NO. 8 THE POLYCHAET — RAW I5 



Stages i to 10 of table 2 

 Stages. 



I. Brain and left ventrolateral nerve cord of the annelid ancestor of the 

 polychaet. (The nerve cord was possibly not ganglionated at this stage. 

 The podial-longitudinal and the nuchal nerves are omitted.) 



2. The oesophageal invagination has produced open nerve loops, along 

 which lie an unknown number of ganglia. 



3. These oesophageal nerve loops have been closed by the advance of their 

 terminal ganglia to the brain to form mid-brain I. 



4. The oesophageal nerve loops have been simplified as suggested by the 

 present visceral oesophageal nerves. 



5. The 1st pharyngeal or upper jaw invagination has produced open nerve 

 loops, along which lie a considerable number of ganglia. 



6. These upper jaw nerve loops have been closed by the advance of their 

 terminal ganglia to the brain to form mid-brain II, which has pushed I 

 to the rear. 



7. The upper jaw nerve loops have been shortened by a coalescence with 

 the visceral oesophageal cord. 



The 2nd pharyngeal or lower jazv invagination has produced an open 

 nerve loop without, however, involving any ganglia. 



9. These loops have been closed by the advance of their terminal ganglia 

 to the brain to form mid-brain III, which has pushed II and I to the 

 rear. (In this way I has been pushed up to the ist nuchal ganglia which 

 will join it as hind-brain.) 



10. These nerve loops have aborted and the hind-brain is added. 



