“Meionemertes polygonimos sp. nov.
Type specimen: Holotype mature male, deposited with the United States National Museum, Smithsonian Institution, Washington, Registration Number USNM 98553, full series of transverse and oblique sections stained with Mallory trichrome.
Type locality: Approximately 140 miles east of Christchurch, South Island, New Zealand (43°22'S, 175'20'E to 43°24'S, 175°15'E: USNS Eltanin Cruise 25, Station 370), collected by 10' Blake trawl from 95 m depth 19 November 1966.
External features: The single specimen of Meionemertes polygonimos gen. et sp. nov. is approximately 6.5 mm long and 0.85 mm in maximum diameter. The body is oval in cross section and of a more or less uniform width for most of its length, narrowing only slightly in the posterior regions (Fig. 1) to end in a blunt tail. The head is rounded; neither the single pair of shallow oblique ventrolateral cephalic furrows nor the two eyes are distinguishable in the intact individual. The colour and appearance in life are unknown; the preserved specimen is off-white to very pale brown with no trace of a pattern.
Body wall, musculature and parenchyma: The epidermis (Fig. 2), mostly 30-50 µm thick, is unusual in that its gland cells, predominantly basophilic in nature, are far less abundant than in most species of nemerteans. The glands are more or less regularly distributed on all sides of the body, but are reduced in number in the cephalic region and are completely missing from the posterior extremity. Acidophilic glands are present but are smaller and much more irregularly dispersed. There are no rhabditous glands. GIBSON (1984) notes that the nature and disposition of epidermal gland cells in nemerteans are dependent upon the species concerned.
The dermis (Fig. 2), composed of concentric layers of delicate connective tissue fibrils embedded in a basophilic matrix, is well developed and 5-20 um thick.
The body wall muscles have the usual hoplonemertean arrangement (Fig. 2), with no diagonal layer between them. Both layers are best developed in the foregut regions of the body where they are about 10 µm (outer circular) and 60 µm (inner longitudinal) in maximum thickness. Reductions in muscle developmobare evident in the posterior half of the body and in the cephalic region. Pre-cerebrally many of the longitudinal fibres form bundles which turn inwards (Fig. 3) to form an open septum, as defined by CORRÊA (1954), leading to the proboscis insertion, but a slender zone of scattered fibres, at most about 5-8 µm thick, continues towards the tip of the head. Throughout the cephalic region isolated fibres lead inwards from the longitudinal layer to provide a loosely arranged network of radial and oblique muscles.
Dorsoventral muscles are completely missing from all parts of the body, and there is no splanchnic musculature associated with any part of the gut.
Parenchymatous connective tissues are extensive, especially in the region between the cerebral ganglia and the anterior limits of the gonads.
Proboscis apparatus: The proboscis pore is just subterminal and ventral. It opens into a tubular rhynchodaeum, lined by a delicate epithelium which neither ciliated nor glandular, loosely flanked by the radial and oblique fibres of the cephalic muscle network.
The proboscis insertion is located immediately in front of the brain. The rhynchocoel extends almost to the extreme posterior tip of the body. Its wall contains separate circular and longitudinal muscle layers which are respectively 5-30 µm and 3-15 µm thick.
Relative to the size of the worm, the proboscis is a large and well developed organ. It possesses four distinguishable regions. A short anterior portion, leading from the proboscis insertion, is about 170-175 µm in maximum diameter and consists of an outer epithelium 10-12 µm thick which contains few gland cells a well developed connective tissue zone 8-15 µm deep which peripherally projects into the epithelial folds, a longitudinal muscle layer 10-15 µm across in which large proboscis nerves are quite distinct, a slender inner connective tissue coat and a delicate lining endothelium. The poorly developed circular muscle layer of this portion of the proboscis is largely masked because its fibres run in the outer connective tissue layer. Towards the rear of the anterior region this connective tissue zone forms a thick (up to 45-50 µm across) transverse septum (Fig. 4) which marks the beginning of the largest portion of the proboscis, the main anterior chamber (Fig. 5). This is 270-300 µm in diameter, its wall comprising a glandular epithelium developed into simple conical papillae, an outer connective tissue coat 4-8 µm thick, a slender zone of outer circular muscle fibres, an outer neural sheath 6-8 µm across from which delicate radial tracts lead inwards to the proboscis nerves, an inner longitudinal muscle layer up to about 30 µm wide divided unequally into distal and proximal portions by the main neural plexus, a thin inner connective tissue stratum and a slender endothelium. The neural supply is particularly well developed; the main plexus bears 10 distinct primary nerves, each 15-20 um in diameter, between which occur smaller secondary nerves 8-12 µm across (Fig. 5). This degree of proboscis neural development, with inner and outer plexi and alternate primary and secondary nerves differing in size, is more typically associated with polystiliferous hoplonemerteans than with monostiliferous forms (COE 1926, GIBSON 1972). At the posterior end of this part of the proboscis, just in front of the stylet bulb, the outer connective tissue layer disappears and the nerves fuse to form a single thick neural ring.
