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Comprehensive Description

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Description

Dokonemertes magellanensis sp. nov.

 

Holotype

 

Female, USNM 80910, full series of stained transverse sections; Paratypes: Male, USNM 80911, sex not determined (anterior body regions only), USNM 80912, 80913, series of transverse or longitudinal sections.

 

 

Additional material examined

 

Seventy-four specimens, intact or consisting only of anterior body fragments, USNM 80547; 5 complete specimens, AM W.5909; 5 complete examples, BM(NH) 1983.10.1-1983.10.5.

 

 

Type locality

 

Near San Juan, Peninsula de Brunswick, Chile, 53º30'48"S, 70°50'33"W (Station 69-11), collected by hand intertidally 24 April 1969.

 

 

External features

 

The shape of preserved individuals is variable. Smaller specimens are usually slender (Fig. 18A) and oval or rounded in cross-section, whereas larger or strongly contracted examples may appear rather bulkier and be somewhat dorsoventrally compressed in the intestinal regions (Fig. 18A, B). The length ranges from 9 to 137 mm, the maximum width from 0.5 to 7.0 mm. The head is characteristically long and thin, much narrower than the trunk, and tapers to a bluntly pointed tip (Fig. 18A, C). It bears shallow but distinct lateral horizontal cephalic slits (Fig. 18A, B, D) which do not quite reach forwards to the tip of the head. The eyes are situated laterally near the anterior regions of the cephalic slits; they are not always distinguishable in unsectioned individuals.

 

The mouth is large; it may be gaping (Fig. 18A, C) or appear as a

 

compressed longitudinal slit (Fig. 18D). Behind the mouth the body is generally of a more or less constant width for most of its length, tapering only in the posterior regions to end in a bluntly pointed tail. There is no caudal cirrus.

 

The colour in life is not known. Preserved individuals vary from a dark dull olive green, greyish-brown or greenish-grey to olive brown or a pale dirty cream or light tan hue. Often the dorsal surface is much darker than the ventral. The head may be lighter in colour than the remainder of the body, or the darker dorsal pigmentation may extend anteriorly to the tip of the snout, when the cephalic slits are usually evident because of their paler coloration (Fig. 18A, D). In specimens with a gaping mouth the oral borders are typically only lightly coloured (Fig. 18C) and emphasize the shape of the aperture. The extreme posterior tip of the body may also be very pale and appear almost transparent.

 

 

Body wall, musculature and parenchyma

 

The epidermis is generally 15-45 µm, occasionally up to 60 µm or more, thick. It contains enormous numbers of glands; rhabditous acidophils are especially abundant and are predominantly restricted to the proximal half of the epidermal layer (Fig. 19A), whereas basophilic glands are far less numerous and are mainly distributed distally. The epithelial lining of the cephalic slits entirely lacks gland cells. Internally the epidermis is bounded by a thin but distinct connective tissue basement membrane, overlying outer circular and inner longitudinal subepidermal muscle layers which are each some 15-30 µm or more thick. Both subepidermal muscle layers can be followed well into the head.

 

The dermis is well developed. Its glands are arranged into lobules 45-60 µm or more tall and 15-25 µm wide; the lobules are largest and most abundant in the region between the cerebral ganglia and mouth. The glands are separated from the body wall outer longitudinal muscle layer by a stratum of laminated connective tissue (Fig. 19B). This is mostly 30-40 µm thick but varies according to the degree of local body contraction. Pre-cerebrally there is a reduction in dermal development; the glandular lobules become both less numerous and less well organized, and the connective tissues lose their laminate formation and instead appear typically parenchymatous in nature (Fig. 19A). In the ventral half of the head the dermis tends to be intermingled with the peripheral portions of the main cephalic longitudinal musculature and often does not form a distinct layer at all, but dorsally a definitive dermis can be distinguished nearly up to the tip of the snout. Radial muscle, nerve and connective tissue fibrils which cross the body wall outer longitudinal muscle zone extend distally between the dermal gland lobules (Fig. 19B).

