NO, 8 THE POLYCHAET — RAW I9 



Mid-brain III, the anterior section, does not carry an eye in 

 polychaets. Perhaps it is because by this time the eyes of the head 

 were so much more efficient than the segmental eyes of the trunk that 

 the pair belonging to the lower jaw segment has aborted. On the 

 other hand, it should be noted that the present first body segment is 

 without an eye-spot, so the abortion may have been effected earlier. 



It will be agreed that the presence of five (the equivalent of six) 

 similar antennae, as is still seen in the Eunicidae and the Amphi- 

 nomidae, is the primitive condition in the polychaet. The development 

 of so great a number by nature on part of a unit brain, such as is 

 conceived either by Racovitza or by Hanstrom, is extremely im- 

 probable, if not indeed unimaginable; whereas, on the theory here 

 advanced, it follows quite naturally. 



The parallel evolution of the mid-brain, the stomodeum, and the 

 visceral nervous system have now, it is hoped, been read at least in 

 outline. 



COMPLETION OF THE BRAIN 



The mid-brain has now been evolved and added to the fore-brain. 

 Arguments have already been advanced for the addition of the hind- 

 brain or nuchal brain last of all. Its incorporation is probably to be 

 attributed to the rearward extension of the brain due to the successive 

 additions of mid-brains I, II and III, which brought mid-brain I to 

 aggregated ganglia of the nuchal organs. This suggests that the 

 incorporation of the hind-brain and of mid-brain III took place at the 

 same time. This completed the basic plan of the polychaet brain. 

 Soderstrom, however, who first claimed that it was an addition from 

 the body, claimed also that for this reason it could not form part of 

 the prostomium. But had he espoused the theory here advanced, on 

 the same principle the whole of the mid-brain would have to go. 

 Gustafson, though he accepted the first claim of Soderstrom, retained 

 the nuchal brain as part of the brain and presumably of the pros- 

 tomium. On the theory here presented there is still greater reason 

 to accept the hind-brain as an integral part of the brain and of the 

 prostomium. This question is further discussed below (p. 25). 



SUMMARY OF THE AUTHOR'S THEORY 



Now that the intimate relationships between these three systems of 

 the brain, the stomodeum, and the visceral nervous system have been 

 deduced and outlined, it is advisable to picture in greater detail the 

 modus operandi of these supposed changes which are illustrated by 

 table 2, stages i to lo. 



