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Six specimens, collected by 5' Blake trawl between 646 and 845 m depth east of the Falkland Islands, 51°58'S, 56°38'W to 52°01'S, 56°38'W (USNS Eltanin Cruise 7, Station 558), 14 March 1963: USNM 81538, 81539; AM W.5910. W.5911; one specimen collected by diving from 7 m depth southwest of Anvers Island, Antarctic Peninsula, 64°46'36"S, 64°03'29"W (R/V Hero Station AH4- 25), 25 January 1969: USNM 81540; 42 specimens collected by Blake trawl between 13 and 29 m depth in Canal Whiteside, between Isla Dawson and Isla Grande de Tierra del Fuego, 53°49'18"S, 70°25'00"W (R/V Hero Cruise 70-2, Station 488), 29 April 1970: USNM 81541-81543; one specimen collected by Petersen grab from 46 m depth in Bismarck Strait, south of Anvers Island, Antarctic Peninsula, 64°53'06"S, 64°04'00"W (R/V Hero Cruise 72-1, Station 964), 6 December 1971: USNM 81554; seven specimens collected by Blake trawl between 38 and 74 m depth near Nelson Island, South Shetland Islands. 62°16'42"S, 58°34’00"W to 62°17'30"S, 58°34'36"W (R/V Hero Cruise 72-1, Station 703), 21 December 1971: USNM 81544-81546; eight specimens collected by Blake trawl between 55 and 78 m depth near Nelson Island, South Shetland Islands, 62°17'30"S, 58°34'36"W to 62°17'54"S, 58°34'36"W (R/V Hero Cruise 72-1, Station 704), 21 December 1971: USNM 81547-81549; 10 specimens collected by Blake trawl between 64 and 82 m depth near Robert Island. South Shetland Islands, 62°19'18"S, 59°11'48"W to 62°19'12"S, 59°11'42"W (R/V Hero Cruise 72-1, Station 726), 26 December 1971: USNM 81550-81553; two specimens collected by 10' Blake trawl between 27 and 40 m depth north of South Georgia, 53°57'30"S, 37°20'42"W (ARA Islas Orcadas Cruise 575, Station 21), 16 May 1975: USNM 81555, 81556; one specimen collected by 5' Blake trawl from 90 m depth near Saunders Island, South Sandwich Islands, 57°47'12"S, 26°22'12"W (ARA Islas Orcadas Cruise 575, Station 56), 27 May 1975: USNM 81557.
Serna de Esteban & Moretto (1968: 172) noted that the general appearance and colour of Parborlasia fueguina "están ligadas al estado de actividad de los individuos. Contraidos, son rollizos y oscuros; relajados, acintados y mas claros . . . La coloration del cuerpo, más pálida en la región ventral, varía del borravino al castaño-rojizo". Though not specifically stated, their comments presumably refer to the living worms. The preserved examples examined during the present studies are quite variable in shape (Fig. 44); they range in length from 3 to 75 mm, the largest having a maximum width of about 7 mm.
The head is either blunt and slightly tapering or rounded; the condition observed by Serna de Esteban & Moretto (1968: 172) that, "Una ligera constricción limita la extremidad cefálica" applies to few of the present specimens. The body is rounded, the epidermal surface usually appearing slightly to moderately wrinkled. On the ventral surface the mouth is mostly small and rounded but occasionally appears as a compressed longitudinal slit. Serna de Esteban & Moretto (1968: 172) commented that "La boca es una abertura muy contractil, que se muestra como un largo y fino surco con bordes más pálidos, o una abertura conspicua de gruesos labios".
The colour of the preserved animals ranges from a dirty white to dull fawn-yellow, fawn-brown or mid-brown. No trace of the yellow band associated with
the posterior parts of the cephalic slits (Serna de Esteban & Moretto, 1968) could be distinguished.
The internal anatomy of Parborlasia fueguina has been neither fully described nor adequately illustrated; the following account is thus supplementary to that provided by Serna de Esteban & Moretto (1968).
Body wall, musculature and parenchyma
The organization of the epidermis, subepidermal muscle layers, dermis and main body wall muscles have been described by Serna de Esteban & Moretto (1968) and are illustrated in Fig. 45A.
The musculature of the head principally consists of loosely arranged bundles of longitudinal fibres posteriorly continuous with the main outer longitudinal muscles. A distinct central cylinder is present (Fig. 45B), comprising outer circular and inner longitudinal muscle layers which together enclose the cephalic lacuna and rhynchodaeum. Oblique and radial muscle strands lead peripherally from the circular layer of this central cylinder.
