Comprehensive Description

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Description

Zodionemertes chilensis sp. nov.

 

Holotype

 

Sexually immature, USNM 81598, full series of transverse sections; Paratypes. Two sexually immature individuals, USNM 81599 and 81600, full series of transverse sections. Three additional examples from the type locality, USNM 81601, have also been examined.

 

Type locality

 

Straits of Magellan, east of Peninsula de Brunswick, 53°30'48"S, 70°50'33"W (Station 69-6A), collected by hand (dipnet after rotenone treatment) between 0 and 1 m depth 16 April 1969.

 

External features

 

The nemerteans range from 2 to 4 mm in length and 0.5 to 0.75 mm in maximum width. Their bodies are rounded at both ends (Fig. 54), the cephalic tip in some being slightly notched. The lateral cephalic slits are shallow but distinct. In general the body surface is fairly smooth but some slight to moderate epidermal furrowing is apparent in places. There is no caudal cirrus. The preserved colour is mahogany with no trace of pattern.

 

Body wall, musculature and parenchyma

 

The epidermis is 15-35 µm thick and possesses large numbers of acidophilic rhabditous glands (Fig. 55A) which are especially abundant on the dorsal surface in the anterior half of the body. The epidermal basement membrane is thin but distinct. It overlies outer circular and inner longitudinal subepidermal muscle layers which, although together only about 6-8 µm thick, are easily distinguished; the longitudinal layer in particular mostly comprises a single or double row of muscle fibres more or less regularly scattered along the distal surface of the dermis (Fig. 55A). These longitudinal muscles are not evident pre-cerebrally, but the circular layer extends to the tip of the head.

 

The dermis (Fig. 55A) is an unusually compact and well defined layer, in places thicker than the epidermis (a maximum of 50-55 µm). It is composed of densely packed, predominantly acidophilic, gland cells with finely particulate cytoplasmic contents. Irregularly scattered between these glands are cells containing a homogeneous pale orange-brown to grey-green secretion (with Mallory). Such cells are more numerous in the anterior body regions; pre-cerebrally, where a distinct dermis is no longer visible, the cells attain their greatest numbers and extend deep into the cephalic tissues amongst the muscle fibres and acidophilic cephalic glands. The dermal gland cells are separated from the body wall outer longitudinal muscles by a thin (3-4 µm) but distinct layer of connective tissue.

 

The main body wall muscles exhibit the usual heteronemertean arrangement, with no diagonal layer. The outer longitudinal stratum is the most strongly developed and attains a maximum width of about 60 m. In contrast the circular and inner longitudinal layers are respectively at most some 15 µm and 30 µm thick. In the extreme posterior parts of the body the dermis and all three main muscle layers are considerably reduced and may together be only 45 µm or less across.

 

The cephalic musculature consists mainly of scattered longitudinal fibres posteriorly continuous with the main outer longitudinal muscles. Throughout the length of the head many of the longitudinal fibres turn inwards, particularly in the dorsal half, to form radial or oblique muscle strands. A number of these lead to an ill-defined central layer of circular muscles, at most 15-20 µm thick, which loosely surrounds the rhynchodaeum, cephalic lacuna and their associated longitudinal muscle fibres. This constitutes an indistinct central muscle cylinder.

 

The longitudinal splanchnic muscles are derived from slender radial muscle strands which cross the various body wall layers to end internally between the foregut epithelium and its subepithelial gland cell zone.

 

Dorsoventral muscles occur in the intestinal regions, forming distinct bundles of fibres (Fig. 55B) which cross the body between the main gut canal and its shallow lateral pockets.

 

Apart from forming the various distinct basal membranes connective tissues are poorly developed and there is little or no parenchyma.

 

Proboscis apparatus

 

The ventral and subterminal proboscis pore opens into a thin-walled tubular and ciliated rhynchodaeum (Fig. 55C).