 

The main body wall muscle layers (Fig. 19B) possess a normal heteronemertean arrangement. The outer longitudinal layer ventrally and ventrolaterally is up to about 370-375 µm thick in the post-oral regions of the body, but only 120-150 µm deep on the dorsal margins. It is divided into more or less regularly arranged blocks of fibres by radial muscle, connective tissue and nerve strands (Fig. 19B). The circular muscle layer is well developed and generally 100-150 µm thick. Its fibre bundles are interleaved with the 'layers' of longitudinal muscle fibres (Fig. 19C) which give rise to the radial muscles; these radial strands may extend peripherally towards the body surface or inwardly towards the gut. The inner longitudinal muscle layer is also strongly formed and up to 120 µm or more thick. In the foregut regions it too is subdivided into bundles of fibres by the radial muscles which extend towards the splanchnic musculature. There is no diagonal muscle layer in the body wall.

 

Pre-orally the circular and inner longitudinal muscle layers are somewhat reduced in thickness (to about 45-60 µm each) and form a distinct central muscular cylinder enclosing the rhynchocoel and post-cerebral blood supply. Both layers extend into the cerebral ring, gradually becoming thinner until they are each only one to a few fibres thick. The outer region of the circular muscle layer, between the mouth and cerebral ganglia, splits up to give a complex network of radial, oblique and tangential fibres which criss-cross the outer longitudinal muscle layer (Fig. 19D). The innermost fibres of the circular musculature, after passing through the cerebral ring, contribute to the proboscis insertion and do not extend into the head, but the longitudinal muscles join with the central cephalic layer of longitudinal fibres which enclose the blood supply and rhynchodaeum (Fig. 19E). The bulk of the cephalic musculature, however, is derived from pre-cerebral extensions of the body wall outer longitudinal musculature. Throughout its length the head is criss-crossed by well developed oblique and tangential muscle fibres which centrally surround the median longitudinal muscle core (Fig. 19E).

 

Over the anterior parts of the mouth two bundles of radial and oblique muscle strands on each side of the body unite to form a loosely organized transverse muscle plate, up to about 150 µm in overall thickness, which passes between the oral and rhynchocoel walls. The plate extends for a short distance in front of the mouth but then disappears. Posteriorly, however, it reaches to about the mid-oral level before splitting up to form the origin of the circular splanchnic muscle layer. The splanchnic muscles surrounding the foregut are extremely well developed. The circular layer, up to about 100-110 µm thick, encircles the gut outside the subepithelial gland cell layer, between it and the foregut vascular plexus (Fig. 19F). The inner (longitudinal) splanchnic muscles, in contrast, run immediately below the foregut epithelium (Fig. 19F) and separate this from the subepithelial glands. The longitudinal splanchnic muscle layer is up to about 60-75 µm thick in the folds of the foregut; in contrast it is only 15-20 µm wide at the bases of the folds where the subepithelial gland cell zone is reduced or absent. The two splanchnic muscle layers in these regions may run adjacent to each other but their fibres do not intermingle.

 

In the intestinal regions the dorsoventral muscle strands passing between the lateral gut pouches are numerous but not especially strong.

 

Parenchymatous connective tissues are moderately well developed, particularly between the inner longitudinal muscle layer and the foregut vascular plexus, and in the intestinal regions of the body.

 

 

Proboscis apparatus

 

The proboscis pore opens ventrally and subterminally. It leads into a long, slender and tubular rhynchodaeum (Fig. 19E) whose epithelium, 15-20 µm thick, lacks gland cells but is ciliated throughout its length. The rhynchodaeum is enclosed by fibres of the cephalic longitudinal muscle core. Near the rear of the rhynchodaeum a few of the oblique muscle fibres of the head lead inwards towards the rhynchodaeal epithelium and form a circular muscle coat adjacent to the epithelial basement membrane. This muscle layer is at first only about 12-15 µm thick, but gradually becomes stronger more posteriorly and, just in front of the proboscis insertion, is developed into a loose sphincter-like layer 45 µm or more deep (Fig. 19G). Some of the longitudinal muscle fibres enclosing the rhynchodaeum penetrate the peripheral regions of the circular muscle layer, so that some interweaving of the two muscle zones is apparent. The proboscis insertion is situated immediately in front of the brain lobes and is formed from bundles of radially orientated muscle fibres leading inwards from the general cephalic longitudinal musculature (Fig. 19H).

