22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



mium traveled the tentacular cirri of the time to form the antero- 

 lateral tentacles. No eyes accompanied this third pair of cirri ; and it 

 has been mentioned that eye-spots have been lost from the first body 

 segment (that with tentacular cirri) in Eunice punctata. 



The added ganglia, following the established rule, addressed them- 

 selves to the fore-brain, as had their predecessors, whom they in turn 

 forced to the rear. They trailed again the ventral cords and by 

 applying themselves to the fore-brain caused the double roots of the 

 oesophageal connectives. They brought also with them new visceral 

 nerves — the present fore-brain roots of the pharyngeal visceral cords. 

 It is significant that these arise from the ventral roots of the oesopha- 

 geal connectives — both were brought together, and at the time of the 

 addition of mid-brain III. 



How exactly the simplification of this visceral system was effected 

 may not be clear ; but the three courses of stage 9 imply redundancy, 

 and one course alone seems to have survived, namely, the most 

 ventral, because this alone would be associated in origin with the 

 new oesophageal connectives. 



One other change and the basic plan of the brain was completed: 

 mid-brain I had now been pushed so far to the rear that it has 

 approached the nuchal ganglia and these also have added themselves 

 in stage 10 to constitute the hind-brain. With this the brain of the 

 ancestral polychaet has been evolved. The stomodeum is completed, 

 and this has involved the completion of the brain. 



LIMITS AND SIGNIFICANCE OF THE PROSTOMIUM OF THE 

 POLYCHAET 



Regarding the extent of the prostomium, there has been much 

 difference of opinion: it has indeed been a subject of keen contro- 

 versy. But there is perhaps no need to recall here the numerous 

 divergent views. Considering the great antiquity of the polychaets, 

 it is not surprising that the head should present great variety, and 

 suggest different opinions. The ideal prostomium, the prototype, is 

 the region in front of the mouth in the common ancestor of the group; 

 and this may be expected to be indicated by what the most primitive 

 polychaets have in common there. By previous writers this seems 

 generally to have been equated with the prostomium of the hypo- 

 thetical primitive annelid ; but this is opposed by the claims advanced 

 in this paper. 



As to its nature and origin, two markedly contrasted conceptions 

 have been held : ( i ) that in its origin it was segmental in some way 

 or other; (2) that it was a unit, and unsegmented (see table i. 



