Comprehensive DescriptionRead full entry
“Praxilla kerguelensis, n. sp. (Pl. XLVI. fig. 7 ; P1. XXVA. fig. 6).
Habitat.—Dredged at Station 149G (off London River, Kerguelen Island), January 29, 1874 ; lat. 48° 50' S., long. 69° 18' E., in 110 fathoms ; surface temperature, 40°·2 ; sea bottom, volcanic mud.
None of the specimens are perfect, but it seems to be a species of some size, reaching a diameter of 2·8 mm.
The cephalic plate considerably differs from that of Praxilla prætermissa, having a much more expanded margin, with a notch (in lateral views) at the commencement of the upper (or posterior) third. The margin then becomes a little narrower toward the median dorsal notch, which is somewhat shallow. Anteriorly (ventrally) the central conical process is well marked, and it has a pit at the base posteriorly, where it abuts on the central ridge of the flat cephalic plate. The ridge stretches from the conical process almost to the dorsal margin of the plate, is acute, and clearly defined by a deep groove on each side. The posterior part of the ventral median process is comparatively isolated, so that it looks like a disk appended to the boldly defined median ridge. The proboscis is studded with proportionally large and distinct papillæ of a slightly conical or globular shape.
The first three segments behind the buccal form the anterior region, and have simple stout spines inferiorly, which in the third amount to four. The spines are slightly oblique at the tip. These three segments are of moderate length, and the second and third have free anterior margins, forming in each case a collar for the preceding segments. The succeeding five segments are conspicuously marked by a white band at the anterior margin, in front of the rows of hooks, and nearly the whole of this in each constitutes the collar round the base of the preceding segment. The anterior segments (so far as they are present) are short, probably from contraction in spirit. The ventral surface is marked by the usual pale band, which in a few of the anterior segments merits the name of a ridge.
The rows of bristles present for the most part an alternation of stout and slender forms. The former have peculiarly modified, narrow wings, followed by a slight enlargement, and very long and finely tapered tips. On the other hand, the latter are simple tapering bristles without evident wings.
The uncini (Pl. XXVA. fig. 6), in profile, show about five teeth above the great fang. The fibres arise close under the latter. The distal region of the hook is comparatively long, and the shoulder well marked.
The food of this form is peculiarly rich in various Diatoms, small and large, sponge-spicules, and other organic debris. Few sand-grains occur in the pulpy greyish mass.
The contrast between the foregoing food and the same materials composing the exterior of the tube is striking. The coarser, larger, and rougher spicules of sponges glisten all over the latter like minute hairs, while Diatoms in great numbers, rounded bodies (Radiolarians ?) like ova filled with coarse yellowish granules, and the chitinous walls of which are devoid of sculpturing other than the very minute dots seen in many ova, and a few sand particles are amongst the most conspicuous features. The tube is friable and mostly membranous. Its anterior region is formed chiefly of the secretion of the animal.
In the general configuration of the body-wall on section, this form approaches Praxilla köllikeri. The circular muscular layer, however, appears to be thicker, and the nerve-area is kept outside the tense line of this coat ventrally. The bulky region of both dorsal and ventral longitudinal muscles is inferior, each tapering in its progress upward, a feature prevalent in the group. The oblique muscles are inserted just outside the nerve-area. The perivisceral corpuscles are mulberry-like with nodular surfaces.”