The nervous systems of Pilidiophora (Nemertea)

Beckers, Patrick

doi:10.1007/s00435-014-0246-3

The nervous systems of Pilidiophora (Nemertea)

Original Paper
Published: 31 October 2014

Volume 134, pages 1–24, (2015)
Cite this article

Download PDF

Access provided by Autonomous University of Puebla

Zoomorphology Aims and scope Submit manuscript

The nervous systems of Pilidiophora (Nemertea)

Download PDF

Patrick Beckers¹

476 Accesses
11 Citations
1 Altmetric
Explore all metrics

Abstract

Today molecular data recover three higher taxa in nemerteans, Palaeonemertea, Pilidiophora and Hoplonemertea. Hubrechtella dubia, a former palaeonemertean taxon was excluded from palaeonemerteans and is now supported as the sister group to heteronemerteans. This taxon is called Pilidiophora, because H. dubia and Heteronemertea share a pilidium larva. To find additional morphological evidence for Pilidiophora, the nervous system of 13 species of this taxon were investigated using different histological techniques. The central or medullated nervous system of nemerteans consists of a ring-shaped brain and lateral branching medullary cords. While the brain of basally branching nemerteans shows no or only slightly posterior enlargement, the brain of Pilidiophora expands caudally. Pilidiophora possess a conspicuous sensory organ, the cerebral organ. Although this organ is also found in hoplonemerteans and certain palaeonemertean species, only in Pilidiophora this organ is directly connected to the dorsal lobe of the brain. Additionally, this organ terminates in a layer of neurons close to the blood vessel in all pilidiophoran species. The analysis indicates that although in H. dubia some characters of the nervous system show the plesiomorphic state, the morphology of the cerebral organ provides an apomorphic character for a taxon Pilidiophora.

The nervous systems of Hoplonemertea (Nemertea)

Article 02 July 2018

Development and Structure of the Nervous System in Phoronids: Evolutionary Significance

Article 13 January 2022

The central nervous system of Oweniidae (Annelida) and its implications for the structure of the ancestral annelid brain

Article Open access 12 March 2019

Use our pre-submission checklist

Avoid common mistakes on your manuscript.

Introduction

In the last decade, the nervous systems of trochozoan taxa such as annelids and molluscs have intensely been studied using immunohistochemistry in order to reconstruct the phylogenetic relationships among these taxa (e.g., Heuer and Loesel 2008; Faller and Loesel 2008; Rothe and Schmidt-Rhaesa 2008; Wollesen et al. 2008; Heuer and Loesel 2009; Richter et al. 2010). For nemerteans, immunohistological data of the nervous system are still rare and restricted to a single heteronemertean and several palaeonemertean species Beckers et al. (2011, 2013). Palaeonemerteans are basally branching, while the remaining taxa, Hetero-, Hoplo- and Bdellonemertea show derived characters (Bürger 1895; Gibson 1972; Andrade et al. 2012, 2014; Kvist et al. 2014).

The central (or medullated) nervous system of nemerteans basically consists of a ring-shaped brain surrounding the proboscis and two laterally situated medullary cords that originate in the ventral part of the brain and are interconnected caudally (Bürger 1895; Gibson 1972). In few palaeonemertean species as well as in all heteronemerteans and some hoplonemerteans, the brain ring is posteriorly enlarged and forms a dorsal and a ventral pair of lobes (Bürger 1895; Gibson 1972; Beckers et al. 2011, 2013).

The central nervous system comprises a central neuropil, that is composed of the neurites of the neurons. The somata of the neurons surround the neuropil, which is regarded as typical for spiralians (Bullock and Horridge 1965; Schmidt-Rhaesa 2007). Several minor nerves, such as a dorsal nerve, proboscidial nerves, esophageal nerves and nerve plexus in different parts of the body, may be present (Bürger 1895; Gibson 1972; Turbeville 1991; Beckers et al. 2011, 2013; Beckers 2012 ). Nemerteans possess several sensory organs at the tip of their head. Photoreception is perceived by pigmented eyes of which up to hundred may be present in one animal (Gibson 1972). While frontal organs probably have a tactile function (Gibson 1972), cerebral organs most likely are chemoreceptors (Scharrer 1941; Ling 1969, 1970; Ferraris 1985; Amerongen and Chia 1982). The former comprise one dorsal and two lateral canals that are open to the exterior via small pits. The cerebral organs of heteronemertean species are attached to the dorsal lobe of the brain and gain contact to the environment by ducts, which are lined with sensory cells (Bürger 1895; Gibson 1972; Scharrer 1941; Beckers et al. 2011; Beckers 2012).

Some recent molecular studies do not support the monophyly of palaeonemerteans (Thollesson and Norenburg 2003; but see also Andrade et al. 2012). Most palaeonemertean groups actually form a basally branching grade, while the former palaeonemertean taxon Hubrechtidae is most closely related to Heteronemertea. Since species of both, Hubrechtidae and Heteronemertea, possess a pilidium larvae, Thollesson and Norenburg (2003) introduced the name Pilidiophora, indicating that a pilidium evolved in the common stem lineage of Heteronemertea and Hubrechtidae (Thollesson and Norenburg 2003; Andrade et al. 2012; Kvist et al. 2014). Molecular data, however, do not unequivocally provide support for Pilidiophora. But the taxon Pilidiophora was also supported in a recent morphological analysis by the arrangement of the musculature of the proboscis (Chernyshev et al. 2013). In order to find additional morphological support for this taxon, the neuroanatomy of twelve heteronemertean species and Hubrechtella dubia was compared to palaeonemertean nervous systems that were recently studied in more detail (Beckers et al. 2013). To facilitate comparisons with previous studies, the same set of markers, terms and methods used in Beckers et al. 2013 were also applied here. Additionally, transmission electron microscopy was used to further characterize neurons, neuropil and glia. Classical Azan staining of histological sections was used to generate 3D reconstructions of the nervous systems.

