54 



PART I. GENERAL ACCOUNT 



The sides of the nerve plate, extending a little beyond the edges of the 

 ciliated band, are formed by thicker bundles of nerve fibres (Figs. 33, 68). 

 Behind the ciliated band the nerve tract becomes narrower once more and it 

 is made up of fewer fibre bundles (Ivanov, 1958b). 



The dorsal nerve trunk of Spirobrachia in the region of the mesosoma is 

 worthy of consideration (Ivanov, 1958b). The mid-dorsal part of the epidermis 

 is here invaginated to form a narrow median groove at the bottom of which 

 lies the nerve trunk (Fig. 39). The groove is particularly strongly developed in 

 the front part of the mesosoma and gradually flattens out towards its hind 

 end (Fig. 1715). It is perhaps worth remarking that the sinking-in of a dorsal 

 nerve trunk by the formation of a nerve groove, which is then followed by its 

 separation from the overlying epidermis as a nerve tube, is a process ex- 

 tremely characteristic of the Enteropneusta and the Chordata. An analagous 

 and equally characteristic process occurs in the formation of the epineural 

 canals of the Echinodermata, which contain the trunks of the ectoneural 

 system in their epithelial walls. 



The dorsal nerve trunk of Lamellisabella, Polybrachia and Spirobrachia 

 possesses giant nerve fibres or neurocords (Ivanov, 1958b). They are found 

 along the whole length of the preannular region of the trunk (Fig. 40) and 

 extend also into the mesosoma (Fig. 39). The ganglion cells to which they 



Fig. 39. Dorsal nerve furrow of Spirobrachia grandis: transverse section in the mesosomal region. 

 ce - chlorogogenic cells ; cut - cuticle ; ep - epidermis \f— keel of bridle ; mb - basement membrane ; 

 mc - circular musculature; mf- muscle fibres of the ridge of the bridle; ml - longitudinal muscu- 

 lature ; nc - neurocords (giant nerve fibres) ; nd - dorsal nerve trunk ; per - peritoneum ; vd - dorsal 

 blood vessel. (After Ivanov, 1958b.) 