Behind the cerebral ganglia the origin of the circular splanchnic musculature (Fig. 45C) is as described by Serna de Esteban & Moretto (1968). These muscles (Fig. 45D) are 30 µm or more thick in the anterior foregut region and enclose both the epithelium and underlying subepithelial glands. As noted by Serna de Esteban & Moretto (1968), the circular splanchnic muscle coat is gradually diminished posteriorly and terminates at the start of the intestine without forming a sphincter. Gibson (1983) reported that in Parborlasia corrugatus loosely arranged longitudinal muscle fibres running between the foregut subepithelial glands constituted an inner splanchnic muscle stratum; traces of a comparable layer can also be distinguished in Parborlasia fueguina. Neither the inner splanchnic muscles nor the subepithelial gland cell zone were mentioned by Serna de Esteban & Moretto (1968).
Weakly developed dorsoventral muscles cross between the lateral diverticula in the intestinal regions of the body.
Apart from comprising the usually well developed boundary or basement membranes which invest or underlie the various organs and epithelia, connective tissues also form an abundant parenchyma in the intestinal regions of the body; this is especially evident in sexually immature individuals.
The subterminal proboscis pore opens into a tubular rhynchodaeum. The rhynchodaeal epithelium, 15-35 µm thick, is non-glandular but possesses cilia in its more anterior regions (Fig. 45E); in a few individuals the epithelium is pouched in a manner similar to that described for Parborlasia corrugatus. Just in front of the brain the rhynchodaeum narrows and is enclosed by a sphincter of circular muscles 25-30 µm thick (Fig. 45B). Immediately behind this is the proboscis insertion.
The rhynchocoel reaches almost to the hind end of the body. For most of its length it possesses a normally constructed wall with separate muscle layers, the circular fibres not being interwoven with the adjacent body wall inner longitudinal muscles. Serna de Esteban & Moretto (1968: 175) noted that the circular layer "se entrecruza con el músculo circular corporal"; in only a few places could similar fibre links be observed among the present specimens. The principal feature of the rhynchocoel, briefly described by Serna de Esteban & Moretto (1968), is its large ventral diverticulum which protrudes into the foregut lumen (Figs 45F, G, 46A). In most examples the diverticulum extends for about 75% of the foregut length, but may be somewhat shorter. The wall of the diverticulum differs from that of the main rhynchocoel in being very much thinner and lined by a delicate epithelium and slender muscle layers; the thin connective tissue stratum situated between the longitudinal muscles and epithelium of the rhynchocoel is also not distinguishable in the diverticular wall. In some individuals the diverticulum is not as well developed and is reduced to a spacious ventral outpushing of the rhynchocoel wall; in these instances, however, similar reductions in the construction of the pouch wall are still apparent.
The morphology of the main proboscis region, shown in Fig. 46B, agrees with that described by Serna de Esteban & Moretto (1968). In its extreme posterior region (Fig. 46C) the proboscis narrows, its epithelium is reduced in height, only longitudinal muscle fibres are distinguishable and the endothelium, though remaining thin, is rather more distinct. A short anterior region, joining the main chamber with the proboscis insertion, is also recognizable (Fig. 46D). This consists of a flattened epithelium which entirely lacks gland cells, a well developed longitudinal muscle layer enclosing two large nerves, and a delicate endothelium.
The proboscis retractor muscle is attached to the dorsal wall of the rhynchocoel some distance in front of its posterior end.
The mouth opens immediately behind the brain. Serna de Esteban & Moretto (1968: 174) said of the foregut that "El epitelio del esófago es simple, cilíndrico, ciliado y está formado por células muy alias que alternan con dos tipos de glándulas, unas basófilas y las otras conteniendo gruesos gránulos acidófilos. Ambas ocupan todo el espesor de los pliegues que regularmente limitan el lumen esofágico, y se extienden hasta el músculo esplácnico, no advirtiéndose membrana basal". In all the present specimens, although no clear basal membrane can be distinguished, it is evident that the foregut wall comprises an outer epithelium up to 30 µm or more thick and an inner subepithelial gland cell zone of variable depth depending upon whether it is between or below epithelial folds (Fig. 45D). The epithelium is predominantly composed of acidophilic glands interspersed with ciliated support cells, whereas the subepithelial layer contains approximately equal numbers of both acido- and basophilic glands. In the posterior regions the subepithelial glands may extend outside the circular splanchnic muscle layer.
