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Comprehensive Description

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Description

Chitinopomoides wilsoni, n. sp. (Pl. V, figs. 162-173.)

 

Generic Characters.—Without thoracic membrane ; operculum with dark, horny, concave plate, collar chaetae with one large knob below the blade ; thoracic chaetae with " Salmacina " bristles ; abdominal chaetae " en cornet " with one angle produced ; uncini with more than a dozen denticles above the large blunt fang.

 

A single specimen contained within its tube was found on the outer surface of the theca of a simple coral (? Flabellum). around which it coiled, close to the upper margin, with the free end of the tube projecting freely at right angles to its main course. The tube measures 1·5 mm. in diameter, has a compressed keel running along its middle line and one or two low rounded, indistinct and ill-defined ridges along the sides ; the wall is thick and the mouth circular with its margin somewhat everted. I did not draw it before crushing it to remove the animal.

 

The operculum of the worm was projecting beyond the mouth of the tube ; in attempting to extract the animal by pricking away the tube-wall with needles the peduncle was broken off at its origin , the gills which adhered closely to the wall were also broken, as was the body behind the thorax.

 

The total length of the worm was 25 mm. of which the gills accounted for 5 mm. and the thorax for 3 mm. ; the diameter was 1.5 mm.

 

The gill plume appears to consist of about 14 filaments on each side, though as they are broken it is impossible to be sure of the exact number. The filament is broad and contains no skeleton ; the barbules are long, but do not project beyond the end of the filament, which is here relatively short, thick, and slightly swollen at its apex.

 

The operculum is brown, horny, and concave with a black line round its edge. It is borne upon a subconical enlargement of the peduncle, which is partly constricted at two places by deep grooves which, however, extend only for about three-quarters of the circumference, and may be artifacts. The peduncle is smooth throughout and bears no processes of any kind ; I am unable to determine where it springs from the gill base, but on the right side of the animal there is a loose fleshy process which may be the lower end of the peduncle.

 

The collar is flattened forward against the gills ; it is a continuous membrane except for a deep incision in the dorsal mid-line ; the ventral region is not incised but is here thin and pleated, forming as it were a flexible lower lip. There is no thoracic membrane ; the thorax contains seven chaetigerous segments, including the collar. The collar chaetae form a greyish bundle which misled me, for under a dissecting microscope of low magnification I supposed at first that they were linear groups of eye­spots. The collar chaetae are of two kinds : (a) long, curved, and delicate with two narrow wings, of which the broader is coarsely toothed on the convex margin ; and (b) about half a dozen long, stout bristles in the upper part of the bundle, each of which has a large blunt knob some distance below the point or commencement of the wing.*

 

In the other thoracic segments the bristles are yellow and are arranged in horizontal bundles without being in definite projecting parapodia. In these groups there are also two kinds of chaetae : (a) the majority are long, straight, two-winged, though only one wing is visible in a certain aspect ; and (b) in the lower part of each bundle a few bristles "en faucille," but the denticulated margin is on the convex side so that it is scarcely a true "sickle." The blade, broader at its commencement where it is finely striated, tapers to a delicate point. It seems to be what de Saint Joseph terms a "Salmacina" bristle. The six uncinigerous neuropods are separated ventrally by the gland shields, paired and quadrate : on the first segment the gland shield is trans­versely divided into two so that there are four narrow elongated pads, an anterior and a posterior on each side.

 

The tori are flush with the surface of the body. The uncinus has a high basal plate, truncated below, with nearly vertical sides, one much higher than the other ; it is trans­parent, colourless, finely striated, and bears 13-15 denticles above the large stout blunt tooth or process, which may be grooved on its under-face, but of this I am not sure.

 

When the uncinus is viewed from above there are seen to be two rows of denticles near the large tooth, increasing to four transverse rows at the opposite end.

 

The abdominal uncini, of which there are about thirty in a notopod, agree with this except that there are four to five transverse rows of denticles.

 

The neuropodial bristles are two per bundle, pale and almost colourless ; the end is widened out to one side and terminates in a point. It seems to be "cupped" or a bristle “en cornet," one margin of which is prolonged to a fine point, the free edge being very finely denticulate or rather "frayed out."

 

Locality.—McMurdo Sound, Station 348, depth 200 fathoms.

 

Remarks.—In the absence of a thoracic membrane this worm agrees with the genera Chitinopoma and Hyalopomatus ; from the latter it differs in having other than capilliform bristles in the thorax and abdomen, and from Chitinopoma in the form of the collar chaetae. The collar chaetae agree with those found in Serpula, Hydroides, and Crucigera, but from each of these it differs in possessing chaetae "en faucille" as in Filoyrana, from which, on the other hand, it differs in the form of the collar chaetae. The abdominal bristles being "en cornet " with one margin prolonged resemble those in the group Pomatoceros, Spirobranchus, etc., whereas in Serpula they are not so produced.

 

From these, however, it departs in the form of the collar bristles.

 

The uncini are quite unlike those of Serpula and others, but in the form of the large tooth it recalls Pomatoceros, though I do not feel sure that the tooth is here grooved.

 

In short, the worm seems to combine the characters of two or more of the known genera and I have to make a new one.

 

In the form of the operculum it agrees, as I have noted above, with Spirobranchus and Chitinopoma and differs from some of the genera to which in its chaetal characters it approaches.”

 

(Benham, 1927)

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© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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