 

The rhynchocoel reaches almost to the posterior end of the body. It is enclosed by separate circular and longitudinal muscle layers, each 15-30 µm thick. The circular fibres are not interwoven with the adjacent body wall inner longitudinal muscles.

 

The proboscis is extremely slender; in an individual with a body 3.5-4.0 mm wide the proboscis has a maximum diameter of 280-400 µm, i.e. approximately 8-10% of the body width. Several morphologically distinct regions can be discerned in the organ. Its most anterior part, joined to the proboscis insertion, consists of a thin epithelium 5-8 µm tall bordered by a slender but distinct connective tissue basement membrane, longitudinal muscle fibres, and a delicate lining endothelium (Fig. 20A). Two obvious nerves run amongst the muscle fibres. This portion of the proboscis is about 140-150 µm in diameter and approximately 1 mm long.

 

The second proboscis region is that in which the two nerves split up and the neural sheath becomes established. This encircles the epithelium but is separated from it by a layer of longitudinal muscle fibres, i.e. outer and inner longitudinal muscle layers can be distinguished. At the same time the proboscis lumen becomes enlarged and the organ itself increases to an overall diameter of 200-220 µm (Fig. 20B). There is no sharp division between the two regions and for some distance back after the neural sheath is formed a bilateral thickening of the sheath is indicative of the main course of the nerves. The epithelium of these two anterior proboscis regions is ciliated but lacks gland cells. The second part is some 650-700 µm long.

 

Four features mark the onset of the third and major proboscis region. The epithelium begins to thicken, at the same time developing gland cells but losing its cilia. Commensurate with these epithelial changes the proboscis circular musculature becomes established between the neural sheath and the inner longitudinal muscle layer (Fig. 20C). The circular stratum is initiated by the outermost fibres of the inner longitudinal layer changing their orientation. The circular muscles are at first weakly formed and only 3-5 pm thick, but they gradually become more strongly developed as they extend posteriorly. For most of this proboscis region, which accounts for some 75-80% of the total length, the epithelium is folded, up to 60-90 µm thick and contains both baso- and acidophilic gland cells. Many of the acidophils appear to be responsible for the distal pads of barbs which are approximately 6-7 µm thick and 14-15 µm in diameter; each pad contains more than 100 individual and tightly packed barbs. The outer longitudinal muscle layer is progressively reduced in development along the length of the proboscis; in the extreme anterior parts of this major region (Fig. 20C) it is complete and up to about 15-16 µm thick, but for most of the proboscis length appears only as two thin blocks of fibres, at most 4-6 p.m deep, running on opposite sides of the organ. Often this muscle layer is only a single fibre thick and is extremely difficult to visualize even when viewed under oil. The remaining muscle layers, however, are distinct, the circular zone being generally 12-15 µm thick and the inner longitudinal muscles 20-30 µm across. Muscle crosses are present in this region of the proboscis but are so slender that they cannot always be seen. When both are discernible (Fig. 20D) one is often more strongly developed than the other.

 

Towards the rear of the proboscis a fourth region can be recognized (Fig. 20E) in which the epithelium is only 15-30 µm thick and lacks gland cells but is sparsely ciliated, the outer longitudinal musculature is completely missing, the neural sheath is either very much reduced or absent, the circular muscle layer is at most only 2-3 µm wide, the inner longitudinal muscle coat is 6-7 µm thick, and the endothelium, up to 6-8 µm deep, is unusually well developed and composed of small cuboidal cells. No trace of a proboscis retractor muscle could be found.

 

The total length of the proboscis is between one-third and one-half that of the body.

 

 

Alimentary canal

 

The mouth is located some distance behind the cerebral ganglia at the end of the slender snout. Typically it appears as a rather peardrop-shaped aperture (Fig. 18C), the narrower end anterior, with thick and distinct lips. Its length varies from 0.5 to 8.0 mm depending both upon the size of the individual and the degree of contraction; in most of the specimens measured it was 1-3 mm long. The mouth is capable of a very wide gape.