 

The proboscis insertion, situated immediately in front of the cerebral ganglia, is primarily formed from radial muscles leading inwards from the dorsal half of the head, but some ventrolateral fibres also contribute. The rhynchocoel extends to the extreme posterior tip of the body. Throughout its length the rhynchocoel wall is extremely thin and mainly comprises a flattened lining epithelium and a delicate circular muscle layer which rarely exceeds a thickness of about 6-8 µm; the circular muscles are not interwoven with the adjacent body wall inner longitudinal muscle fibres. The longitudinal musculature of the rhynchocoel wall is restricted to a ventral and ventrolateral plate of fibres (Fig. 55D) at most 5-7 µm deep, situated between the epithelium and circular muscle layer. These longitudinal muscles extend throughout the foregut and anterior intestinal regions, but cannot be distinguished in the posterior portion of the rhynchocoel; here the wall is exceptionally delicate and the circular muscle coat is only one or two fibres thick.

 

The proboscis is comparatively small, approximately 12-18% of the body width. Anatomically it is divisible into three regions. The short anterior portion, leading from the proboscis insertion, comprises an epithelium 5-20 µm thick which does not contain gland cells, a layer of longitudinal muscle fibres and a flattened endothelium. Connective tissue membranes lining these layers are thin but well developed (Fig. 55E). A pair of large proboscis nerves runs between the epithelium and muscle stratum. For most of its length, however, the proboscis epithelium is up to 45-60 µm tall and contains large numbers of strongly acidophilic gland cells and barbs, the remainder of the proboscis wall containing a delicate neural sheath 5-6 µm deep, a circular muscle layer 10-15 µm across, an inner longitudinal muscle layer 15-18 µm in maximum thickness, and a thin endothelium (Fig. 55D). A weakly developed and incomplete outer longitudinal muscle layer is present, but is reduced to isolated bundles of fibres running on three sides of the organ, located between the epithelium and neural sheath. In the more posterior parts of this main proboscis region the outer longitudinal muscle fibres cannot be distinguished. The short hind part of the proboscis (Fig. 55F) lacks both the epithelial glands and barbs and a circular muscle layer; it comprises an outer epithelium 8-12 µm thick, a very thin neural sheath, a layer of longitudinal muscle fibres up to about 30 µm in maximum thickness, and an endothelium. No muscle crosses can be distinguished in any part of the proboscis.

 

The proboscis retractor muscle is attached to the dorsal wall of the rhynchocoel about mid-way along the length of the intestine.

 

Alimentary canal

 

The small mouth opens a short distance behind the cerebral ganglia. It leads into a buccal chamber whose walls are deeply folded and bulge forwards towards the rear of the cerebral sensory organs. The bulk of the foregut comprises a slightly to moderately folded ciliated epithelium 20-25 µm thick externally surrounded by a zone of subepithelial gland cells (Fig. 55A). The epithelium contains only acidophilic glands interspersed between its ciliated interstitial cells; these glands are elongate to pyriform in shape, whereas the subepithelial acidophils are large and irregularly shaped. Basophilic glands are confined to the subepithelial region, although their secretory tracts can be distinguished leading towards the foregut lumen and penetrating the epithelial border. Between the epithelium and its underlying glandular tissues there is a weakly developed stratum of longitudinal splanchnic muscles. Isolated fibres from this layer sometimes run outside the subepithelial glands and occasional oblique or circular fibres can also be discerned in a similar position; there is, however, no circular splanchnic muscle layer formed.

 

The intestine opens from the dorsal wall of the foregut (Fig. 56A). The rear portion of the foregut extends posteriorly for a short distance (between 60 and 70 µm in the three specimens examined histologically) below the intestine as a distinct dorsoventrally compressed caecum (Fig. 56B). Although a few heteronemertean species are known in which a caecum is found at the junction between foregut and intestine (Friedrich, 1960a), the foregut caecum, complete with its subepithelial gland cells, in the present species is a unique feature quite different from the intestinal caecum of other taxa.

 

The intestine itself is a simple tubular canal, lined by a glandular gastrodermis up to 45-60 µm thick which possesses a normal heteronemertean construction. Shallow lateral pouches or diverticula are present for most of the intestinal length but are missing from the extreme posterior portions. The anus is terminal.

