Comprehensive DescriptionRead full entry
“Exogone heterosetosa, n. sp. (Pl. XXXIII. figs. 15, 16 ; Pl. XXIVA. fig. 11).
Habitat.—Dredged at Station 144A (off Marion . Island), December 26, 1873 ; lat. 46° 48' S., long. 37° 49' 30" E. ; depth, 69 fathoms ; surface temperature, 41°€·0 ; sea-bottom, volcanic sand. It was found amongst the bristles of Lagisca antarctica during the examination of that form.
A minute species, about 3 mm. in length and a third of a mm. in breadth.
The body consists of thirty-four segments, which have a greater transverse than antero-posterior diameter, the latter attaining its greatest proportion posteriorly. The tail has a pair of cirri of considerable length.
The head (Pl. XXXIII. fig. 15) has anteriorly a pair of bluntly conical frontal lobes, which have a thin bridge along the middle line. A pair of eyes occur on each side, the axis of the pairs being directed forward and outward. The anterior on each side is the larger, and has a conical ," lens." A short lateral cirrus is placed on each side of the head anteriorly, and there seems to be a median of similar length, though the state of the preparation renders its presence doubtful. The pharyngeal region presents the usual anterior tooth. The proventriculus shows only the transverse striation of the compressed muscles in the preparation, but glandular papillæ are probably present internally.
Each foot has dorsally a short and somewhat cylindrical cirrus, which, in the compressed specimen, does not project so far outward as the setigerous region. The latter anteriorly bears a tuft of jointed bristles, the shafts of which are for the most part curved below the dilated tip. The distal appendage is minute, with a terminal hook and a basal projection, so that it appears bifid (Pl. XXXIVA. fig. 11). Two other singular bristles occur in the same group : one is a long and slightly curved form with an oblique tip like a beak (probably a modification of the shaft of the jointed kind) ; the other has a shaft of similar length, which distally is flattened out so as to form a spathulate tip with a terminal filament, as in certain Sabellidae and in Magelona. In addition to these, at and behind the twelfth foot, a tuft of long and very fine hair-like bristles is present. These are often broken, as in the figure.
Seven segments (viz., from the sixteenth to the twenty-second bristled segments) of the body posteriorly are filled with the reproductive products, and are densely granular and opaque, while two early embryos are in proximity externally (Pl. XXXIII. fig. 16). The latter are somewhat conical, one end being blunt, with two lateral bosses and three median cirri ; the other, apparently the posterior, being pointed and furnished with at least one cirrus above the termination (the other perhaps having fallen off). The central region of the embryo is occupied by the same deep yellowish granules as in the body of the adult, while externally there are traces of several feet.
The embryos of this genus somewhat resemble those of Syllides pulligera, Krohn. The minute size of this form renders its diagnosis somewhat difficult, but it leans rather to Exogone than to any of the allied genera.”