 

The foregut epithelium, 30-75 µm thick, contains large numbers of gland cells. Although many of the glands bordering the mouth exhibit slightly different staining reactions from those of the foregut itself, they do not appear to form a 'salivary gland' region. The wall of the foregut is developed into longitudinal folds and furrows; the basophilic glands tend to be concentrated at the bases of the furrows, whereas the remainder of the epithelium is primarily composed of acidophilic glands and ciliated interstitial cells. The subepithelial gland cells, which appear to be composite in form and arranged in lobules, are most numerous in the folds of the foregut (Fig. 19F) and are either sparsely arranged or totally missing below the furrows.

 

The foregut splanchnic musculature, which is strongly developed, has been described earlier.

 

The posterior quarter of the foregut is narrower and less folded than the anterior regions. Its junction with the intestine is marked by a distinct valve-like and anteriorly-directed bulge of the ventral foregut wall (Fig. 20F). The dorsal wall extends for a short distance behind the valve and forms a distinct constriction where it meets the intestinal epithelium. Both circular and longitudinal splanchnic muscle layers extend into and terminate at the valve and its dorsal counterpart, but neither contains any branches of the blood system. Such a structure is very unusual in nemerteans but has been reported for other southern hemisphere species such as Parborlasia fueguina Serna de Esteban & Moretto, 1968, and Parborlasia hutchingsi Gibson, 1978. The appearance of the valve in the present species suggests that it works by a combination of muscular and mechanical mechanisms; a large mass of food in the lumen of the posterior foregut would tend to push the valve backwards and close it against the dorsal constriction (Fig. 20F), but valve closure could be modified or prevented by contraction of its muscle layers. It seems most likely that the purpose of the valve is to retain food material in the foregut with a consequent prolonged exposure to the various epithelial and subepithelial secretions.

 

The intestine possesses a normal heteronemertean construction, with a gastrodermal lining 45-60 µm or more thick. Its walls are developed into shallow pseudometamerically arranged lateral diverticula. The anus opens terminally.

 

 

Blood system

 

A single thin-walled and spacious lacuna comprises the cephalic blood supply. In front of the proboscis pore it forms a large median sinus, but for most of its length is enclosed by the longitudinal muscle fibres of the central cylinder and is slung saddle-like over the rhynchodaeum (Fig. 19E). A short distance in front of the proboscis insertion connective tissues and longitudinal muscle fibres divide the lacuna dorsally, forming two lateral blood channels which flank the rhynchodaeum (Fig. 19G). These vessels become so compressed as they enter the cerebral ring that in the anterior brain regions they are difficult or impossible to distinguish. Behind the ventral cerebral commissure, however, they expand and meet ventrally to form a U-shaped duct curving around the rhynchocoel. The base of this duct is irregularly subdivided by muscle and connective tissue strands so that in some sections three separate vessels (one mid-ventral and two lateral) can be recognized. This separation is increasingly evident in the more posterior cerebral regions and three distinct vessels can be followed through several successive sections before they again temporarily join.

 

Behind the cerebral organs the three vessels are firmly established; the lateral channels, expanded to form post-cerebral lacunae, run on either side of the rhynchocoel and pass close to but do not directly bathe the rear of the cerebral organs, whilst the ventral vessel extends between the roots of the lateral nerve cords and is separated from the rhynchocoel floor by the transverse muscle plate. The ventral vessel is irregularly crossed by dorsoventral connective tissue and muscle strands but essentially branches to form a pair of ducts (Fig. 20G) which continue back towards the mouth. The lateral channels are also irregularly traversed by muscle and connective tissue fibres. Immediately in front of the mouth the ventral vessels unite to produce a large median lacuna (Fig. 21A) but over the anterior buccal borders this further divides to form two ventrolateral vessels; these irregularly communicate with the appropriate lateral ducts on each side of the rhynchocoel. All four vessels are then subdivided to provide the very well developed foregut vascular plexus. This comprises a series of interconnected longitudinal blood spaces running between the body wall inner longitudinal muscles and the splanchnic circular muscle layer (Fig. 21B) on all except the dorsal margins of the foregut. The plexus extends the full length of this gut region but is somewhat reduced in the posterior half.

