NO, 8 THE POLYCHAET — RAW 13 



able, for nature in such an unimaginable time would simplify a 

 system complicated by repeated invaginations. And in any case a 

 more detailed knowledge of the visceral nervous system is needed 

 than is furnished by Heider, if we are to trace the history as com- 

 pletely as is still possible. The development of these interrelations is 

 illustrated by table 2, stages i to 10, pp. 14-15. 



The effect of the most ancient, the oesophageal invagination, on the 

 disposition of the nervous system is to be seen in the oesophageal 

 nervous system; but it is perhaps difficult to interpret. Heider de- 

 scribed a considerable system, all of which, excepting the connection 

 with the brain, lies in the oesophageal hypodermis (Heider, 1925, pp. 

 yy and 78). From the posterior lappets of the fore-brain the visceral 

 oesophageal nerves (m, fig. 3), freed from the hypodermis, and soon 

 apposed to one another, extend to the supra-oesophageal visceral 

 ganglion (figs. 4 and 5, gi) ; thence the separated pair of nerves 

 continue rearward (figs. 4 and 5; n co i), and both then branch, 

 one branch of each {n oe /, figs. 4 and 5) continuing presumably for 

 the length of the oesophagus in the side pads (fig. 2), the other 

 {n CO 2, figs. 4 and 5) continuing rearward to join its fellow in the 

 infra-oesophageal visceral ganglion (g^, figs, i, 4, and 5). According 

 to Heider the visceral oesophageal ring, thus completed, has the 

 character of a long-stretched ganglion. From the infra-oesophageal 

 visceral ganglion a single cord in the ventral wall of the oesophagus 

 extends rearward to a terminal ganglion gs (at the level of the 4th 

 podial nerves). From this a pair of nerves go to the sides of the 

 upper jaw sac, and so join up with the pharyngeal system. 



The oesophageal visceral nerve ring mimics the oesophageal central 

 nerve ring ; but the union of the visceral nerves in the midventral line 

 of the oesophagus calls for explanation, in view of the evidence for 

 the lateral position of the nerve cords at that early period. Might it 

 be due to mechanical drag imposed by the upper jaw sac invagina- 

 tion? 



The oesophageal visceral nervous system is much more com- 

 plex than our ideal oesophageal loop ; but some redistribution may 

 well have taken place, since the time when the oesophagus was the 

 whole of the fore-gut. If appendages were present, and were in- 

 vaginated, as is probable, they have disappeared, having been super- 

 seded by those of the jaw sacs. The narrowness of the visceral 

 oesophageal ring in contrast with the great width of the correspond- 

 ing ring of the central nervous system can be attributed to the total 

 absence of oesophageal appendages in contrast with the size and the 

 action of those of the pharynx. 



