NO. 8 THE POLYCHAET — RAW 21 



antennae and a pair of eyes. For after the invagination the parapodia 

 of this segment may well have become restricted to a pair of dorsal 

 cirri — "tentacular cirri" — as in the first body segment of Eunice, 

 associated with a pair of eyes, present on all the other body segments ; 

 and both cirri and eyes advanced to the prostomium with their ganglia. 

 The oesophageal loops, though represented in stage 4 as simplified, 

 may perhaps have continued in existence until the simplification of 

 the first pharyngeal, stage 7. 



The 1st pharyngeal invagination (stages 5 to 7). The pair of nerve 

 loops due to this (stage 5) are apparently represented by the lower 

 half of the oesophageal ring and the cords joining gr,, g^, and the 

 X-form body, together with part of the pharyngeal cords (n ph of figs. 

 4 and 5). The unknown primal ganglia along it are again suggested 

 by a few plus signs. 



The loop was closed (stage 6) by the advance of the foremost 

 uninvaginated ganglia to the brain to form mid-brain II, which also 

 carried with it pharyngeal cords of the visceral nervous system and 

 the ventral chains. But, as after the oesophageal invagination, it was 

 to the fore-brain that they came, entering between this and mid-brain 

 I and pushing the latter to the rear. As yet, there was no fusion 

 between those brain parts. Again the fore-brain appropriated these 

 added nerves, which arrived outside of the previous system. Mid- 

 brain II again, like I, had to preside over the pair of antennae (the 

 previous "tentacular cirri") and the pair of eyes, which had advanced 

 with these ganglia. 



After this stage came a simplification of this ist pharyngeal visceral 

 nerve loop, and not improbably of both the oesophageal and 1st 

 pharyngeal loops together (stage 7). If two complete loops still 

 existed, much was redundant, and the dorsal side of the first pharyn- 

 geal seems to have joined the dorsal side of the oesophageal ; whereas 

 other parts aborted. 



The 2nd pharyngeal invagination. This, the lower jaw invagina- 

 tion (stage 8), may be expected to have produced small nerve loops. 

 No ganglia are associated with the lower jaw, so no ventral ganglion 

 pair was involved. The invagination was probably due to the use 

 by the mouth of the previously foremost pair of extra-stomodeal 

 appendages, the dorsal cirri of which at the same time functioned as 

 tentacular cirri ! These appendages are represented by the lower jaw 

 plates and pads. 



The new visceral nerve loop was closed by the advance of the fore- 

 most pair of ventral-chain ganglia to the brain, in the same way as 

 before, to form mid-brain III (stage 9). With them to the prosto- 



