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 versa. 



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 



