PLUMES OF CEPHALODISCUS, 251 



the plumes of the first pair in Cephalodiscus. The second 

 and later plumes of Cephalodiscus are not represented in 

 Rhabdopleura. Besides the figures of buds of Rhabdo- 

 pleura published by Lankester and reproduced here as text- 

 fig, llj those by AUmau ^ and Schepotieff ^ may be consulted 

 with advantage. 



Summary. 



1. The torsion of the axes of the first and second plumes 

 of the buds of Cephalodiscus described by Mastermau 

 does not take place. The grooved faces of the axes of these 

 plumes are in the first instance directed towards the dorsal 

 face of the buccal shield, and they maintain this relation 

 through life. 



2. The last two pairs of plumes do not arise between the 

 first two pairs of plumes and the buccal shield, as described 

 by Masterman, but they arise on the dorsal side of those 

 plumes, the side remote from the shield. Their grooves are 

 directed, not towards the shield, but away from it. 



3. The evidence derived from a study of the buds of 

 Cephalodiscus bears out the contention of Harmer that 

 the series of plumes and post-oral lamella are continuous. 

 The plume-axes develop in pairs successively, the median 

 pair first, then a pair lateral to these, and so on. When the 

 post-oral lamella appears, usually at the time Avhen the third 

 or fourth pair of plume-axes are beginning to develop, its 

 edge is in contact with the last-developed pair of plume-axes. 

 The fifth plume-axis appears between the fourth plume-axis 

 and the end of the margin of the post-oral lamella, the sixth 

 between the fifth and the end of the post-oral lamella, and, in 

 the case of Cephalodiscus nigrescens, in which there are 

 seven pairs of plumes in the adult, the seventh arises 

 between the sixth and the end of the margin of the post- 

 oral lamella. 



1 'Quart. Jouru. Mic. Sci.,' vol. 9, 1869, pi. 8, Hgs. 7 and 8. 

 * ' Zool. Anzeiger,' vol. xxviii, 1905, p. 802, fig. 6, 