The single central stylet is about 35 µm long. It is mounted on a conical basis 35-40 µm long and 30 µm in maximum diameter, i.e., the stylet to basis ratio is approximately 1 : 1. There are two large accessory stylet pouches, each containing five reserve stylets in various stages of styletogenesis. The spacious stylet bulb chamber is lined by a non-glandular epithelium some 30 µm in maximum height, enclosed by a layer of loosely arranged circular muscle fibres. At its rear the chamber narrows sharply to form a canal 10-12 µm wide leading to the posterior portion of the proboscis.
The posterior proboscis forms a swollen elongate sac, up to about 300 µm in diameter. Its wall consists of an epithelium 60 µm or more thick enclosed in turn by
thin outer longitudinal and inner circular muscle layers and a flattened endothelium. The proboscis retractor muscle is attached to the dorsal wall of the rhynchocoel some distance in front of its posterior end.
Alimentary canal: The oesophagus opens from the anterior end of the rhynchodaeum, immediately behind the proboscis pore; its slender epithelium is neither ciliated nor glandular. The stomach is a spacious but comparatively short region, lined by a densely ciliated and moderately folded glandular epithelium 20-25 µm thick. The dorsal portion of the stomach continues posteriorly as a broad, somewhat dorsoventrally compressed pyloric region, approximately 150 µm long, which narrows only as it nears its junction with the intestine. The general organization of the foregut thus resembles the Type A illustrated by FRIEDRICH (1956).
The intestine is simple. It possesses a short anterior ventral caecum extending forwards to about mid-stomach level; the caecum has neither anterior pouches nor lateral diverticula. The main intestinal canal is broad and curves below the rhynchocoel. Though lacking distinct diverticula, it bears irregular shallow lateral lobes for most of its length. A short rectal region is distinguishable from which gland cells are missing. The anus opens at the posterior tip of the body.
Blood system: The blood vessels are thin-walled and possess valves, but lack associated muscle fibres. In the head a pair of slender cephalic vessels runs on either side of the rhynchodaeum, meeting anteriorly via a suprarhynchodaeal loop. Behind the brain a single mid-dorsal and paired lateral vessels extend to the posterior end of the body, where they join at a supraintestinal connective; there are no additional transverse vessels in the intestinal regions. The origin of the mid-dorsal blood vessel could not be traced but, as in several species of Tetrastemma (KIRSTEUER 1963), the vessel does not form a vascular plug. The arrangement of the blood system thus represents a simple hoplonemertean grade.
Nervous system: The cerebral ganglia are well developed. They are ensheathed by a distinct outer neurilemma, but there is no inner neurilemma separating the fibrous and ganglionic cerebral components. Dorsal and ventral commissures, respectively about 15 µm and 40 µm in diameter, are comparatively long, the cerebral ganglia as a consequence being set rather far apart. On each side the dorsal and ventral brain lobes are posteriorly separated by parenchymatous connective tissue. There are no neurochord cells.
The lateral nerve cords run in a normal position to meet posteriorly via a supra-intestinal commissure. There is no accessory lateral nerve.
The peripheral nervous system, other than that associated with the proboscis, is not particularly well developed. There is no mid-dorsal nerve, as occurs in many hoplonemerteans, and only a small number of cephalic nerves can be distinguished leading forwards from the cerebral ganglia.
Eyes: A single pigment cup ocellus, 35-40 µm in diameter, occurs on either side of the head, situated ventrolaterally near the tip.