 

The mid-dorsal blood vessel arises from a dorsal branch at the base of the U-shaped duct where this passes through the cerebral ring. An unusual feature of the dorsal vessel, in comparison with other heteronemertean species, is that it runs in the ventral rhynchocoel wall for a considerable distance (i.e. reaching far behind the mouth) before protruding into the rhynchocoel to form the villus. The villus (Fig. 21C) is about 1.5 mm long and projects into the rhynchocoel lumen for up to 250-300 µm.

 

In the intestinal regions the blood system consists of three longitudinal vessels, linked by pseudometameric transverse connectives. Both the vessels and connectives are thick walled. The vessels are enclosed by a delicate layer of circular muscle fibres and may be considerably dilated.

 

 

Nervous system

 

Unusually for heteronemerteans the cerebral ganglia are located far in front of the mouth, approximately two-thirds of the distance back between the tip of the slender snout and the anterior limits of the oral aperture. The ganglia are compact (Fig. 21D) and elongate, with dorsal and ventral lobes of about the same size. Both the inner and outer neurilemma are distinct. The dorsal and ventral commissures are situated at the same level, towards the front of the brain, and both contain tracts of densely staining material (Fig. 21D); the tracts do not, however, seem to constitute neurochords as they are not found elsewhere in the nervous system. The fibrous core of the dorsal ganglionic lobes is posteriorly forked into short upper and long lower branches. The ganglionic tissues are freely penetrated by fibres running inwards from the surrounding cephalic musculature; they also contain enormous numbers of large vacuolated cells, 15 µm or more in diameter, which house a distinct nucleus possessing a single acidophilic nucleolus. The abundance of these cells suggests that they are not equivalent to the neurochord cells found in some heteronemertean taxa.

 

The lateral nerve cords lead directly from the rear of the ventral ganglionic lobes, gradually moving further apart as they run the remaining length of the head. Radial fibres of the body wall musculature pass through the outer ganglionic layer of the nerves but do not enter their fibrous cores.

 

The peripheral neural system is well developed. Anteriorly the brain lobes split up into a number of thick cephalic nerve roots (Fig. 19H), each invested by an extension of the cerebral outer neurilemma. A branch from the most ventral root on each side leads to the proboscis insertion and forms the origin of the proboscis nerve supply. The remaining cephalic nerves continue forwards, branching to innervate the various structures and regions of the head.

 

Several other nerves emerge from the cerebral ganglia. Thick lateral branches of both dorsal and ventral lobes lead to extraganglionic nerve tissues, situated between the brain and the cephalic slits (Fig. 21E), which are enclosed by connective tissue membranes continuous with the outer neurilemma. Fibres from these ganglionic masses innervate the posterior parts of the cephalic slit epithelium.

 

A pair of stout nerves leading inwards from the ventral brain lobes, near the origin of the main longitudinal nerve cords, comprises the start of the foregut nerve plexus. They run posteriorly near the lateral cords and are occasionally linked to them by transverse neural strands. Just in front of the mouth the two nerves are joined by a thick transverse connective (Fig. 21A); behind this the nerves reach ventrally to the buccal borders and extend alongside the mouth, meeting behind it to complete a circum-oral nerve ring. Branches of this ring extend towards the gut borders and can be followed for much of the foregut length amongst or internal to the circular splanchnic muscle fibres (Fig. 19F).

 

The peripheral neural sheath and mid-dorsal nerve are well developed. Throughout the length of the body large numbers of radial nerve fibrils lead from the neural sheath towards the body surface, many of them reaching the epidermal border (Fig. 19B). All regions of the body are thus well provided with a nerve supply.

 

 

Sense organs

 

There are about 35-45 irregularly distributed eyes on each side of the head, situated either immediately below the epidermis or more deeply alongside the shallow anterior regions of the cephalic slits (Fig. 21F). Each eye is 45-60 µm in diameter and possesses a simple pigment cup ocellus type of construction. The eyes extend from the extreme tip of the snout back to the posterior rhynchodaeal region of the head; they do not reach the anterior brain margins.

 

Neither a frontal organ nor cephalic glands could be distinguished in any of the specimens examined.