 

Blood system

 

A median cephalic lacuna arches over the dorsal margins of the rhynchodaeum (Fig. 55C). It separates into two lateral vessels just in front of the proboscis insertion; these enter the cerebral ring and rejoin over the ventral cerebral commissure to form a U-shaped channel flanking the ventral and lateral rhynchocoel borders. Behind the ventral commissure muscle fibres and connective tissue strands split the channel into dorsal, ventral and paired lateral vessels. The lateral vessels move dorsally, expand to bathe the rear portions of the cerebral sensory organs and then continue backwards above the buccal wall on either side of the rhynchocoel as spacious post-cerebral lacunae. They are soon traversed by radial muscle and connective tissue fibres, the branches so formed providing the origin of the foregut vascular plexus. The ventral vessel meanwhile runs posteriorly towards the forward bulge of the buccal region, where it separates into two, each half merging with the appropriate post-cerebral lacuna.

 

Soon after its origin the dorsal vessel enters the rhynchocoel wall to form a single villus (Fig. 56C); this just reaches behind the level of the mouth. The villus then ends and the mid-dorsal vessel leaves the rhynchocoel wall to continue posteriorly in the usual position.

 

Towards the rear of the foregut the branches of its plexus become increasingly restricted to the ventral borders, continuing to the end of the foregut caecum. Here they coalesce on either side, develop thicker walls and form the main ventrolateral vessels. These extend to the posterior end of the body where they and the mid-dorsal vessel meet via a supraintestinal connective. No evidence of pseudometameric transverse channels linking the three longitudinal vessels can be found in any other part of the intestinal region.

 

Nervous system

 

The cerebral ganglia are well developed, their slender (15 µm thick) dorsal cerebral commissure being located some way in front of the ventral, which is 60 µm or more in diameter. Both an inner and outer neurilemma are discernible, but the inner sheath is not strongly developed and often the ganglionic and fibrous tissues of the cerebral lobes abut directly against each other with no evident intervening neurilemma. The fibrous core of the dorsal lobes is anteriorly simple, posteriorly forked into short upper and long lower branches. The anterodorsal and lateral regions of the cerebral ganglia are enclosed by successive layers of ganglionic cells separated by connective tissue lamellae continuous with the outer neurilemma(Fig. 56D); the arrangement is very similar to that described for Dokonemertes magellanensis gen. et sp. nov. (p. 146). Neither neurochords nor neurochord cells can be distinguished in any part of the nervous system.

 

The main longitudinal nerve cords meet posteriorly via a subintestinal commissure.

 

The peripheral nervous system is well developed. Several nerve tracts leading anteriorly above and on either side of the cephalic lacuna can be followed for some distance towards the tip of the head, although their ultimate fate cannot be discerned. One of the nerves leading from the anterior surface of each ventral brain lobe loops upwards and inwards between cephalic nerve roots and enters the proboscis insertion; these nerves provide the proboscis neural supply.

 

The peripheral neural sheath is delicate but bears a single well-defined mid-dorsal nerve.

 

A small nerve leads from the inner surface of each lateral nerve cord root and runs to a pre-oral commissure. The two thick nerves which extend posteriorly from this commissure comprise the foregut nerves; these nerves, in places 35-40 µm across and in direct communication with the peripheral neural sheath, pass on either side of the mouth and then join at a post-oral connective. Branches of the circum-oral nerve ring provide the foregut neural plexus. The nerves of the plexus mainly run adjacent to the proximal border of the foregut subepithelial gland cell zone, although isolated fibres also pass between the glands. The plexus terminates below the junction between foregut and intestine, at the anterior end of the foregut caecum, via a thick (15 µm or more) transverse ventral commissure (Fig. 56E).

 

Sense organs

 

There are 8-12 eyes on either side of the head. Most are grouped near the cephalic tip, close to the inner borders of the lateral slits, but a few are found further back and may reach the anterior brain region. Each eye is oval to spherical, up to 40-45 m in maximum diameter, and appears composed of finely granular cytoplasm enclosed by a very thin and often indistinct connective tissue membrane (Fig. 56F). In a few cases suggestions of an indistinct 'retinal cup' can be discerned; the eyes thus appear to be of a simple type, lacking the pigmented layer typical of most nemertean ocelli.

 

There is no frontal organ. Cephalic glands are, however, moderately well developed and consist of irregularly-shaped acidophils distributed between the muscle fibres and other tissues of the head. The glands, which are most numerous in the ventral cephalic region and reach back to the brain, discharge to the exterior via long independent ducts extending through the epidermis.

