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 (7.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, Il, SS) 