Cerebral sensory organs: The cerebral sensory organs, though moderately large, possess a simple construction. They open ventrolaterally from shallow oblique cephalic furrows situated just in front of the anterior borders of the brain. The ciliated sensory canals lead upwards and inwards before turning posteriorly to enter the cerebral organs, where they are flanked above and below by neuroganglionic tissue (Fig. 6). Glandular tissues are restricted to a small posterolateral cap of cells (Fig. 7). The cerebral organs are enclosed by a delicate neurilemma, posteriorly continuous with that investing the brain lobes. The organs are innervated by short nerves leading from the ventral borders of the ventral cerebral lobes.
Frontal organ and cephalic glands: The frontal organ comprises a single small ciliated pit opening from the dorsal wall of the rhynchodaeum adjacent to the proboscis pore. The cephalic glands are well developed, but consist of variably shaped and sized basophilic lobes with a diffuse arrangement. Near the anterior tip they are principally restricted to the dorsal half of the head, but further back spread to reach the ventral borders too. The glands extend back to the anterior margins of the brain.
Excretory system: All that can be distinguished of the excretory system are small and thin-walled collecting tubules running on either side of the body in the vicinity of the stomach. No trace of efferent ducts or nephridiopores could be found.
Reproductive system: The single specimen of Meionemertes polygonimos gen. et sp. nov. is a mature male. The testes are irregular in size and shape. Except for the extreme anterior and rectal regions, they are freely distributed throughout the intestinal parts of the body, extending from the dorsolateral borders adjacent to the rhynchocoel to the mid-ventral position (Fig. 8). Up to eight or nine testes can be counted in a single section. Larger testes are 200 µm or more in maximum dimension.
The testes contain sperm in various stages of development. Many possess aggregations of mature spermatozoa (Fig. 9); under oil immersion individual sperm appear to have a round head and a tail some 20-25 µm in length. Similar sized spermatozoa have been reported from other species of nemerteans (FRANZEN 1956), the round-headed type being regarded as primitive (FRANZEN 1983). Several of the testes possess gonoducts open to the body surface and from many of these the sperm have been discharged. This indicates that at the time of collection (November) the species was reproductively active and that it therefore breeds during the austral summer.
Systematic Discussion
Although in need of revision, seven families of monostiliferous hoplonemerteans are currently recognized (GIBSON 1982, 1985). Amongst these the Carcinonemertidae are exclusively ectohabitant/parasitic on decapod Crustacea, possess well developed submuscular glands throughout the length of the body and have a short rhynchocoel and proboscis; the Cratenemertidae are characterized by a rhynchocoel wall composed of a meshwork of interwoven circular and longitudinal muscle fibres (a feature shared by the terrestrial and transitional prosorhochmid genera Acteonemertes, Antiponemertes, Argonemertes, Katechonemertes and Leptonemertes); the Emplectonematidae, though a poorly defined group, include genera in which the rhynchocoel is less than half the body length, the proboscis is short and the cerebral sensory organs are situated anterior to the brain; and the ototyphlonemertidae are mesopsammic forms with cerebral ganglia containing statocysts and a short proboscis and rhynchocoel. The present specimen, with a long rhynchocoel enclosed by two distinct muscle layers, a large and well developed proboscis, cerebral organs located mainly below the brain, and neither sub-muscular glands nor statocysts, can thus be excluded from any of the above taxa.
The Amphiporidae too is a poorly defined family. Most members of the type genus Amphiporus (including the subgenus Intestinonemertes), however, are large
stout nemerteans with an intestinal caecum bearing anterior as well as lateral pouches, lateral diverticula along the length of the intestine, eyes which are typically numerous and arranged in groups, dorsoventral muscles and a mid-dorsal blood vessel forming a single vascular plug. Particular characters of other amphiporid taxa include a mouth and proboscis pore which either open separately (Duosnemertes) or close together into a common ventral bay (Communoporus), a rhynchocoel provided with paired lateral or unpaired dorsal or ventral caeca (Amphiporella, Gurjanovella, Proneurotes, Valdivianemertes), an anteriorly divided body wall longitudinal muscle layer (Correanemertes, Dananemertes, Paramphiporus, Poseidonemertes and also the Amphiporus hastatus group), caecal diverticula on the oesophagus or stomach (Alaonemertes and the subgenus Intestinonemertes), large numbers of eyes extending post-cerebrally above the lateral nerve cords (Zygonemertes), a closed pre-cerebral septum (Tagonemertes), and branched gonads and a complex multi-pouched blood system (Arctonemertes) FRIEDRICH 1955, 1957, CORREA 1957, KIRSTEUER 1965, 1974, GIBSON 1982); none of these features occurs in the New Zealand specimen, which does not therefore belong with any of the amphiporid genera.
