PLUMES OF CEPHALODISCUS NIGRESCENS. 29 



Post-oral Lamella. 



The post-oral lamella consists of a pair of flaps projecting laterally fr6m the 

 right and left sides of the mouth. The flaps are continuous with one another behind 

 the mouth ajDerture, 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 [i.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 Imnds 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 pre.ssure that the apex breaks up, and a side view 

 is difiicult 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 diff"erent from those pigmented 

 cells of the central face of the plume-axis, whereas in C. dodecalop/ms 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 extension, and in this one and in the 



