“Euchone pallida Ehlers. (Pl. IV, figs. 126-130.)
Ehlers, 1908, p. 158, pl. XXI, figs. 10-15 ; pl. XXII, figs. 1-4 ; Ehlers. 1913, p. 574.
As Ehlers has remarked, the discovery of a representative of this distinctly Arctic and Northern genus in these Antarctic Seas is noteworthy ; and though the species appears to be distinct it is more nearly related to E. rubrocincta Sars than to other species, especially in the absence of definite spatulate chaetae from the dorsal bundles in the thorax. Southern, however (1914, p. 144), states that the resemblance between the shorter and longer bristles is exaggerated by Malmgren ; the shorter ones have wider wings.
Several specimens were obtained by the "Terra Nova," and, except in a few details, they agree with the original description. The points of apparent difference are either matters of interpretation or may be due to the larger size or to better preservation of the material at my disposal.
The worm is at once recognised by the naked eye from the other Sabellids of the region by these four features : (a) the shortness and relative stoutness of the body ; (b) the length of the gill plume in relation to that of the body ; (c) the high collar ; and (d) above all by the curious "caudal membranes" forming the lateral boundaries to the ventral pre-anal groove.
(a) The largest individual measures 55 mm., of which 35 mm. account for the body and 20 mm. for the gill plume ; the thorax is 8 mm. and the abdomen 27 mm. in length ; the breadth of the former is 6 mm. and of the latter 4 mm.
This individual had been removed from its tube before preservation so that it is well hardened and the gills are loosely spread outwards.
The thorax contains 8 segments ; the abdomen 26. I find this number to be constant in each of the specimens examined, whereas Ehlers found 32 segments in a smaller worm.
(b) The length of the gill plume is more than half that of the body, indeed in one specimen the body only just exceeds the gills, as the following figures show :
Length in mm.
Body. Gill.
A .. .. .. 35 20
B .. .. .. 27 22
C .. .. .. 25 15
Ehlers.. .. 22 14
The worm B had also been removed from its tube before preservation, the gills are extended and the body perhaps somewhat contracted. The number of gill filaments is 17 or 18 on each side, whereas Ehlers counted only 14. The gill filament is supported by a single series of skeletal cells, and each barbule has likewise its skeleton of long narrow cells, each of which has at its end a short tubular outgrowth on one side.
Of the five pairs of intra-branchial filaments, which Ehlers regards as vestigial gill filaments, I find that two pairs spring from the base of the gill near the ventral region, of which the more ventral is two-thirds the length of a gill filament and the other about half the length of a filament ; the more ventral is connected with the interfilamentar membrane, but I do not see, even under a microscope, the serrations along its edge that Ehlers figures. The other three pairs lie dorsal of the hood-like tentacles, and are stouter and much shorter than the ventrally-placed structures, being about the same length as the tentacles.
(c) The delicate collar arises from the 1st and 2nd segments and reaches the same level all round its circumference ; it is directed forwards and is not reflected ; its height laterally is 3.5 mm. in the largest individual.
There are two points on which I think that emendations to Ehlers' account must be made. The collar is divided by dorsal and ventral clefts into a right and left lateral lobe. On the dorsal surface there is a deep but narrow cleft ; at its posterior end the dorsal convex margin of the lobe, where it passes on to the surface of the body, becomes thickened and continuous with the outer edge of an elongated gland, the "Nuchal Gland," occupying the dorsal surface of the first two segments of the thorax ; it is traversed by the faecal groove. The gland itself extends forwards to the base of the gill support and ends at about the level of the anterior margin of the collar.
In another individual the right and left lobes of the collar overlap in the middle line so that the greater part of the gland is concealed from view, and only its anterior end is seen between the gill bases. This is what is shown in Ehlers' figure (pl. XXI, fig. 11), and he seems to have misinterpreted this anterior end of the nuchal gland as representing the "Kopflappen."
Although the anterior margin of the collar keeps the same level all the way round, yet, owing to the forward extension of the first segment on the ventral surface, the collar is here only half the height of its lateral region ; and whereas Ehlers finds only a slight notch in the median line on its anterior border, I find that there is a deep cleft extending down to the base and reaching to the first ventral gland-shield.
(d) The description and figure given by Ehlers of the pre-anal groove and its neighbourhood scarcely do justice to this structure, which is more accurately shown in Malmgren's figure of Euchone analis Krover (pl. XXVII, fig. 88 f.).
The pre-anal groove is bounded by a relatively high and thick membrane, the "caudal membrane" as Moore terms it, which commences posteriorly on each side of the anus and extends forwards along the ventral surface of the animal for 10-12 segments. It increases in height towards its anterior limit where it curves towards the mid-line to meet its fellow ; just before they meet, each lateral membrane bends rather sharply backwards so as to form with its fellow a "spout " which is continuous in front with the faecal groove. Posteriorly the caudal membrane extends behind the anus to end in the supra-anal portion of the last segment of the body—the periproct. On the floor of the groove there is only the very slightest indication of the segmentation of the body, and the nerve cord is visible through the thin body-wall. In the first specimen studied the two caudal membranes stand upright so as to be nearly parallel to one another, but in the other individuals they are spread outwards so as to be nearly flat and the whole structure then resembles the “head" of some Maldanid.
It is evident that these membranes are mobile and perhaps muscular in nature. What is the function of this peculiar structure?
The worm agrees, except for the above details, completely with the account given by Ehlers ; but the tube inhabited by it differs from that described by him.
At Kaiser Wilhelm II Land the species inhabits a thick-walled tube of mud ; for Ehlers found some of the animals within their tubes. In his original account, though there was such a tube in the same phial as the worm he had no certainty that it belonged to it ; but in 1913 he writes : " Eines dieser Tiere wurde aus einer sehr dickwandigen Schlammröhre herausgelöst; damit ist die Beschaffenheit der Wohnröhre dieser Art sichergestellt."
The worm, however, from McMurdo Sound inhabits a tube which is not unlike that of Potamilla antarctica in that it is horny-yellow in colour with its upper region sometimes covered with grains of mud or of sand, but not to such a degree as to hide the horny membrane. The tube is thinner than that of Potamilla and more brittle. Some of the specimens were still within their tubes, others had been extracted. This difference in the nature of the tube is not sufficient to demand the formation of a new species, although, so far as I am aware, such differences in the nature of the tube inhabited by different individuals has not been recorded for any other species of Sabellid. The European species forms a tube of mud ; but Euchone alicaudata, from Japan, forms a stiff horny tube (Moore and Bush, 1904, p. 165), similar to that from McMurdo Sound. I cannot think that Euchone pallida in the latter region would leave its own mud tube and enter one formed by Potamilla, for against such a supposition is the fact that the worm fits its tube precisely, and a tube of this diameter would, if it belonged to Potamilla, be much longer than are those of Euchone. Nor do I know of any record of a Sabellid migrating in such a way from one tube to another.
Localities.—McMurdo Sound, Stations 314, depth 222-241 fathoms ; 316, depth 190-250 fathoms (4) ; 321, depth 180-250 fathoms (2, coll. Nelson) ; 355, depth 300 fathoms (three) ; Ross Sea, Bay of Whales. Station 191, depth 194-250 fathoms (one, coll. Lillie).
Distribution.—Kerguelen, in 88 metres ; Kaiser Wilhelm II Land, in depth of 385 metres.”
(Benham, 1927)