Where the dorsal foregut wall is penetrated or distended by the rhynchocoelic diverticulum both the splanchnic and subepithelial gland cell layers are missing and the epithelium is reduced to a flattened sheet of cells.
Serna de Esteban & Moretto (1968) describe a conical valve between the foregut and intestine. A similar structure occurs in many of the present specimens (Fig. 46E), whilst in others the dorsal gastrodermal wall extends further forwards than the ventral and the valve appears like a ventral flap-like fold (Fig. 46F). Such differences are probably caused by contraction or extension at the time of preservation.
The intestine exhibits a normal heteronemertean construction except in its extreme posterior region, where it is involved with the blood system (p. 200). For most of its length the intestine bears deeply lobed lateral diverticula. The anus is terminal. Gut contents in some of the specimens include accumulations of diatom and dinoflagellate fragments, inorganic debris, broken spines and chitinous structures somewhat similar to molluscan radula teeth.
In the head the median cephalic lacuna (Fig. 45B, E) separates into two lateral vessels at the proboscis insertion. These vessels extend into the cerebral ring and meet below the rhynchocoel in a U-shaped duct which almost immediately separates off the mid-dorsal branch (Fig. 46G). Connective tissue and muscle fibres then split off the base of the U so that near the rear of the ventral commissure there are four longitudinal blood channels, one mid-ventral, one mid-dorsal (and here forming the anterior portion of the rhynchocoelic villus) and two lateral (Fig. 46H). Behind the brain the mid-ventral channel expands (Fig. 45C) and then divides medially, each half joining the lateral vessel on the appropriate side of the body (Fig. 47A). The two spacious lacunae so formed are then subdivided by radial muscle and connective tissue strands to form the origin of the foregut vascular plexus; this extends the full length of the foregut between the body wall inner longitudinal and circular splanchnic muscle layers (Fig. 45D).
The mid-dorsal vessel forms a short rhynchocoelic villus (Fig. 46H) which ends a short distance behind the cerebral organs. The course of this vessel where it runs in the wall of the rhynchocoelic diverticulum could rarely be seen in the present specimens but appears to conform with that described by Serna de Esteban & Moretto (1968).
The branches of the foregut plexus posteriorly coalesce to provide a pair of thick-walled ventrolateral vessels. Both these and the mid-dorsal vessel possess distinct connective tissue epithelial basement membranes (Fig. 47B). Throughout the intestinal length the lateral vessels are irregularly dilated and then appear thinner walled; they may even be temporarily subdivided (Fig. 47C). Slender pseudometameric transverse connectives linking the mid-dorsal and lateral vessels can be seen in several places.
In the posterior intestinal regions of some specimens the mid-dorsal vessel is either enormously dilated (Fig. 47D) or forms a slender ventral branch leading towards the gut lumen (Fig. 47E, F). Although the gastrodermis bordering these ventral extensions of the vessel is extremely thin (often only 2-3 µm), the blood spaces retain their distinct lining membranes and do not directly communicate with the intestinal cavity. Near the anus both the mid-dorsal and lateral vessels are enlarged and lacuna-like (Fig. 47G), vacuolate regions often being distinguishable in the gastrodermis immediately adjacent to the lacunae. This arrangement was briefly noted by Serna de Esteban & Moretto (1968), who subsequently (1969b) concluded that connections between the rhynchocoel, blood system and vacuolate parenchyma in the posterior end of the body were indicative of rectal respiration.
The general organization of the cerebral ganglia and main lateral nerve cords was described by Serna de Esteban & Moretto (1968).
The peripheral nerves are well developed. Amongst these nerves leading inwards near the anterior roots of the lateral nerve cords (Fig. 47H) meet just in front of the mouth at a thick transverse commissure (Fig. 45C), then continuing posteriorly, completing the buccal nerve ring and separating to provide the foregut neural plexus. The branches of this plexus form a circum-alimentary commissure at the junction between foregut and intestine.
Serna de Esteban & Moretto (1968: 172) stated that, "En estos animales no se advierten ocelos". Though not consistently distinguishable, several of the present specimens contain up to 22 oval or spherical simple eyes, about 45-50 µm in maximum diameter, on each side of the head situated near the innermost margins of the cephalic slits (Fig. 48A).
A frontal organ similar to that described by Serna de Esteban & Moretto (1968) can be distinguished in some of the present nemerteans. The cephalic glands are irregularly scattered between the muscle fibres of the anterior head region.
