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Unresolved name

Craticulineus novaezealandiae

Description

provided by NMNH Antarctic Invertebrates

“Type species: Craticulineus novaezealandiae sp. nov.

Etymology: The generic name is derived from the Latin craticulus (lattice work) and refers to the complex wickerwork of connective tissue and muscle fibres which criss-crosses the central longitudinal muscle plate. The specific name indicates the country near which the nemerteans were found.

Diagnosis: Heteronemertea with a single pair of horizontal lateral cephalic slits; caudal cirrus absent; proboscis simple, unbranched, for most of its length con­taining two muscle layers (outer circular, inner longitudinal) but in anterior re­gions with two additional blocks of outer longitudinal fibres, without muscle cros­ses; rhynchocoel wall circular musculature partially interwoven with adjacent body wall inner longitudinal muscles; dorsal fibrous core of cerebral ganglia poste­riorly branched into upper and lower forks; nervous system without neurochords and neurochord cells; foregut without subepithelial gland cell zone, but with cir­cular splanchnic muscle layer; extremely well developed central longitudinal muscle plate present between rhynchocoel and foregut, posteriorly extending into anterior intestinal regions; body wall without diagonal muscle layer; dermis di­visible into outer glandular and inner connective tissue zones; blood system with single median cephalic lacuna and three post-cerebral longitudinal vessels which are not linked by pseudometameric transverse connectives; excretory system ex­tending throughout most of foregut region, without external nephridiopores and discharging into foregut; cephalic glands poorly developed, scattered; single fron­tal organ present; eyes present; sexes probably separate.

Craticulineus novaezealandiae sp. nov.

Type specimens: Holotype mature female, deposited with the United States National Museum, Smithsonian Institution, Washington, Registration Number USNM 80545, full series of stained transverse and oblique sections; paratype unsectioned, Registration Number USNM 80546.

Type locality: 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, approximately 140 miles east of Christchurch, South Island, New Zealand.

External features: Both specimens are about 18 mm long. Their bodies are rounded in cross section and sharply tapered (Fig. 1), with a maximum diameter of approximately 2 mm in the anterior regions. The head is bluntly rounded and bears short lateral horizontal cephalic slits. The mouth is small and indistinct.

There is no caudal cirrus. Neither individual shows any evidence of a colour pattern;

the colour varies from a uniform dull brown to a pale creamish-brown.

Body wall, musculature and parenchyma: The epidermis (Fig. 2) is thickest

(75-100 µm or more) in the anterior body regions, but is reduced to a height of 45-60 µm in the slender posterior regions of the body. There are no unusual features in the organisation of the epidermis but in several places many of its acidophilic gland cells have discharged their contents and the body surface is covered by a distinct layer of amorphous material (Fig. 3). The thin but distinct epidermal basement membrane is internally bordered in turn by a delicate layer of subepidermal circular muscle fibres, at most 6-7 µm thick, and an incomplete stratum of longitudinal muscles which consists of small isolated bundles of fibres enclosed by connective tissues of the dermis.

The dermis (Fig. 2, 3) is well developed, 120 µm or more thick, and comprises outer glandular and inner connective tissue layers. Its glands are large, irregularly-shaped and composite in form; both acido- and basophilic secretory material can be distinguished within the same gland lobule. In the anterior body regions most of the dermis is composed of glandular tissue, whereas posteriorly the glands are reduced and the inner connective tissue zone is both thicker (60-75 µm) and much more evident.

The body wall muscle layers present a normal heteronemertean arrangement and there is no diagonal muscle coat. In the anterior half of the body the outer longitudinal layer is compact and some 250 µm thick, but posteriorly it becomes very much reduced in development (30-65 µm thick) and is far more loosely or­ganised. In contrast, the circular muscle layer is 30-45 µm thick throughout the length of the body, whereas the development of the inner longitudinal muscula­ture is variable; alongside the foregut and intestine it is often only 10-15 µm thick, but below the intestine it may be 150 µm or more deep. In the intestinal regions fibres emerging from the inner margins of the circular muscle layer form the dor­soventral muscle strands which traverse the body between the intestinal diver­ticula and gonads.

