PLUMES OP CEPHALODISOUS. 245 



like dodecaloplius, lias a tubariiim in whicli the polypides 

 live in a common cavity^ which opens by several ostia to 

 the exterior. In my report on Cephalodiscus in the 

 ' '' Discovery " Expedition Reports ' (pp. 23, 24) I ventured 

 to suggest that, in the species of the former kind (i. e. species 

 of the sub-genus Idiothecia) the late buds severed their 

 connection with the parental stolon, and wandered over the 

 surface of the colony in order to settle down in some con- 

 venient situation, usually the apex of a branch, and to secrete 

 tubes of their own. The fact now recorded, namely, the 

 inability to discover in the material of Cephalodiscus 

 gilchristi any individuals intermediate in growth between 

 buds with small fifth plumes and full-grown polypides with 

 well-developed sixth plumes, supports that suggestion, for 

 the late buds, while developing their fifth and sixth plumes, 

 would be migratory forms on the surface of the colony, and 

 would be brushed off while the specimen was in the trawl. 

 In the species of the sub-genus Demiothecia (e.g. 

 Cephalodiscus hodgsoni and C. dodecalophus) the 

 late buds would complete their development within the 

 common cavity of the tubarium, and while being di-awn up 

 in the trawl, and while undergoing fixation in preservative 

 fluids, would not be more likely to be lost than the younger 

 buds and the adults of the colony. 



General Remarks on the Collar and its Appendages. 



Haimier in his recent ' Monograph on the Pterobranchia 

 of the " Siboga " Expedition ' (p. 30) lays stress on the rela- 

 tions of the plumes (or "arms") to the post-oral lamella (or 

 "operculum") in Cephalodiscus. He points out that 

 "in Balanoglossus the anterior margin of the collar forms 

 a projecting fold encircling the base of the proboscis-stalk. 

 The ventral half of this fold may be regarded as constituting 

 a lower lip, while the dorsal part is connected, in the middle 

 line, with the anterior neuropore. In Cephalodiscus the 



