fixes itself firmly to the walls of the tube, i.e. in the pre- 

 annular region of the body. Correspondingly the neurocords do not extend 

 behind the girdles. 



The ganglion cells in the dorsal nerve trunk are few, but at the level of the 

 girdles they increase in number. In a few forms (e.g. Lamellisabella zachsi) 

 this ganglionic concentration is clearly connected with the innervation of the 

 special musculature of the girdles (p. 44) (Ivanov, 1958b). In the postannular 

 region the narrow dorsal nerve trunk may be traced to the hind end of the 

 body (Fig. 22). 



Over the rest of the body the nerve-net of the epidermis consists of a 

 slender and in places interrupted layer of nerve fibres. At the level of the 

 bridle, however, and at the border between the mesosoma and the metasoma 



