PLUMES OF CEPHALODISCUS NIGRESCENS. 29 
Post-oral Lamella. 
The post-oral lamella consists of a pair of flaps projecting laterally from the 
right and left sides of the mouth. The flaps are continuous with one another behind 
the mouth aperture, but the connecting part is very narrow. The lateral flaps are 
mostly wrinkled at their edges in a manner which suggests that they are now in a 
partially contracted state, and would in the living animal be of greater extent. They 
may cover the collar pores and gill-clefts, but much depends on the degree of 
contraction of the individual examined; their probable function is to direct the 
food-current into the mouth and to separate it from the current that is issuing from 
the gill slit. The food-current coming down the grooves on the outer sides of the 
plumes probably passes into the narrow cleft-like space between the buccal shield and 
the post-oral lamella, and thus into the mouth. 
Plumes. 
The plumes, of which there are usually fourteen, are more or less symmetrically 
disposed. The inner face of each (7.e., that face which is directed towards the middle 
of the bunch of plumes) is convex as regards its length (figs. 25-27, plate 5), and is 
also convex as regards its breadth, except for a shallow and narrow groove which 
extends along the basal two-thirds and separates the two broad bands of pigmented 
epithelium. This groove, which marks the position of the nerve of the plume-axis, 
dies away towards the extremity, and the two black bands approach one another 
and fuse into one broad band. At its middle the axis is about twice or two and 
a half times as broad as it is thick. 
The apex of the plume varies in shape according to the degree of extension 
of the whole plume. In a moderate state of contraction, such as that in which the 
majority of the plumes are found, the apex is rounded, smooth, black all over, and 
bears no pinnules (figs. 23 and 26). In consequence of the curvature of the axis 
the true shape of the apex is not readily made out, for the plume cannot be- got to 
lie flat without applying so much pressure that the apex breaks up, and a side view 
is difficult to obtain on account of the breadth of the axis. A few specially good 
examples of plumes are shown in figs. 23-27. 
The extremity is not differentiated to the same extent as is that of C. dodecalophus ; 
it does not appear like two-thirds of a sphere set upon the end of the plume-axis. 
The cavity of the plume-axis does not enlarge at the extremity as it does in C. 
dodecalophus, and the cells of the extremity are not different from those pigmented 
cells of the central face of the plume-axis, whereas in C. dodecalophus the cells of 
the apical enlargement are much taller and more regularly set than those of the face 
of the plume-axis. 
Plumes found fully extended have the terminations almost invariably damaged, 
but in fig. 24 is shown a plume in moderate extevsion, and in this one and in the 
