5. NERVOUS SYSTEM AND SENSE ORGANS 57 



Pogonophora there is no nerve tube, but it is worth noting that in Spiro- 

 brachia spp. the nerve trunk of the mesosoma lies at the bottom of a deep 

 furrow or median invagination of the epidermis, which may be considered, 

 in all likelihood, as the beginnings of a protected unexposed sub-epidermal 

 nervous system, to wit as the first stage in the formation of a nerve tube. 



Physiological experiments performed upon Balanoglossus and Saccoglossus 

 provided evidence (Bullock, 1940) that the nerve tube of the collar, which was 

 formerly thought to play the role of the central co-ordinating organ or brain 

 (Dawydoff, 1948), had no great spontaneous activity and thus appeared to be 

 a conducting tract on a par with other sections of the nervous system. It has 

 also been stated (Knight-Jones, 1952) that there is a considerable predomin- 

 ance of longitudinal fibres in the nerve tube, similar to those of other longi- 

 tudinal nervous pathways. 



A rather complex system of longitudinal and transverse nerve fibre bundles, 

 including the preoral ring, has been described in the proboscis of Sacco- 

 glossus (Knight-Jones, 1952). Koshtoyants (1957) considers that in this region 

 not only are stimuli conducted but that it also serves a "neuro-reflector" 

 function, and that "nervous impulses concerned with the integration of the 

 motor activity of the animal as a whole" arise here. Nevertheless there is no 

 anatomical expression whatsoever in the proboscis of Enteropneusta of any 

 ganglionic cluster which might be considered as a central part of the nervous 

 apparatus. From the data of Bullock (1945) and Knight-Jones (1952) the 

 nerve cells of Enteropneusta appear to have a more or less diffuse distribution 

 throughout the integumentary epithelium. 



On the other hand, the mass of the dorsal cluster of ganglionic cells in the 

 protosoma of Pogonophora seems to be a true, though very primitive, central 

 section of the nervous system, deserving to be called the brain. Though its 

 fine structure remains unknown it is clear that it possesses the proper archi- 

 tectonics, if only because of the arrangement of the main fibre bundles 

 within it. In this respect the nervous system of Pogonophora is considerably 

 more advanced than that of Enteropneusta. Another indication of its higher 

 differentiation is the comparatively feeble development of the general epider- 

 mal nerve net. 



A last anatomical distinction between the two groups lies in the degree of 

 development of their giant nerve fibres. In Enteropneusta they are confined 

 to the collar, where they run along embedded in the walls of the nerve tube 

 (Spengel, 1884, 1893) and seem to be the axons of large unipolar nerve cells 

 situated in the same part of the nervous system (Bullock, 1944, 1945; 

 Knight-Jones, 1952). In Pogonophora the neurocords or giant nerve fibres 



