l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



the last to arrive, as indicated by the fact that it bears the visceral 

 pharyngeal nerves and the roots of the oesophageal commissures, and 

 further, they all three diminish in size from front to rear. Each of 

 the three parts of the mid-brain carried with it to the head a pair 

 of cirri, and the first two also a pair of eyes; for each segment in 

 Eunice generally bears a pigment spot with the structure of an eye 

 just above each appendage. The cirri survive as the antennae of the 

 prostomium, but usually in Eunice one pair only of the eyes survives, 

 belonging to mid-brain II. These eyes must have superseded earlier 

 eyes innervated by the fore-brain. When a second pair of eyes exists, 

 these are a posterior pair and belong to mid-brain I. The union of 

 the pair of cirri belonging to this last into the median antenna may 

 be connected, first, with the relegation of the brain division serving 

 it to so posterior a position and, second, with its small size as com- 

 pared with mid-brain II and III (see table 2, stages 5 to 7). 



The second pharyngeal or lower jaw invagination comprises the two 

 sacs beneath the "fore-pads." The upper of these sacs is bounded on 

 its ventral side by the lower jaw pair of plates, there implanted. The 

 median edges of these are free and thickened, forming a pair of 

 crushers, between which the two sacs are in communication ; whereas 

 the anterior edges are sharp and form a pair of chisels. Not im- 

 probably the infolds bearing these plates may represent the highly 

 modified parapodia of a single segment, opposed to one another by 

 the invagination. 



No ganglia or considerable nerves are associated with them, at- 

 tributable to this invagination ; and hence its ganglion pair is probably 

 the one which, with its pair of cirri, subsequently advanced to the 

 brain. Perhaps by this time the nerve cords were more ready to free 

 themselves from the hypodermis. Probably the nerve cords were still 

 lateral in position, or at least not closely approximated in the ventral 

 line as now. On our theory the ganglia of the lower jaw segment 

 were the last of the ganglia from the central nerve cords to join the 

 brain, forming mid-brain III, the largest as well as the last of these 

 additions. Like their predecessors, they in their turn addressed them- 

 selves to, and arrived behind, the fore-brain, for they alone of the 

 mid-brain divisions receive the oesophageal connectives; and they, 

 too, pushed their predecessors to the rear. Again, they brought to 

 the fore-brain not only the central nerve cords which in this case 

 persist as the oesophageal connectives with their two roots, but also 

 the ends of a new loop of the visceral nervous system which persist 

 as the pharyngeal nerves, reaching the brain with the ventral roots 

 of the oesophageal connectives. As before, the other end of this loop 

 has been short-circuited and has disappeared. 



