“Parborlasia landrumae sp. nov.
The new species is named after Miss Betty Landrum, Supervisor of Records for the Smithsonian Institution Oceanographic Sorting Center, Washington, D.C., who for many years has been involved in collating data from numerous Antarctic collecting cruises.
Sexually immature, USNM 81595, full series of transverse sections; Paratype. Sexually immature, USNM 81596, full series of transverse sections. Seven additional specimens from the type locality, USNM 81597, have also been examined.
Off Wilkes Land, 66°20'53"S, 110°27'10"E to 66°21'00"S, 110°28'00"E (Station DAB), collected by dredge between 183 and 237 m depth 6 December 1961.
Parborlasia landrumae sp. nov. is 4-14 mm long and up to about 3.0-3.5 mm in maximum width. The body is rounded, the epidermal surface slightly to moderately wrinkled transversely depending upon the degree of contraction. The head is narrower than the trunk and somewhat wedge-shaped (Fig. 50); it possesses distinct lateral horizontal cephalic slits. On the ventral surface the mouth appears as a small rounded or pear-shaped aperture. There is no caudal cirrus. The colour of the preserved individuals varies from pale yellowish-brown to greyish-white, with no trace of a pattern.
The internal morphology of the present specimens generally resembles that of other Parborlasia species in so far as the body wall, musculature, parenchyma, alimentary tract, excretory and nervous systems are concerned. Certain differences can be distinguished, however, and these are described in the following sections.
The rhynchocoel in the mid-foregut regions is ventrally dilated and bulges towards the gut lumen (Fig. 51A); it does not, however, form a distinct diverticulum as in Parborlasia fueguina. In front of and behind the bulge the rhynchocoel wall possesses a normal construction, with separate longitudinal and circular muscle layers enclosing a delicate lining epithelium, but both muscle strata are reduced as they enter the wall of the bulging portion (Fig. 51A, B). The circular muscle layer passes right around the bulge, but is at most only one or two fibres thick between it and the dorsal gut wall, whereas the longitudinal muscle sheath is ventrally incomplete and its fibres do not extend beyond the lining of the paired rhynchocoelic villi (Fig. 51B).
The proboscis, as in Parborlasia fueguina, is divisible into three regions according to the organization of its wall. Anterior and posterior portions (Fig. 51A, C) are very similar to those described for Parborlasia fueguina, but the major portion of the organ differs significantly both from this species and other members of the genus. In Parborlasia landrumae sp. nov. the proboscis, though large relative to the size of the body, nevertheless possesses much more weakly developed muscle layers, the outer longitudinal stratum in particular being reduced to scattered and isolated bundles of fibres, nowhere forming a complete layer, situated between the epithelium and neural sheath (Fig. 51D). For most of the proboscis length this outer muscle layer is restricted to three sides of the organ, one adjacent to each of the two thickened regions of the neural sheath and one diametrically opposite (Fig. 51E). The middle circular and inner longitudinal muscle layers are also reduced, with a maximum thickness of about 8 µm and 18 µm respectively. There are the two muscle crosses typical of the genus, but both are exceptionally delicate, formed only of single muscle fibres and can only be distinguished under oil immersion. The positions of the crosses are indicated in Fig. 51E.
The arrangement of the blood supply in the head and anterior cerebral region is the same as in other members of the genus; the median cephalic lacuna posteriorly divides into a pair of lateral vessels which enter the cerebral ring and then unite ventrally to form a U-shaped duct curving below the rhynchocoel.
This duct in turn becomes subdivided into the four longitudinal channels described for Parborlasia fueguina. Post-cerebrally, however, the dorsolateral and mid-ventral channels comprise enormous lacunae which directly join and bathe the major portions of the cerebral sensory organs (Fig. 51F). Between the cerebral organs and mouth there is a massive central lacuna which, except for the rhynchocoel, occupies the entire region enclosed by the body wall muscle layers (Fig. 52A). A little further back the walls of the buccal chamber ventrally encroach upon this central lacuna, which is then split up by connective tissue and muscle strands to form the foregut vascular plexus.
Almost as soon as it is formed the mid-dorsal vessel penetrates the rhynchocoel wall to form a villus. For most of its length this is single and has a normal appearance (Fig. 52B), but where it runs in the floor of the ventral rhynchocoelic bulge it divides into two longitudinal villar projections (Fig. 51A, B). These unite posteriorly to form a single mid-dorsal vessel as the rhynchocoel wall returns to its normal construction. For a short distance the mid-dorsal reduced muscle development is, however, different enough from the complete musculature of other Parborlasia forms to be regarded as a specific difference.
Additional features can be distinguished in the present nemerteans which are not found in other members of the genus. These are the presence of the massive median post-cerebral blood lacuna, the remnants of a longitudinal muscle plate extending into the intestinal regions on either side of the mid-dorsal vessel, the manner in which the mid-dorsal vessel divides to form two villar projections in the dilated portion of the rhynchocoel (the branched nature of the mid-dorsal vessel in Parborlasia fueguina as described by Serna de Esteban & Moretto, 1968, differs from the present double villus arrangement, and the paired villi found in Cephalurichus gen. nov. (p. 127) is not associated with a modified rhynchocoel wall), the degree of cephalic gland development, and the smaller number of eyes. These differences, when taken in conjunction with the reduced proboscis musculature, are regarded as sufficient justification for placing the present nemerteans in the new species Parborlasia landrumae.
Parborlasia landrumae sp. nov. is at present known only from the type locality (Fig. 49).”
(Gibson, 1985; 204-210)
- Gibson, R. 1985. Antarctic nemerteans: Heteronemertea--descriptions of new taxa, reappraisals of the systematic status of existing species and a key to the heteronemerteans recorded south of latitude 50Â°S. Zoological Journal of the Linnaen Society, 83: 95-227. http://invertebrates.si.edu/antiz/taxon_view.cfm?mode=bibliography&citation=2399
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