PLUMES OF CEFHALODISCUS. 229 



Figure Q shows the developing sixth plume, wedged iu 

 between the base of the fifth and the antero-lateral edge of 

 the post-oral lamella;^ it is already pear-shaped, and with a 

 narrowing base. The preparation here drawn is presenting 

 its dorsal surface to the observer, and the fifth plume is 

 showing its groove facing postero-dorsally. 



In figure r the fifth plumes are larger and with better 

 developed pinnules than in q, but the sixth plumes are not 

 more advanced; indeed, on the left side the sixth is less 

 developed than it is in q. It is worthy of special remark 

 that, while in the case of the first and second plumes the 

 development of the right and left plumes of each pair is 

 regular and symmetrical, in the fifth and sixth plumes the 

 one is very frequently more advanced than the other of its 

 pair. The grooves of the fifth plumes should be facing 

 posteriorly or postero-dorsally; by the pressure of the cover- 

 glass upon the preparation the plumes have twisted round 

 somewhat. For the same reason the first plumes have their 

 grooves facing to the left, while the third plume on the 

 right side is twisted right round so as to present its grooved 

 surface to the observer. The pinnules on the first plumes 

 are now fairly long, and the pinnules of the third and fourth 

 plumes are finger-like, and resemble those of the first plumes 

 in the stage shown in figure o. 



s differs from r in that the sixth plumes are more advanced, 

 the right hand one showing signs of pinnules. The fifth 

 plume shows its groove facing posteriorly. The bud from 

 which the preparation shown in figure s was made was a very 

 late bud, so large, and with such great development of the 

 visceral mass, bringing the base of the stalk from a posterior 

 to a ventral position, that, if it was not still in connection 

 with a full-grown polypide by means of its stalk, one might 

 easily mistake it for an adult polypide. 



^ Tlie close relation obtaining between the base of the last-formed plume 

 a»id tlie edge of the post-oral lamella was, I believe, not known until it was 

 pointed out by Harmer two years ago ('Pterobranchia of tiie "Siboga" 

 Expedition,' 1905, p. 36). 