 

The cephalic slits are anteriorly very shallow but become progressively deeper as they extend back towards the brain. In their posterior regions the slits are internally bordered by a well developed subepithelial 'neural' layer, 15-45 µm or more thick (Fig. 21F), separated from the epithelium by circular muscle fibres. This neural layer is closely linked to the extraganglionic tissues described earlier. At the brain level the deepest part of the slits bears long (30 µm) densely arranged cilia and the epithelial and subepithelial tissues are almost completely intermingled. The outer region of the slits closes, so that a deep intramuscular canal is formed. This almost immediately develops dorsal, inner lateral and ventral pouches which give a characteristic trifoliate appearance in transverse sections (Fig. 21G). The dorsal and ventral pouches continue posteriorly for a short distance alongside the brain and then end blindly; they are surrounded by a distinct zone of extraganglionic tissue 20-45 µm or more thick. The lateral pouch, meanwhile, continues inwards between the outer portions of the dorsal and ventral brain lobes to enter the cerebral organ. Here it forms the ciliated cerebral canal. This turns through 90° to run near the outer lateral margins of the cerebral organ, reaching almost to the rear before ending in a mass of glandular tissue. The large and well developed cerebral organs are located between and behind the posterior regions of the dorsal and ventral brain lobes. They are enclosed by a distinct neurilemma (Fig. 22A) and consist of a central neuroganglionic mass innervated by the ventral fibrous branch of the dorsal cerebral lobes, anteriorly and posteriorly bordered by glandular caps. Extensions of these caps meet on the ventral margins of the organs.

 

 

Excretory system

 

The very well developed excretory system extends for most of the foregut length. It consists of a complex series of interlinked longitudinal tubules running inside the branches of the foregut vascular plexus (Fig. 21B). The tubules are usually attached to the vessel walls, but sometimes run freely in the vessel lumen and are then completely surrounded by the blood fluid. The tubules, up to about 60-70 µm in diameter, are lined by a cuboidal epithelium 4-15 µm thick. There may be more than one tubule in a blood vessel. Tubules in adjacent vessels interconnect when the vessels themselves join and do not appear to penetrate outside the blood plexus. In the anterior foregut regions the excretory tubules are at first confined to the lateral margins, but further back they reach dorsolaterally into the vessels running adjacent to the rhynchocoel and ventrolaterally below the gut. At the rear of the excretory system the collecting tubules lead to a single thick-walled duct in one of the lateral plexus vessels. This duct emerges from the blood vessel, penetrates the body wall inner longitudinal and circular muscle layers (Fig. 22B) and leads over the lateral nerve cord to a large efferent tubule. The efferent tubule (Fig. 22C) does not immediately lead to the body surface but runs obliquely backwards between the fibres of the body wall outer longitudinal musculature for 500 µm or more before finally discharging at a small nephridiopore (Fig. 22D). There is a single nephridiopore on each side of the body.

 

 

Reproductive system

 

The sexes are separate. The gonads are distributed between the intestinal diverticula and possess no unusual features.

 

 

Systematic discussion

 

Coe (1943) and Gibson (1972) indicated that amongst the numerous anoplan nemerteans the mouth was situated immediately behind the brain in all except members of the palaeonemertean family Cephalothricidae, where it is much more posteriorly placed at the end of a long snout. The arrangement in the Magellan Straits nemerteans thus closely resembles that found in the cephalothricids, yet they are undoubtedly lineid heteronemerteans. Other heteronemerteans are known in which the head is long and acutely pointed (e.g. the valenciniids Valencinia longirostris Quatrefages, 1846, and Zygeupolia rubens (Coe, 1895)), but in these the cerebral ganglia are located at the rear end of the head with the mouth just behind them (Quatrefages, 1846; Coe, 1895; Thompson, 1901), quite a different organization from that of the present specimens. Such a feature, although significantly different from any other heteronemertean, is not by itself necessarily significant at the generic level and further anatomical characters must be considered.

 

Gibson (1981a) listed 35 genera of lineid heteronemerteans; three additional genera, Micrurimorpha (Korotkevich, 1980), Craticulineus (Gibson, 1984) and Cephalurichus gen. nov (present paper) bring the total to 38. A further genus, Heterolineus, mentioned by Gibson (1982a), is of dubious validity and according to Correa (1963) should be regarded as a junior synonym of Lineus. Amongst the morphological characters listed by Gibson (1981a) which can be used to differentiate between the various lineid genera are the organization of the proboscis, rhynchocoel wall circular muscles and dermis, and the presence or absence of neurochord cells and a caudal cirrus. The present nemerteans possess a simple unbranched proboscis which for most of its length contains three muscle layers and two muscle crosses, rhynchocoel wall circular muscles which are not interwoven with the adjacent body wall inner longitudinal muscles, a dermis with separate glandular and connective tissue zones, and neither neurochords nor a caudal cirrus. This combination of characters enables the worms to be excluded from all of the above genera except Lineus, Parborlasia, Pseudolineus and Wiotkenia.

