wings. The others in this region have two groups—an upper with longer shafts, more
tapered and slightly curved tips with narrow wings (Plate CXXVIII, fig. 3), and a dense
lower group of spatulate forms (Plate CXXVIII, fig. 3 a) with a process at the tip.
The bristles of the middle region form the usual bristle-pillar of rather short bristles
with striated shafts, comparatively broad striated wings and very finely tapered tips, two
series, a longer and a shorter, being conspicuous. The shaft has a distinct curvature
ab the junction with the tip. The posterior bristles are fewer in number, and have wings
distinctly striated and very attenuate tips (Plate CXXVIII, fig. 3 b). In glancing at
small preserved specimens, it is found that most have the posterior bristles directed
forward, and in several the anterior have the same direction. In the larger forms the
anterior bristles are often directed upward, outward and slightly backward. The anterior
hooks, which occur on all the anterior segments except the first, are avicular (Plate
CXXVIII, fig. 3 c), with a sharp main fang and above it a series of minute teeth on the
crown (Langerhans shows about twenty-four) and a moderately long base. Stride pass
from the neck to the base, after curving round the prow. These hooks are accompanied
by the short bristles with the spatulate tips. The posterior hooks differ only in their
smaller size and the brevity of the base. In comparing the larger with the smaller forms
from Plymouth, the essential characters of the bristles and hooks are as well shown by the
smaller as the larger. The tube is composed of a tough internal lining coated with fine
sand-grains, the whole being firm and resistant—especially in the Canadian examples.
In the spirit-preparations it often happens that the annelid can only be removed from
its tube by the rupture of the tissues. Lo Bianco mentions the occurrence of a colony
of Balani and of Cellepora on the tube, and that occasionally the annelid lives in a sponge
(Hircinia) at considerable depth off Naples.
Reproduction.—Specimens from Port Erin in the second week of September had
large and apparently nearly ripe ova. The great numbers of this species and of all sizes
on certain sites is noteworthy.
The Sabella neglecta of Sars1 appears to be a closely allied if not identical form. It
is devoid of tentacles and the tips of the branchial filaments are short.
Arnold Watson8 (1906) gives an account of a rock-boring Potamilla, probably P.
Torelli, at Tenby, which reproduced both anterior and posterior ends, and which, he states,
formed the anterior region by the addition of one segment only, the adjoining “ abdominal ”
region having its bristles and hooks reversed to suit the normal condition of the parts.
Treadwell8 mentions the occurrence of paired eyes just behind the tip of the
branchiae in an example from Honolulu. Such has not been observed in British waters.
Mesnil and Caullery (1911) give an interesting account of remarkable Protozoan
parasites (Haplosporidium potamilla and levine) which they found in the tissues of this
species, and which they could not readily compare with anything known. The parasites
displaced the alimentary canal in the infected segments. The memoir is illustrated by
excellent text-figures and plates. The same authors* have recently (1920) recorded the
1 ‘ Reise Lofot. og Finm./ p. 83, and ‘ Christ. Selsk. Forhandl./ p. 31 (sep. copy).
2 * Proceed. Roy. Soc./ vol. lxxvii, p. 332, 4 text-figs.
3 ‘U.S. Comm. F. & F./ 1906, p. 1178.
* fComp. Rend. Paris/ October, 1920, p. 683.
occurrence of scissiparity in this species in so far as they found in the tubes adults with
regenerating posterior extremities, whilst beside them were smaller forms of twenty-eight
to thirty:five segments, corresponding to the original tails, and with developing branchiae.
They concluded, therefore, that the species was scissiparous.-
Potamilla incerta, Langerhans, which Dr. Allen procured by the dredge on Yealm
ground, Plymouth, seems to be the young of Potamilla Torelli, and in this Prof. Fauvel
agrees. Indeed, it is difficult td find satisfactory distinctions between Potamilla reniformis
and P. Torelli, for the absence of ocular points on the branchial filaments may not be of
capital importance. P• incerta has five thoracic segments.
3. S abella B.C. Plate CXX, fig. 6—ventral view of anterior region; Plate CXXVIII,
figs. 1—1 c—bristles and hooks.
Specific Characters^?-After removal of the branchiae the cephalic region presents no
deep dorsal fissure. Collar commences as a narrow process on each side of the dorsal
furrow, slopes obliquely forward and outward and incurves laterally and ventrally into
a deep though thin lamella, which attains its maximum in the mid-ventral line, where it is
.separated from its fellow by a fissure. Branchiae reddish brown in spirit, about eleven
filaments on each side, connected only at the base ; pinnas short; a terminal strap-shaped
process. Body distinctly segmented; segments fifty-six or more. Ventral surface with
scutes throughout. Five or six bristled segments anteriorly, the hook-rows being one less
in each case. Bristles with straight shafts, finely tapered, tips and narrow wings, dorsally
and ventrally with spatulate tips, the outline of the wings being more or less elliptical.
Posteriorly the bristles form a tulip-like fascicle. Anterior hooks with a main fang and
a series of serrations above it, a boldly convex prow, and an oblique, tapered base. A
series of modified (beak-like) bristles accompany them. Posteriorly the hooks are
smaller, their necks longer and the bases more oblique.
Tubes of secretion with a few grains of sand attached.
It appears to be a variety of Potamilla Torelli, though presenting certain features of
its own.
Synonym.
1916. Sabella B.C., McIntosh. Ann. Nat. Hist., ser. 8, vol. xvi, p. 13.
Habitat.—Procured at Berehaven in numbers by the Royal Irish Academy’s Expedition,
1886. When the branchiae are absent the cephalic region presents no deep fissure as
in ordinary Sabellids, but the slight dorsal furrow ends in a solid mass, which, with an
incurvation in the middle, passes from side to side, and then bounds the region laterally
to the ventral surface. This rim forms a projecting base to the branchiae. The collar
commences as a narrow process on each side of the dorsal furrow, slopes obliquely
forward and outward, and inclines laterally and ventrally into a deep though thin lamella,
which attains its maximum in the mid-ventral line, where it is separated from its fellow
by a fissure. The branchiae preserve much of their reddish-brown coloration in spirit,
and they are of considerable length. The pigment is arranged on the pinnae so as to