“ISOCIRRUS YUNGI Gravier.
Gravier (1911), p. 122, pl. IX., fig. 109; pl. X, figs. 115-120.
Gravier's type specimens were two fragments, of which one was an anterior portion and the other a short piece of the hinder end. They are a good deal smaller than the specimens collected by the "Aurora," and the tube in which the animal lived was not collected or reported upon.
Eleven individuals of the species, mostly imperfect, were obtained at a depth of 157 fathoms. In most of them the body is encircled by a portion of the mud-tube, which has a very thick wall; thus a worm measuring 7 mm. in diameter fills the lumen of a tube whose external diameter is 11 mm., so that its thickness is 2 mm.
A complete individual studied is 110 mm. in length with a breadth of 5 mm. anteriorly. Another one, lacking only the anal funnel and a portion of the long preanal segment, attains a length of 135 mm. with a breadth of 7 mm.; but judging from some of the fragments still within their tubes, the species may reach even a greater size than this.
The colour is almost uniform pale brown, except that in one or two cases the 5th and 6th segments are darker than the rest; the glandular band at the commencement of the segments is nearly white.
The complete individual first mentioned above consists of the " head," followed by nineteen elongated chætigerous segments and a long preanal segment ; this bears six. glandular half-rings, corresponding in position to the uncinal glands to be described below, and so probably represents six segments; of these glands the three anterior extend further round the body than do the other three. The body terminates as usual in an anal funnel.
The uncini commence on the 5th segment, and the neuropods of the last seven segments are very prominent.
In the 2nd, 3rd, and 4th segments there are one or two short, stout, bluntly-pointed spines below the small bundle of capilliform chætæ : Gravier in speaking of these says that there is "une rang e de crochets ventraux"; if by this he means a vertical row of hooks the statement does not apply to these specimens.
The "head," i.e., the prostomium and peristomium, is equal in length to the 2nd segment ; each of the next five segments is approximately equal to this; but each of the following six is a good deal longer. But it depends on the state of preservation, for in some extended worms this difference between the first five and the next six is hardly noticeable. The chætæ in this genus are inserted near the anterior boundary of the segment; in the first five, at about one-third ; in the following six or more, at about one-fourth the length of the segment.
There is a glandular band surrounding the prechætal region of each segment, this is interrupted on each side by a deep, narrow, horizontal furrow. This glandular band forms, in some individuals, a feeble collar, but in extended specimens the overlap is not apparent. At the segment on which the true hooks appear, namely, the 5th, there is also a post-chætal gland; at first this is narrow, but as the series of uncini becomes longer this gland increases in width as well as in length. By the 8th segment the prechætal band is differentiated into a narrow dorsal and a wider ventral portion; and the ventral gland, which now appears as a large oval patch, overlaps the previous segment more distinctly. By the 10th the dorsal gland has become considerably reduced, and on the 11th has disappeared, so that posteriorly only the ventral or uncinal gland persists ; this enlarges in the segment further back, till in the 17th, for instance, it covers half its length.
I have given these details as Gravier says nothing about them; the arrangement is entirely in agreement with the general character of the glands described by Arwidsson for the genus.
Gravier's account of the prostomium, or cephalic plate, needs no addition, though his figure is somewhat diagrammatic. The dorsal transverse portion of the membrane that surrounds the plate is crenated. He states that there is a dozen low rounded lobes, but I find that the number and form is variable. Sometimes they are uniform in size, though in some individuals they may be larger than in others; in the former case I counted 18 lobes, in the latter as many as 25. In other individuals, the smaller and larger lobes are irregularly alternating.
The anal funnel, as the generic name implies, is surrounded by uniform digitations, of which I count as many as 36.
A figure of the capilliform chætæ is given by Gravier; but his interpretation of the hook is not quite in agreement with what I see. The large fang is surmounted by four others of much smaller size; and there are some laterally situated small teeth at the base of the large fang. Further, the bay between the fang and the bundle of threads is deeper and roughly semicircular in outline.
It may be that these small differences depend on the segment or region of the worm from which the uncinus is taken.
Locality.—
Station 3, 157 fathoms.
Distribution .—Petermann.”
(Benham, 1921)