 

The genus Lineus, although the oldest and one of the largest heteronemertean taxa, still lacks a definitive diagnosis because very few of its 90 or more species have been adequately described (Friedrich, 1960a; Cantell, 1975; Gibson, 1981a). Gibson (1981a: 206) nevertheless provided a provisional definition for the group which includes the following features, "foregut with splanchnic musculature composed either of only longitudinal fibres or mixed longitudinal and circular layers ... cephalic glands usually well developed; most species with frontal organ". The present nemerteans have neither a frontal organ nor cephalic glands and, whilst their splanchnic muscles comprise both circular and longitudinal fibres, they are arranged into two quite distinct and separate layers and are not intermixed as in Lineus species. On the basis of these differences, together with the unusual relative positions of the cerebral ganglia and mouth, it is concluded that the Magellan Straits nemerteans do not belong in the genus Lineus.

 

The present worms in some ways resemble species of Parborlasia, two of which are found in Antarctic and Subantarctic waters. Gibson (1983) recently found that in at least one of these, Parborlasia corrugatus, as in the Magellan Straits nemerteans, the foregut possesses a subepithelial gland cell zone and both longitudinal and circular splanchnic muscle layers, and eyes are present in the head. A further similarity which can be found is the presence of a valve-like extension of the dorsal gut wall at the junction between foregut and intestine in Parborlasia fueguina and Parborlasia hutchingsi (Serna de Esteban & Moretto, 1968; Gibson, 1978), although in the latter species at least it is the anterior intestinal, rather than the posterior foregut, wall which forms the valve. Other features of the present nemerteans, however, are quite different from those found in Parborlasia species. These include the absence of a frontal organ and cephalic glands, the far less strongly developed dermal connective tissue layer, the degree of regional differentiation of the proboscis, the incomplete nature of the proboscis outer longitudinal muscle zone, and the mouth being located far behind the brain lobes. The combination and nature of these differences are regarded as sufficient to exclude the present nemerteans from the genus Parborlasia.

 

The genus Pseudolineus, established by Friedrich (1960a), is poorly described and the number of species is not known (Gibson, 1982a). Apart from noting that the genus lacks neurochords and neurochord cells, has no lateral rhynchocoelic pouches, possesses rhynchocoel wall muscles which are quite separate from the body wall musculature, and contains an unbranched proboscis, all that Friedrich (1960a: 58) indicated was that the "Faserkern der Dorsalganglien vorn and hinten gegabelt; Seitennerven biegen vor den Cerebralorganen seitwärts; Cutisdrüsen durch sehr starke Bindegewebsschicht gegen äLm abgegrenzt; unpaarer Schlundgefäβstamm ist nicht in zwei nebeneinander laufende Äste geteilt, sondern am Ende durch zwei direkt seitlich aufsteigende Äste mit den Seitengefäβen in Verbindung". An anteriorly and posteriorly forked fibre nucleus in the dorsal ganglia is an uncommon feature in heteronemerteans and this, together with the arrangement of the foregut blood vessel, differs significantly from the condition occurring in the present nemerteans, which do not thus belong in the genus Pseudolineus.

 

The Magellan Straits nemerteans can be excluded from the remaining genus for comparison, Wiotkenia, because of one unique feature of this taxon, namely that the proboscis insertion is not pre-cerebral but is located in the intestinal regions of the body (Serna de Esteban & Moretto, 1969a).

 

Since the present nemerteans cannot be placed in any of the existing lineid genera, the new genus Dokonemertes is established for them.

 

 

Geographical distribution

 

Dokonemertes magellanensis gen. et sp. nov. is known only from the type locality (Fig. 23).”

 

 

(Gibson, 1985; 138-152)

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© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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