 

For most of their length the cephalic slits are shallow but distinct. Their epithelium (Fig. 56F), which lacks gland cells, is internally bounded in turn by an obvious connective tissue basement membrane, a delicate layer of circular muscle fibres and a well developed neuroganglionic cellular zone 15-30 µm deep which is posteriorly continuous with the extracerebral nerve tissues investing the brain lobes.

 

At their rear the cephalic slits close externally, forming a deep intramuscular chamber lined by a densely ciliated epithelium 15 µm or more thick. From the innermost borders of these chambers the ciliated cerebral canal on each side leads directly inwards, enters the ganglion cell layer of the dorsal cerebral lobe and immediately turns through 90° to run upwards alongside the outer margin of the fibrous core (Fig. 57A). The canal then arches over this core and enters the cerebral organ. Unusually for heteronemerteans the cerebral sensory organs are neither well developed nor clearly delimited from the dorsal brain lobes; instead they comprise small dorsoposterior caps of gland cells (Fig. 57B) which lack their own neural tissues. Some of the glands extend along the inner and ventral borders of the dorsal cerebral lobes, so that the cerebral organs are effectively tightly wrapped around the rear of the brain. The hind portion of the cerebral organs projects a short way into the post-cerebral blood lacunae.

 

Excretory system

 

The excretory tubules extend throughout most of the foregut region, anteriorly reaching on either side of the buccal chamber. The collecting tubules are small, mainly lateral and run in the branches of the foregut vascular plexus. Efferent ducts are broad (10-15 µm in diameter) but thin walled. They open laterally near the posterior limits of the excretory system; there may be one or two nephridiopores on each side of the body (Fig. 57C) which may open close together or be fairly widely separated.

 

Reproductive system

 

All the specimens are sexually immature, with no trace of gonad formation.

 

Systematic discussion

 

The main anatomical features of the present nemerteans are a simple unbranched proboscis containing three muscle layers (the outermost reduced and incomplete) and no muscle crosses, rhynchocoel wall circular muscles which are not interwoven with the adjacent body wall inner longitudinal muscles, a dermis which is unusually compact but separated from the outer longitudinal musculature by a thin sheet of connective tissue, no caudal cirrus, and a nervous system which contains neither neurochords nor neurochord cells. This combination of characters excludes the worms from all the known heteronemertean genera (Korotkevich, 1980; Gibson, 1981a), including the new taxa described in the present paper, except Evelineus, Lineus and Pseudolineus.

 

Evelineus is a monotypic genus, the only known species (Evelineus tigrillus Corrêa, 1954) occurring intertidally on the coast of Brazil. Several differences can be found, however, between Evelineus and the Chilean nemerteans. In particular Evelineus has extremely well developed frontal organs and cephalic glands, a distinct central cephalic muscle cylinder, a proboscis in which the outer longitudinal muscle layer is both complete and extends the full length of the organ, a foregut which has neither a subepithelial gland cell zone nor a posterior caecum, no eyes, and a blood system in which the cephalic supply consists of several vessels. These differences, particularly in combination, clearly indicate that the present nemerteans do not belong in the genus Evelineus.

 

The genus Lineus, as noted earlier, still cannot be adequately defined. Two features of the Chilean nemerteans, however, have not been reported for any species of Lineus and justify their exclusion from this taxon; these are the presence of the foregut caecum (Friedrich, 1958, included the occurrence of an intestinal caecum as one of the reasons for establishing the genus Lineopsis; this generic name was subsequently changed to Lineopsella by Friedrich, 1970), and the manner whereby the rhynchocoel wall longitudinal muscle fibres are confined to the ventral and ventrolateral borders in the anterior body regions and are completely missing from the posterior portion.

 

The principal features of Pseudolineus, as defined by Friedrich (1960a: 58) are "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". The present nemerteans can thus be excluded from the genus Pseudolineus by the fibrous core of their dorsal cerebral lobes being forked only at the rear, by the absence of a well developed dermal connective tissue layer, and by differences in the organization of the blood system. Pseudolineus also has no foregut caecum.

 

Geographical distribution

 

Zodionemertes chilensis gen. et sp. nov. is at present known only from the type locality (Fig. 58).”

 

 

(Gibson, 1985; 212-222)

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Creative Commons Attribution Non Commercial 3.0 (CC BY-NC 3.0)

© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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