The internal epithelium of the cephalic slits (Fig. 48B) is identical to that reported by Serna de Esteban & Moretto (1968). At their rear the slits form deep intramuscular canals with three pouches (Fig. 48C); the dorsal and ventral pouches end blindly, but the median chambers lead to the ciliated cerebral canals. These enter the large and well developed cerebral organs, which consist of central neuroganglionic tissue surrounded by extensive and primarily acidophilic gland cells (Fig. 48D). Some of the glands anteriorly intrude into the dorsal ganglionic lobes (Fig. 48E).
The collecting tubules of the excretory system extend from just behind the mouth along the lateral borders of the foregut. They are thick-walled (8-15 µm) and intimately involved with branches of the foregut vascular plexus, which they frequently penetrate (Fig. 48F). Towards the rear of the excretory system, at about mid-foregut level, one of the collecting tubules on each side leads to an efferent duct. This passes over the lateral nerve cord and discharges via a lateral nephridiopore.
The sexes are separate. The gonads form spacious chambers situated between the pouches of the intestinal diverticula. No unusual features can be distinguished in the reproductive system.
One individual contains large numbers of parasites encysted in the body wall muscle layers, cerebral ganglia, lateral nerve cords and foregut subepithelial gland cell zone. The parasites appear to represent two developmental stages. Smaller examples (Fig. 48G) are spherical, 45-70 µm in diameter and comprise an acellular core of coarsely granular material enclosed by tightly packed concentric layers of an acidophilic substance. Larger, and presumably older, stages are spherical to ovoid, 180-200 µm in diameter, with a similar but bigger granular core, the concentric lamellae interspersed with a 'sandwich filling' of finely granular cytoplasm, and a peripheral zone in which separate cells containing nuclei 5-8 µm across can be distinguished. Both stages are
encapsulated in a structureless layer 2-3 µm wide; in some of the larger parasites this outer layer contains minute refractile granules. The identity of the parasites remains unknown.
The organization of the cephalic slits, proboscis, blood system, dermis and
foregut, together with the absence of a caudal cirrus and neurochords, clearly establishes the Antarctic worms as members of the genus Parborlasia; other genera share some of these characters (Gibson, 1981a), but the present combination is not found elsewhere amongst the Lineidae. Of the four species of Parborlasia known only Parborlasia fueguina possesses both a rhynchocoelic diverticulum and a conical valve at the junction between foregut and intestine; the presence of these features in the Antarctic nemerteans thus identifies them as Parborlasia fueguina.
A comparison between the present description and that originally given by Serna de Esteban & Moretto (1968), however, reveals a number of discrepancies which require discussion.
Neither the longitudinal splanchnic musculature nor foregut subepithelial gland cell zone were mentioned by the original authors. Although no definite basal membrane can be distinguished in the present examples, the occurrence of separate epithelial and subepithelial layers is confirmed by the way in which the former has come away from the foregut wall in less well preserved individuals. Radial muscle fibres which traverse the body wall of most heteronemerteans often end internally between the glands of the foregut subepithelial layer, when one is present, but never penetrate the epithelium itself. Serna de Esteban & Moretto's (1968: 174) comment that the radial fibres "perderse entre las glándulas del esófago" thus suggests that their species does possess a subepithelial glandular layer around the foregut which was probably overlooked because of the absence of a discernible basement membrane. Similarly, the inner longitudinal splanchnic musculature, consisting of isolated fibres, is often difficult to detect between the subepithelial glands and could easily have been missed; it is only recently (Gibson, 1983) that these splanchnic muscles and subepithelial glands have been described from Parborlasia corrugatus, despite this being the best known Antarctic heteronemertean species.
Eyes are distinguishable in many of the present nemerteans, yet Serna de Esteban & Moretto (1968) found none in their specimens. Nemertean eyes often cannot be demonstrated except in histological preparations and, in the present specimens, close microscopic examination was often needed to distinguish between the eyes and cephalic nerves in sections through the head. It is concluded that the original authors either overlooked the ocelli in their material or based their observation purely on external appearances.
Parborlasia fueguina has previously been recorded from only one locality, the Bay of Ushuaia, Tierra del Fuego (approximately 54°49'S, 68°18'W (Serna de Esteban & Moretto, 1968). The present studies thus extend the known range of the species (Fig. 49), which has now been recovered from depths down to 646-845 m.”
(Gibson, 1985; 194-204)