58 PART I. GENERAL ACCOUNT 



run rather a long way back from the edge of the mesosoma and occupy all 

 that region of the body which lies in front of the girdles of the trunk. 



Thus the nervous system of Pogonophora, while displaying to some extent 

 a common architectonic line with that of Enteropneusta, at the same time 

 shows a whole series of significant divergencies. 



Sense organs 



Besides the solitary sense cells of the cephalic lobe possibly only the 

 dorsal ciliated band of the metasoma may be regarded as a sense organ. 

 Dawydoff (1948), however, conjectured that the bridle of Siboglinum weberi 

 (which, in agreement with the original description of Caullery, he considered 

 to be an oblique groove) might be fringed by narrow cells homologous with 

 the sensory organs of the proboscis of Saccoglossus cambrensis (Brambell and 

 Cole, 1939). From description given above (pp. 13 and 35), it is clear that 

 neither in its structure nor by the location of the cuticular keels is it possible 

 to compare the bridle with the proboscidial sensory organ of Enteropneusta 

 or to ascribe to it a sensory function. 



The dorsal ciliated organ of the trunk, with its well-developed basal nerve 

 layer furnished with ganglionic cells is, in all probability, a chemo-receptor. 

 According to Jagersten (1956) this is the rudiment of a special organ of 

 locomotion which was developed in the ancestors of Pogonophora. The strong 

 development of the ganglionic elements, however, imparts to the ciliated 

 band the character of a functional rather than of a rudimentary organ. 



