PLUMES OF CEPHALODISCUS NIGRESCENS. 31 



colourless, transparent granules. Similar granules are found adhering to the surface 

 of the pinnules. Extended pinnules such as these have sand-grains not merely 

 entangled among them, but actually adherent to them. The other plumes of such an 

 individual are in moderate extension only, and one is led to conclude from these 

 relations that, at the time when the colony was placed in the preservative solution, 

 the pinnules of the extended plumes were in the act of secreting one of those 

 increments to the margin of the tube which a longitudinal section of the tube shows 

 so sharply marked off from one another (fig. 12, plate 4). The secreted material, being 

 presumably of a tenacious character, prevented the retraction of the plumes in 

 question, whereas the other plumes were free to contract, and did so more or less. 

 The sand-grains also which came up with the dredge became embedded in the newly- 

 secreted material before it had had time to solidify. 



Each pinnule is roughly circular in section (fig. 29, plate 5), and has a very 

 slightly expanded extremity (fig. 33). A single unilateral series of the yellow-brown 

 cells with one or two black dots occurs along the pinnule. There are none at the actual 

 extremity, but at a short distance from the end there occurs a group of five or six. 



A transverse section of a pinnule shows tall epithelium on one side and low 

 epithelium on the other. The pigment cells occur among the latter (fig. 30). The 

 high epithelium is on that side of the pinnule which is in relation with the aponeural 

 surface of the plume-axis. The low epithelium with occasional pigment cells is 

 continuous with the pigmented neural face of the axis. 



Within the pinnule are two tubular cavities bounded by the skeletal basement 

 membrane, and separated the one from the other by a curved wall of the same 

 substance. The tube which is next the high ciliated epithelium is continuous with 

 the coelomic space of the plume-axis, and contains here and there solitary coelomic 

 nuclei. The other tube is probably a blood-space, although its communication with 

 the main blood-vessel of the plume-axis has not been established by the careful 

 examination to which the sections were submitted. 



The plumes, although disposed in nearly radial symmetry at the anterior end of 

 the body, are clearly collected into two groups, right and left. The lophophoral arm 

 that bears the seven plumes of each side is short and nearly semi-circular. It is 

 broadly attached to the body on the posterior side of its ventral half or more (viz., 

 that nearest the buccal shield). The other part stands free from the body and bends 

 back slightly, so that the sixth and seventh plumes appear to be set at a more 

 posterior level than the first and second (i.e. those nearest the shield). Two 

 consecutive plume-axes touch one another at their bases, whereas the end members 

 of the right and left series (viz., first and first and seventh and seventh) are separated 

 from one another by a slight interval. The fourteen plume-axes are set around the 

 margin of an elliptical area in a fairly orderly fashion, and the whole series of plumes 

 can be laid out flat on a glass slip, radiating from a central point or from the two foci 

 of an ellipse, without separating their bases. 



VOL, II, H 



