PLUMES OF CEPHALODISCUS HODGSONI. 55 
some cases retarded—the width of the posterior lobe is less than that of the anterior 
lobe (text-fig. 17, A, B, C). 
Except in being somewhat larger, and in that the width of the posterior lobe 
is a little greater in proportion, the shield does not differ materially from that of 
C. dodecalophus. Text-figure 17, F shows a composite figure made from seven 
well-expanded shields of the latter species. The width of the front lobe is °73 
to *83 mm., and that of the hind lobe rather less. The centre of the red line is 
‘60 to ‘66 mm. from the front edge, and ‘11 to ‘16 mm. from the hind edge, the 
average antero-posterior diameter being *77 mm. In young buds the red line is 
relatively farther forward than in the adult polypide, as in C. hodgsoni. 
Plumes. 
The normal number of plumes is twelve, but the sixth pair develop late, and 
a full-sized polypide, with buds of its own, and with well-developed ovaries, may 
have only ten fully-grown plumes. Close inspection, however, usually reveals the 
presence in such polypides of a sixth pair, these being very small outgrowths, with 
no pinnules, situated at the most anterior part of the line of attachment of the 
post-oral lamella to the rest of the body. The late-developed sixth plumes of such 
individuals are rarely exactly equal in size; it almost always happens that one is 
larger than the other. No polypide of C. hodgsoni has been found possessing more 
than twelve plumes. 
The plumes bear a fairly close resemblance to those of C. dodecalophus. Each 
consists of an axis, about 1°5 mm. long when well extended, with a central coelomic 
space continuous with the collar cavity, with a hollow end-bulb with high epithelial 
cells, and with pinnules arranged in two lateral series (figs. 31 and 32, plate 5). The 
axis is flattened, and grooved along the surface more remote from the central nervous 
system, and the pinnules, which usually number thirty, sometimes as many as forty, 
along each edge are not merely attached to the edge of the axis, but are continued 
across the groove almost to the median line. The pinnules of one side as they 
become free from the edge of the axis are not set exactly in one plane, but they 
alternate slightly, so that when a plume is laid flat on a glass slip under the 
microscope, the pinnule bases will not be all in focus at the same time. Assuming 
that the pinnules are denoted by consecutive numbers along one edge, those bearing 
odd numbers will be out of focus when those bearing even numbers are clearly 
defined, and vice versd. 
The base of each pinnule is enlarged, and this is especially noticeable if 
the plumes are in a state of contraction. New pinnules are produced at the 
base of the plume-axis for a considerable time after the bud has become 
separated as a free polypide, if not for the whole life of the polypide; a few 
small basal pinnules in graduated series are almost invariably to be found. All 
