PLUMES OF CEPHALODISCUS. 235 



of plumes arise between the first three pairs and the shield 

 and have their grooves directed towards the shield (see text- 

 fig. 1 of this paper), I am in a position to confirm Harmer's 

 opinion^ that this statement is without foundation, and that 

 Masterman's earlier enumeration of the plumes^ is the 

 correct one. In Cephalodiscus dodecalophus, as in 

 C. hodgsoni, the grooves of the first and second pairs of 

 plumes, developed in such a position that they face the 

 buccal shield, continue to face the shield, and the last two 

 pairs of plumes are not developed between the first two 

 pairs and the shield, but on the side of the first two plumes 

 remote from the shield, i.e. to their dorsal side; and the 

 grooves of the last-formed plumes are not directed towards 

 the shield, but away from it. The grooves of the third and 

 fourth plumes face somewhat laterally. 



The six pairs of plume-bases are set on an ellipse, and the 

 grooves of the twelve plumes face away from the foci of the 

 ellipse. The six grooves of the right side lead down into 

 the space between the shield and the post-oral lamella on the 

 right side of the mouth; similarly the six grooves of the left 

 side lead into the left side of the mouth (see text-fig. 8, 

 C. nigrescens). 



Development of the Plumes in Buds of Cephalodiscus 



nigrescens. 



While in buds of Cephalodiscus hodgsoni and 

 Cephalodiscus dodecalophus the first pair of plumes 

 begin to appear at about the same time as the buccal shield, 

 in those of Cephalodiscus nigrescens the buccal shield 

 is well differentiated, and with a well-defined posterior lobe 

 (text-fig. 6, a) before there is any sign of plume development. 

 When the first pair of plumes make their appearance (b, 1) 

 they are small as compared with the bud as a whole, and 

 when the second plumes begin to develop (c, 2) the buccal 



1 • Pterobranchia of the "Siboga" Expedition/ 1905, pp. 36, 37. 

 - 'Quart. Journ. Mic. Sci.,' vol. 40, 1897, p. 346, pi. 26, fig. 36. 