Between the rhynchocoel and dorsal foregut wall there is an extremely well developed longitudinal muscle plate, up to 180-200 µm thick. The plate is criss­crossed by connective tissue and muscle fibrils which give it a latticework ap­pearance (Fig. 4). Isolated fibres of the rhynchocoel wall circular musculature are also laterally interwoven with bundles of the adjacent body wall inner longitudinal muscles (Fig. 5). The central plate of fibres extends posteriorly for about one-third the length of the intestine, but in this region is reduced and rarely exceeds a thick­ness of about 40-45 µm.

There is no distinct central muscle cylinder in the head. Pre-cerebrally the body wall outer longitudinal muscles separate into bundles of fibres, surrounded by connective tissue, from which oblique and radial strands lead towards the rhynchodaeum and median cephalic blood lacuna.

Parenchymatous connective tissues are moderately well developed throughout the body; they are especially evident amongst the various structures of the cephalic region, between the muscle fibres of the longitudinal plate, alongside the lateral borders of the foregut, and around the intestine, intestinal diverticula and gonads.

Proboscis apparatus: The proboscis pore opens from the tip of the head into a slender tubular rhynchodaeum lined by an epithelium, 5-25 µm thick, which pos­sesses neither cilia nor glands. For most of its length the rhynchodaeum is enclosed by a loose layer of longitudinal muscle fibres (Fig. 6), but immediately in front of the proboscis insertion it is encircled by a sphincter of circular muscles, 30 µm thick, derived from the general cephalic musculature. The proboscis insertion is located just anterior to the brain lobes. It comprises isolated bundles of radial fibres which lead inwards from the deeper parts of the body wall outer longitudinal musculature.

The rhynchocoel is approximately 88% of the body length. In the foregut regions its wall contains a delicate epithelium, a longitudinal muscle layer 6-15 µm thick and a

circular muscle stratum 15-45 µm across; fibres leading from the lateral portions of the circular musculature are interwoven with the adjacent longitudi­nal muscle bundles as described earlier. Throughout the foregut region the cir­cular muscle layer becomes progressively thicker, especially on the dorsal and dorsolateral margins where it does not contribute to the meshwork of fibres; near the junction between foregut and intestine the layer is more than 150 µm thick and forms a strong sphincter encircling the rhynchocoel. Behind this sphinc­ter there is a sudden decrease in the development of the circular muscles and for the remaining rhynchocoel length they are only 15-25 µm or less thick. The longi­tudinal muscle layer is extremely delicate in the posterior rhynchocoel wall and often no trace of its fibres can be distinguished; in these regions it has a maxi­mum thickness of about 2-3 µm.

The proboscis is not strongly developed and has a maximum (introverted) dia­meter of about 300 µm. Near its insertion it is composed of a thin and non-glan­dular epithelium, an incomplete outer longitudinal muscle layer organised into two blocks of tissue, below which run the two distinct nerves (Fig. 7), a thin con­nective tissue layer, an inner longitudinal muscle zone 25-30 µm thick and a deli­cate endothelium. Behind the brain the epithelium begins to increase in thickness and develop gland cells, a circular muscle layer is formed and the two nerves subdivide to provide the neural sheath. In this region (Fig. 8) the proboscis con­sists of an epithelium 45 µm or more thick, an incomplete outer longitudinal muscle layer, at most 15 µm deep, arranged into two separate blocks, a neural sheath which is somewhat enlarged below each of the outer longitudinal muscle bundles but is otherwise only 4-6 µm across, a circular muscle coat 5-8 µm thick, an inner longitudinal muscle zone 15-25 µm wide, a distinct inner connective tissue sheath 3-5 µm deep and a delicate boundary endothelium. The two bundles of outer longitudinal muscle fibres become gradually smaller as they proceed posteriorly, and for most of the proboscis length only circular and inner longitudinal layers can be detected. There are no muscle crosses in any part of the proboscis.

