Comprehensive DescriptionRead full entry
“Thelepus cincinnatus (Fabricius).
Amphitrite cincinnatus Fabricius, 1780 ; Thelepus c. Malmgren, 1865, p. 387; T. antarcticus Willey, 1902, p. 278, pl. XLV, fig. 6 (nec Kinberg) ; T. antarcticus Benham, 1921, p. 91; T. cincinnatus Hessle, 1917, p. 212 (with full literature).
In the account of that species of Thelepus with two pairs of which occurred
in great numbers in the " Aurora " collection from Commonwealth Bay, I referred it to T. antarcticus to which Kinberg ascribed only two pairs of gills. Hessle has since examined the type of that species and finds that it possesses a third pair, which Kinberg seems to have overlooked. Consequently this antarctic worm is not that species. As I indicated in my report on the " Aurora " worm, this antarctic species is very similar to the northern T. cincinnatus Fabr., as Willey has pointed out, but there seem to be a few differences judging by the recent account given by McIntosh.
Hessle has had the opportunity of studying a large series of this form both from the Arctic Seas and from Graham's Land, and he writes " Trotz sorgfältiger Bemühungen habe ich keinen einzigen konstanten Unterschied zwischen den arktischen and antarktischen Exemplaren gefunden."
As I noted. I was unable to find any specimen with three pairs of gills, such as Fauvel had suggested might occur as a variation ; and in the present lot there is in every case only two pairs of gills, so that we may regard the possibility of such a variation as excluded.
In striking contrast to the very great abundance of this worm in Adelie Land is the paucity of specimens gathered during the expedition to McMurdo Sound, from which only some dozen tubes—some of them empty—were collected ; these are all of small size and the only large one comes from Cape Adare in the same region as that recorded by Willey. This large individual reached me without its tube. It is not nearly of the size commonly attained by those at Commonwealth Bay.
As I had not paid attention to the nephridial papillae when making that report, I take this opportunity of supplementing that account. Hessle, who has investigated the distribution of the nephridia in the various genera of Terebellids, does not describe the position of the papillae in all cases. This individual is a male, filled with sperm morulae ; there are four papillae on each side in the usual position on the chaetigerous segments 2, 3, 4, and 5, but the last is scarcely noticeable. These papillae are relatively smaller than one would expect for a worm of this size, but this is explained, I think, by the highly glandular character of the skin, by reason of which the papillae do not project as far as in worms in which this thickening is absent. I am able, also, to give the facts about the female for, owing to the kindness of the Trustees of the Australian Museum, I have some specimens at hand from the former expedition. In a female the papillae are depressed, glandular and very difficult to see, so closely do they resemble the surrounding tissue ; unless searched for, they would be overlooked. However, on the segments 3, 4, and 5 the region between notopod and neuropodial torus is of a paler hue and smoother than the surrounding body-wall ; the papillae have the same position and appearance as in other genera.
The worm attains sexual maturity while still of small size, for, in one measuring 55 mm. in length, excluding the tentacles, I find the body cavity filled with eggs.
In this and other smaller worms the glandular dorsal surface differs in appearance from that noted in the larger individuals. Each segment is marked by a transverse row of large circular translucent spots, with smaller ones interspersed amongst them ; this glandular dorsal region is very distinctly marked off at the sides, and stands up as a sort of cushion extending from side to side, but not reaching the notopods. It lacks the roughness of the larger forms. I notice that Hessle has a statement to the same effect. In the tubes of this species, the basal membrane which supports the sand grains is often seen when the worm is within ; for the wall of the tube is stretched and the sand grains appear arranged in transverse or circular lines round the tube.
Some of the tubes have, as usual, foreign bodies of various sorts attached. In the specimen from Station 348 portions of the calcareous Polyzoan Salicornaria are attached, mostly horizontally and projecting beyond the edge of the tube.
Localities.—Cape Adare, Station 220, depth 45-50 fathoms. McMurdo Sound, Stations 314, 316, 348, 355, in depths of 200-300 fathoms.
Distribution.—In the Antarctic and Arctic Seas ; Mediterranean ; warmer and colder parts of the Atlantic ; Japan.”