Recent studies concerned with the systematic relationships of terrestrial nemerteans (MOORE and GIBSON 1981, 1985) conclude that the genera presently included in the Prosorhochmidae (GIBSON 1982) have evolved from at least two ancestral families. Any relationship between the New Zealand species and most of the terrestrial or transitional 'prosorhochmid' genera has already been discounted on the basis of rhynchocoel morphology. Several other characters, however, not found in the present specimen, enable it to be excluded from any of the remaining taxa in the family. Such features include the presence of neurochords (Prosadenoporus, Prosorhochmus) and accessory lateral nerves (Antarctonemertes, Friedrichia, Gononemertes, Obargeria, Oerstedia, Oerstediella, Paroerstedia) in the nervous system, a cephalic neuroglandular complex in place of cerebral organs and ganglionic swellings on the lateral nerve cords (Divanella), and binucleate flame cells reinforced by cuticular support bars (Geonemertes, Pantinonemertes) (CORREA 1954, FRIEDRICH 1955, KIRSTEUER 1965, MOORE and GIBSON 1981, GIBSON 1982, GIBSON and MOORE 1985). In addition, Prosorhochmus possesses a split pre-cerebral septum and an exceptionally well developed frontal organ, Pantinonemertes has a divided body wall longitudinal musculature and no pre-cerebral septum, Geonemertes is a terrestrial genus, and the two remaining taxa, Campbellonemertes and Potamonemertes, are exclusively freshwater.
The final monostiliferous family, the Tetrastemmatidae, comprises mostly small species in which the rhynchocoel is as long as the body, or nearly so, the cerebral organs are in front of but close to the brain, and the intestine usually bears shallow and unbranched lateral diverticula. Although in its general organization the present species more closely resembles the tetrastemmatid hoplonemerteans than the members of the other families, and is thus placed with the Tetrastemmatidae, several characters enable it to be distinguished from any of the existing genera. Thus Africanemertes, Itanemertes, Prostomiopsis and Tetrastemma have a closed pre-cerebral septum (FRIEDRICH 1955, CORRÊA 1957, KIRSTEUER 1963, 1965), the excretory system extends for most or all of the body length in Prostoma (an entirely freshwater taxon), Sacconemertes, Sacconemertelia and Sacconemertopsis (KARLING 1933, IWATA 1970), the cephalic glands are large but compact, the excretory system is well developed, the intestine bears anterolateral pouches, and the cerebral organs open far in front of the brain in Algonemertes (CORRÊA 1954), Amphinemertes is hermaphroditic, possibly commensalistic with tunicates, and has voluminous cephalic glands (COE 1905), Arenonemertes species have three anterior pouches in place of an intestinal caecum, few gonads and large adhesive gland cells distributed over the body surface (FRIEDRICH 1933), Austroprostoma possesses a median brain lobe, from which the lateral nerves arise, situated between the dorsal and ventral ganglia, and cerebral organs with a vermiform appendage (STIASNY‑WIJNHOFF 1942), Nemertellina has simple cerebral organs located far in front of the brain, a mid-dorsal nerve, lateral intestinal diverticula and gonads restricted to the hind end of the body in a single row on either side of the alimentary canal FRIEDRICH 1955), Nemertellopsis possesses a mid-dorsal nerve, a small number of gonads but no oesophagus (FRIEDRICH 1955), and Prostomatella species possess dorsoventral muscles, compact rather than diffuse cephalic glands, paired anterolateral intestinal pouches bearing diverticula, large cerebral organs and a well developed excretory system (CORRÊA 1954, FRIEDRICH 1955).
Two additional monotypic genera are not at present placed in any of the monostiliferous families; both, however, differ from the present form. Elcania contains neurochords in its nervous system and has an intestinal caecum provided with both anterior and lateral diverticula (MORETTO 1970); this species also possesses caecal pouches on its stomach and rhynchocoel, although MORETTO did not regard these features to have a taxonomic significance. Annulonemertes is segmented, has a short rhynchocoel and proboscis, no cerebral organs and no pyloric region in the foregut (BERG 1985).
The combination of anatomical characters possessed by the present species from New Zealand waters thus does not allow it to be placed in any of the existing
monostiliferous genera; the new genus Meionemertes is accordingly established for it.”
(Gibson, 1986; 280-287)