The epithelium contains small distal pads of acidophilic barbs, mainly aggregated

to one side of the organ. Each pad is about 6-7 µm in diameter and 3-4 µm thick; individual barbs are less than 1 µm wide. Comparable structures have been reported from the proboscis of other lineid species by, for example, JENNINGS and GIBSON (1969) and GIBSON (1981). The proboscis retractor muscle is attached to the dorsal wall of the rhynchocoel near the circular muscle sphincter.

Alimentary canal: The mouth is a minute longitudinal slit, about 100 µm long,

opening just below the rear of the brain lobes. It leads into a tubular foregut

which becomes progressively narrower in its posterior half. Anteriorly the foregut is spacious and curves below the rhynchocoel; its epithelium is deeply folded, up to about 150 µm thick, and is penetrated by isolated circular muscle fibres derived from the splanchnic muscle layer. Further back, however, as the foregut both narrows and becomes dorsoventrally compressed (Fig. 9), the epithelium is less deeply folded, only about 45-75 µm in height and the splanchnic muscles form a distinct stratum 5-10 µm thick located entirely outside the gut wall. The narrowest part of the foregut occurs just before it joins the intestine; here the splanchnic musculature increases to a thickness of some 60 µm and forms a distinct sphincter enclosing the alimentary tract. There is no subepithelial gland cell layer associated with any part of the foregut.

Behind the sphincter there is a considerable overlap between the foregut and intestinal epithelia, the foregut opening into both the most anterior lateral intestinal pouches and the main intestinal canal. The lateral diverticula themselves extend forwards for a short distance on either side of the foregut.

The intestine possesses a typically heteronemertean construction, with deeply lobed and branched lateral diverticula. The gastrodermis is up to 100 µm or more thick. The anus is terminal.

Blood system: The blood system exhibits a simple grade of organisation. The cephalic supply consists of a single median lacuna, arching saddle-like over the rhynchodaeum (Fig. 6), enclosed by a layer of loosely packed longitudinal muscle fibres 30-40 µm in maximum thickness. Near the rhynchodaeal sphincter the ce­phalic lacuna separates into two lateral vessels. These enter the cerebral ring and run for only a short distance before meeting below the rhynchocoel to form a median ventral vessel, surrounded by longitudinal muscle fibres which lead for­wards from the central muscle plate. A dorsal branch of the ventral vessel runs towards the rhynchocoel wall and forms the mid-dorsal duct. Behind the brain the ventral vessel divides into two. Each branch leads dorsolaterally, passes be­low one of the cerebral sensory organs and then curves around the anterior wall of the foregut to run the remaining foregut length in a lateral position (Fig. 9). There is no foregut vascular plexus.

Soon after its formation the mid-dorsal vessel penetrates the rhynchocoel wall to form a rhynchocoelic villus (Fig. 10). This extends for approximately 250— 300 µm, then re-enters the rhynchocoel wall and passes through the circular muscle layer (Fig. 11) to run the remaining foregut length between the fibres of the longitudinal muscle plate (Fig. 12), gradually moving closer to the dorsal gut wall as it extends posteriorly.

In the intestinal regions the lateral blood vessels move to a ventrolateral posi­tion below the intestinal diverticula, whilst the mid-dorsal vessel runs immedia­tely adjacent to the dorsal gut wall. All three vessels possess thick (15 µm) walls. No evidence of pseudometameric transverse connectives linking the mid-dorsal and ventrolateral vessels could be distinguished in the intestinal regions.

