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Comprehensive Description

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Description

Baseodiscus antarcticus, sp. n. (Pl. I, figs. 4, 6.)

 

Stations 314 and 355: McMurdo Sound, 222-300 fathoms.

 

A fairly distinct constriction immediately behind the mouth, when the head is not retracted. Cephalic grooves lateral and vertical. Mouth small and circular. Primary basement-membrane of cutis deep, but loose, and with many radial muscle-fibres. A well-developed layer of gland-cells in connection with the cutis. Bundles of fibres in outer longitudinal muscle-layer of body separated by much gelatinous tissue. Circular muscle-layer thin. Walls of gut not folded. Proboscis slender, and proboscis-sheath thin-walled.

 

Two specimens which I refer to this form occur in the collection.

 

The larger of the two measures 5.5 cm. in length, and has a maximum thickness of 9 mm. The smaller, which is apparently a young female, measures only 2 cm. in length and 5 mm. in thickness.

 

There is no trace of colour or markings upon either individual.

 

EXTERNAL FEATURES.

 

In the small specimen the characters of the head (Pl. I, fig. 4) can be fairly well made out ; it is marked off from the body by a moderately distinct constriction, immediately behind the mouth. The proboscis-pore (P.P.) is a well-marked vertical slit just below the apex of the head. The shallow cephalic grooves (G.) are lateral and vertical, and apparently do not form a complete ring. The mouth (M.) is small and circular, with regularly wrinkled margin.

 

In the larger example the head is much retracted, and little of these features can be made out with certainty.

 

INTERNAL ANATOMY.

 

A small piece was taken from about the middle of the body of the small specimen, and cut into transverse sections (Pl. I, fig. 6). These reveal the following features :‑-

 

The external epithelium of the body (Ep.) consists of tall cells, resting on a secondary basement-membrane (B.M2.), succeeded by two thin layers of muscle-fibres, an outer circular and an inner longitudinal. Beneath the latter is a well-developed layer (G1.) of large glandular cells. Next comes the thick primary basement-membrane (B.M1.), consisting of a rather loose connective tissue, through which many bundles of muscle-fibres pass outward radially.

 

The outer layer of longitudinal body-muscles (L.M2.) comes next in order. The

 

bundles of fibres belonging to this layer are somewhat scattered, and are embedded in a considerable amount of gelatinous and solid-looking connective tissue.

 

The circular muscle-layer (C.M.) is thin. Between it and the outer longitudinal muscles lie the large lateral nerve-stems (L.N.).

 

The inner layer of longitudinal muscles (L.M.) is comparatively thick and dense. Beneath it lie the proboscis-sheath (P.S.), the gut, and a certain amount of loose connective tissue. In this connective tissue, between the muscles and the gut, there are numerous large spaces, some of which are probably blood-sinuses, but others appear to be the gonadial sacs. The former are situated dorsally and ventrally, the latter at the sides. In the anterior part of the series of sections these gonadial spaces are empty, but more posteriorly ova are beginning to be developed from their walls. The ova always appear on that side of the sac which is towards the exterior. The sex of the larger specimen was not determined, and I am unable to give any particulars as to the arrangement of the gonads in the male.

 

The gut (Pl. I, fig. 6, Int.) is simple and spacious, and its wall is not folded.

 

The proboscis is feebly developed, and its sheath is thin-walled, and not abundantly provided with muscles.

 

With such scanty material available, nothing further can be said at present of the anatomy of this species. Its chief interest lies in the fact that it is the only member of the genus as yet recorded from truly Antarctic waters, unless we accept Eupolia punnetti as a "good" species. I shall further state my views with regard to this question under the heading of Linens corrugatus ; but I may be permitted to remark here that I can see no reason for referring that form to the genus Eupolia (or Baseodiscus). If this view be correct, the present species will be the only one, I believe, hitherto recorded from a latitude further south than 42°.”

 

(Baylis, 1915; 120-121)

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© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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