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 liy fine trabeculje, 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 (b.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. 



