On the Development of the Plumes in Buds of Cephalodiscus

Abstract

1. The torsion of the axes of the first and second plumes of the buds of Cephalodiscus described by Masterman 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 Harrner 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 when 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 postoral lamella. 4. The line of plume-bases in the adult, when viewed from the front, is an ellipse, incomplete on the upper side (text fig.8); and the line of the attached edge of the post-oral lamella is another ellipse, similarly incomplete. The two ellipses join where they are incomplete. 5. Separate accounts are given in this paper of the plume-development in buds of Oephalodiscus hodgsoni, C. dodecalophus, C.nigrescens, and C.gilchristi. 6. The plumes develop relatively later in buds of C.nigrescens and C.gilchristi than those of C.hodgsoni and C.dodecalophus.