258 BISPIRA VOLUTACORNIS. 
extends on each side of the middle line into a triangular reflected flap, the anterior surface 
of the whole being of a rich deep brownish purple bordered with white. If the base of the 
separated branchial system be examined, a facet marks the dorsal end of each of the semi- 
circles of firm tissue for attachment, and the facet fits on the chordoid pillars on each side 
of the dorsal furrow. The firm basal mass of each branchial fan is bridged on the ventral 
side of the facets by a narrow but firm band. From the ventral aspect (Plate CXIII, fig. 5, 
and Plate CXV, figs. 1—1) the basal mass on each side curves forward and inward to end 
in a thick inrolled edge in the centre of the spire. In the middle line, and attached by its 
basal web to the bridge of connecting tissue on the one hand and the dorsal edge of the 
base of the fan on the other, is the tentacle on each side. These webs are of importance 
in directing currents, and the ventral surface is incurved. The tentacle is short, broad at 
the base and tapered distally. The exterior of its base is tinted brown, and a ridge, formed 
apparently by the prolongation backward of the thickened margin of the flap guarding the 
dorsal edge of the channel from the centre of the spire, keeps the base of the tentacle stiff. 
It strictly belongs to the branchial apparatus and separates with it. The mouth itself shows 
deeply folded walls ventro-laterally, viz. an inner or dorsal smaller fold, and a considerably 
larger fold or lamella running into the body-wall ventrally. 
From the firm, muscular spiral base the branchial filaments pass forward to the number 
of forty-five to eighty on each side, the number apparently increasing with age. ‘They are 
united at the base by a short web, and are comparatively long. Hach filament has the 
camerated chordoid axis, which extends to the base of the terminal process, but does not 
seem to enter it, for only an opaque granular central region with a short web at each side 
is present. On the outer edge of many a pair of well-marked pigment-specks, and in some 
two or three, occur, but their position is irregular. Where three are present, the first is a 
short distance above the base, and the others at irregular distances. A considerable portion 
of the tip is always devoid of them. They are dense masses of blackish pigment apparently 
enclosed in some cases in a capsule. The pinne (barbules, De St. Joseph) are short, and 
form a dense double row along the inner edge of the filament, becoming shorter as they 
approach the tip, where they end abruptly (Plate CX XVIII, fig. 5e). Their colour is brownish 
violet. In young forms with twenty branchie in each fan, a pair of black ovoid eyes occurs 
on the dorsal filaments about a third from the end, whereas the specks are situated near the 
middle of the ventral filaments. 
A section through the anterior region behind the origin of the branchize shows the great 
development of the chordoid supporting tissue (Plate CX X XVI, fig. 22), and the arrangement 
of the parts about the mouth. 
The body is of moderate length for a Sabella, ranging from two inches to six or more 
(13 cm.—I cm. broad, De St. Joseph), and the segments vary from eighty to ninety or more. 
it is rounded dorsally and flattened ventrally, the mid-ventral lime bemg marked by the 
groove from the posterior edge of the anterior region to the tail. The body tapers from the 
posterior third (in spirit) to the tip of the tail, at which the anus is, whilst beneath the tip 
are two somewhat ovoid papillae with pigment-dots. The anterior region has nine bristled 
segments, but the number, as De St. Joseph shows,’ varies much, e. g. from five to eleven, 
and the numbers on the respective sides may differ. The setigerous processes are large, 
1 «Ann. Se. nat.,’ 7° sér., xvi, p. 288. 
