30 W. G. RIDEWOOD. 
remnants of injured, fully-extended plumes the apex is seen to be pointed. A 
plume in the ordinary condition is about ‘8 mm. or 1 mm. in length; a fully 
extended plume is 4 mm. long. 
On the outer face of the plume-axis, between the two series of pinnules, is a 
ciliated groove, broad, deep and V-shaped in section at the base of the axis, narrow 
and about as deep as wide along the greater part of it (fig. 28). Towards the tip 
the groove dies away, and the outer face of the terminal portion of the axis is slightly 
convex. 
The pinnules arise from the edges of the outer face of the plume-axis, and form 
a regular and close-set series from the base of the axis to near the tip (fig. 23). The 
pinnules arise obliquely along the edges of the axis, so that in a transverse section of 
a plume almost all of the sections of the pinnules are oblique. Further, the epithelium 
of the pinnule is continued along the grooved face of the plume-axis as an oblique 
ridge which stops only just before reaching the median plane of the axis. A transverse 
section of the plume-axis, therefore, always shows the ciliated epithelium of the 
aponeural groove in the form of an irregular sinuous line (fig. 28, c.e.). 
The interior of the plume-axis is occupied by a cavity, directly continuous with 
the collar-cavity, and traversed by fine trabecule, irregularly placed, and with small 
nuclei adhering to their sides (fig. 28). There is an important tract of longitudinal 
muscle fibre on the aponeural side of the plume-axis, lying to the inner or coelomic 
side of the layer of skeletal matter that underlies the superficial epithelium. On the 
neural side of the axis the muscle fibres are less abundant. There seems to be no 
special mechanism for the extension of the plume-axis, and Harmer is probably 
correct when he surmises (10, p. 42) that elongation is effected by fluid pressure in 
the collar coelom. 
The section drawn in fig. 28 does not show the neural groove, being cut too near 
the apex of the plume-axis, and the two masses of pigmented epithelium on the neural 
face of the axis are closer together than they would be in a section taken nearer the 
base of the plume. Lying immediately internal to the nerve tract, and appearing as 
a space in the sub-epidermal skeletal layer, being bounded on all sides by the skeletal 
substance, is the blood-vessel of the plume (0.v.). 
The pinnules form a regular and close-set series from the base of the axis to near 
the tip (figs. 23 and 26). The longest are those arising about half-way along the 
axis. The pinnules at the basal end are sometimes very short, but this is probably 
in plumes which have not yet reached their full development. The number of 
pinnules along each side of the axis varies from seventeen to fifty in plumes which 
have apparently reached their full development. The most distal pinnules may project 
beyond the apex of the plume-axis (figs. 24 and 27) or not (figs. 23 and 26). 
In some cases the most distal pinnules of three or four of the plumes of the 
individual are found to be adherent to the margin of the tube in which the animal 
was living. They are greatly attenuated, and their cells are full of highly refractive, 