Nervous system: The cerebral ganglia are well developed. Both the inner and the outer neurilemma are distinct (Fig. 14, 15). The dorsal cerebral commissure is slender (40-45 µm wide) and anterior to the thick (more than 150 µm) ventral commissure. In the dorsal lobes the fibre core is anteriorly simple, posteriorly bifurcated; the upper branch, together with its surrounding ganglionic tissues, projects some way posteriorly over the cerebral organs, whereas the ventral fork is short and leads directly into the organs. There are neither neurochords nor neurochord cells in the nervous system, ahd no unusual features could be distin­guished in the brain or lateral nerve cords.

Amongst the peripheral nerves the cephalic supply is the best developed. Ap­proximately 20 small separate nerves lead forwards, mainly from the dorsal lobes, on each side of the rhynchodaeum. They soon become dispersed between the cephalic tissues and serve the various parts of the head. The mid-dorsal nerve emerges from the rear of the dorsal cerebral commissure and can be followed the full length of the body in the peripheral neural sheath just outside the body wall circular muscle layer. A single nerve runs from the inner anterolateral margin of each ventral ganglionic lobe, enters the proboscis insertion and provides the pro­boscis nerve supply. From the rear of the ventral commissure two small nerves lead towards the foregut wall but are not developed into a circum-oral ring. Be­hind the mouth the nerves extend posteriorly near the lateral blood vessels, but are not distinguishable in the hindmost foregut regions; no trace of a foregut neural plexus, as found in many heteronemerteans, could de [sic] discerned.

Sense organs: There are 6-8 small and rudimentary eyes on each side and near the front of the head, situated either above or below the cephalic slits. Each eye is oval in shape and has a maximum diameter of about 35-40 µm.

The single frontal organ is very small. It comprises a shallow ciliated pit, 45­-50 µm wide, opening at the tip of the head. The epithelial lining of the frontal organ bears longer cilia than the adjacent epidermis but does not contain gland cells. Cephalic glands are few in number and are scattered between the paren­chymatous tissues, nerves and muscle fibres of the head. They are confined to the anterior cephalic regions.

The cephalic slits are deep, their epithelial lining being bordered by a distinct connective tissue basal membrane. The outer part of the epithelium histologically resembles the neighbouring epidermis, but the deeper portions almost entirely lack gland cells and possess a distinct striated distal border. Towards their rear the slits close externally, but continue towards the brain as deep intramuscular canals (Fig. 13) flanked peripherally by a small pouch-like extension of the glan­dular wall. The ciliated cerebral canals lead from the posterior end of the intra­muscular ducts, pass between the dorsal and ventral brain lobes and enter the front of the cerebral organs.

The cerebral organs are moderately well developed and are enclosed by a distinct outer neurilemma (Fig. 14, 15). Anteriorly the ciliated canal runs along the inner lateral border of the organ, externally flanked by neuroganglionic tis­sues (Fig. 14). These further back are innervated by the ventral branch of the dorsal lobe fibre core, where it forms the cerebral organ nerve. Just behind this nerve the ciliated canals cross to the outer margins of the cerebral organs (Fig. 15) before again extending posteriorly. The canals end near the hind tip of the organs in a mass of gland cells. The glandular component of each cerebral organ is restricted to this posterior cap of cells and the bulk of the organs' tissues are neuroganglionic in nature.

Excretory system: The excretory system is extremely well developed and ex­tends throughout most of the foregut length. It consists of large numbers of thick-walled collecting tubules, up to 40-45 µm in diameter, lying very close to the ventral and lateral foregut margins. In the more posterior regions the tubules are reduced in number and become increasingly confined to the lateral gut bor­ders, near the lateral blood vessels. Enlarged portions of these lateral tubules open into the foregut lumen (Fig. 16) at several places, and there are no external nephridiopores. Such an arrangement is extremely unusual in heteronemerteans and is known in only two other genera, Apatronemertes (WILFERT and GIBSON 1974) and Baseodiscus (PUNNETT 1900, COE 1906 a, b, GIBSON 1974). The excretory system ends just before the appearance of the anterior intestinal diverticula, where these extend alongside the foregut.

Reproductive system: The sexes are probably separate. In the holotype, a mature female, the ovaries extend throughout the intestinal region. They are large, numerous and irregularly distributed between the pouches of the intestine. Intra-ovarian eggs are in various stages of development; mature ova, up to about 250 µm in diameter, contain an oval nucleus 55-60 µm in maximum width in which the single acidophilic nucleolus (Fig. 17, 18) is 15-18 µm across. Several of the ovaries are completely empty.

An extremely unusual feature, which has not apparently been reported for any other nemertean species, is that many of the mature ova have become lodged in extra-ovarian sites (Fig. 17-20). These sites are always external to the body wall circular muscle layer and maturation of the individual ova does not appear to be in any way impaired. The possibility that the structures represent some form of parasite is discounted by histological comparisons between mature ova from ex­tra- and intra-ovarian locations. Whether such a phenomenon is normal for this species or represents some freak condition cannot at present be assessed.

Parasites: Occasional gregarine parasites are present in the intestinal lumen. Each is 75-100 µm long, 15-20 µm in diameter, possesses a single strongly acido­philic nucleus and is filled with a finely granular cytoplasm.

Systematic Discussion

Amongst the lineid heteronemerteans the rhynchocoel wall circular muscle layer is usually quite separate from the adjacent body wall longitudinal muscles and only two previous genera, Micrurides and Micrurinella (FRIEDRICH 1960 a, b), have been described in which the muscles are interwoven as in the present spe­cies. In both of these genera, however, the rhynchocoel wall is normal throughout most of the foregut region and it is only further back that the interwoven devel­opment is found. FRIEDRICH (1960 b) said of Micrurinella that 'In der Magenregion beginnend und nach hinten an Ausprägung zunehmend, tritt eine Verflechtung der Ringmuskulatur mit der umgebendeh Längsmuskulatur ein.', later comment­ing for Micrurides (FRIEDRICH 1960 b) that 'In der Ausbildung der Rhynchocoel­wand besteht Übereinstimmung mit Micrurinella . . . da im Bereich der Mittel­darmregion die Ringmuskulatur mit der inneren Längsmuskulatur des Körpers durchflochten ist.' This arrangement is very different from that found in the pre­sent species, where the rhynchocoel and body wall muscle layers are interwoven throughout the foregut regions.

Other differences, however, can also be distinguished; in Micrurinella the der­mal gland cells are embedded in the body wall outer longitudinal muscles, the excretory system is restricted to the anterior foregut region and opens to the exterior via a pair of lateral nephridiopores, the dorsal fibrous core of the cere­bral ganglia is not posteriorly forked, there are no dorsoventral muscles between the intestinal diverticula, and the proboscis contains two muscle layers and two muscle crosses, and in Micrurides the proboscis has three complete muscle layers throughout its length and possesses two distinct muscle crosses (FRIEDRICH 1960 b). These differences enable the present species to be excluded from either of the above genera, and no other genus is known in which the rhynchocoel and body wall longitudinal muscles are interwoven in this way. FRIEDRICH (1960 b), in referring to the interwoven musculature of Micrurinella, stated that 'ich dieses Merkmal neben anderen als grundlegend für eine neue Gattung ansehe.'

The organisation of the excretory system in the present species is also unique amongst the heteronemerteans. Many species of Baseodiscus (Baseodiscidae) do possess excretory ducts which discharge into the foregut lumen, but in this genus the efferent tubules are of a normal nemertean type and merely run inwards rather than to the body surface (COE 1906 a, b, GIBSON 1974). The only lineid known to possess an internally discharging excretory system is the freshwater Apatronemertes albimaculosa Wilfert and Gibson, 1974, and in this form the ex­cretory system is intimately associated with the post-cerebral vascular plexus. The arrangement of the excretory system in the present species, together with its interwoven rhynchocoel and body wall musculature, are thus sufficient justi­fication for the establishment of the new genus Craticulineus; no other lineid heteronemertean is known at present with this combination of characters.”

(Gibson, 1984; 77–87)