Materials and methods

Animals

Lineus acutifrons Southern, 1913 (Fig. 1a) was collected from a sandy flat at Pouldohan (Britany, France). Micrura purpurea Dalyell, 1853 (Fig. 1b), Riseriellus occultus Rogers, Junoy, Gibson and Thorpe, 1993 (Fig. 1c), Ramphogordius sanguineus Rathke, 1799 (Fig. 1d), Lineus longissimus Gunnerus, 1770 (Fig. 1e) and Ramphogordius lacteus Rathke, 1843 (Fig. 1f) were found on the rocky shore of Le Cabellou close to Concarneau (Britany, France). Lineus ruber Müller, 1774 (Fig. 1g) and Lineus bilineatus, (Renier 1804) (Fig. 1h) were collected in Roscoff (Britany, France). Lineus viridis (Müller 1774) was collected at the Ile de Groix, Britany, France. All animals were collected in March 2011. Notospermus geniculatus (Delle Chiaje 1828) (Fig. 1i) and Cerebratulus fuscus Renier, 1804 (Fig. 1j) were sampled on the isle of Giglio/Italy in May 2013 in Poseidonia sea grass meadows by diving. Hubrechtella dubia Bergendal, 1902, was collected in the Gullmarsfjord, Sweden, in 2009. The structure of nervous system in Apatronemertes albimaculosa Wilfert and Gibson 1974 was recorded from the type series. Baseodiscus hemprichii (Ehrenberg 1831) was collected in Van Phong Bay/Vietnam in May 2005.

Immunohistochemistry and histology

Animals for immunohistochemical investigations were relaxed in a 7 % MgCl₂ solution and fixed afterward in 4 % paraformaldehyde in seawater. Animals were embedded into gelatine–albumen and cut into sections of 60 µm thickness. Afterward, animals were stained with the antibodies FMRF-amide (ImmunoStar, Hudson, WI, USA), Serotonin (Sigma-Aldrich, Saint Louis, MO, USA), conjugated α-Tubulin (Sigma-Aldrich, Saint Louis, MO, USA), Synapsin (Hybridoma Bank, IA, USA) and for nucleus staining Sytox (Invitrogen, Carlsbad, CA, USA). For a detailed description, see Beckers et al. (2013).

For histological studies, the animals were relaxed in a 7 % MgCl₂ solution and fixed afterward in Bouin’s fluid (modified after Dubosq-Basil). Animals were dehydrated in an ascending ethanol series and embedded in Paraplast. The specimen were sectioned into slices of 5 µm thickness and stained with Azan. For a detailed description, see Beckers et al. (2013).

Ultrastructure

The head of Lineus ruber was fixed in a 2.5 % glutaraldehyde solution, buffered in 0.05 M phosphate 0.3 M saline (pH 7.2) at 4 °C for 2 h and kept in the same buffer. The specimen were postfixed in 1 % OsO₄ buffered in 0.05 M phosphate 0.3 M saline at 4 °C for 1 h, immediately afterward dehydrated in an ascending acetone series followed by propylenoxide and embedded in Araldit. Semithin section and ultrathin sections of 1 µm and 70 nm thickness, respectively, were cut on a LEICA UC6. Semithin sections were kept on glass slides and stained with Toluidine blue, while ultrathin sections were stained with uranyl acetate and lead citrate.

Analysis and 3D reconstruction

Azan-stained slices were analyzed with an Olympus microscope (BX-51). Every section was photographed with an Olympus camera (Olympus cc12) equipped with the dot slide system (2.2 Olympus, Hamburg) and aligned using imod (Kremer et al. 1996) and imod align (http://www.q-terra.de/biowelt/3drekon/guides/imod_first_aid.pdf). 3D reconstructions were performed with Fiji (1.45b) (Schindelin et al. 2012)/trakem (Cardona et al. 2012) and Amira (4.0) (see Fig. 3a). The schematic drawings were performed according to the 3D reconstructions with Illustrator and Photoshop (CS6).

Immunochemical investigations were performed with a Leica confocal laser scanning (Leica TCS SPE) microscope using the LAS AF 1.6.1 software. Stacks were loaded into Fiji (1.45b) (Schindelin et al. 2012) and further processed, using the “maximum projection” tool of the Image j software (Schneider et al. 2012). The images were finally processed with Fiji (1.45b) (Schindelin et al. 2012) and Photoshop (CS6). Ultrastructure sections were photographed with an Zeiss Lyra electron microscope using the analysis software and further processed using Fiji (1.45b) (Schindelin et al. 2012). Adobe Illustrator (CS6) was used to prepare all figures.

Statistical analysis

Neuron diameters were measured with Cell*D (3.2 Olympus) and statistically analyzed using the SPSS 15.0 statistical package. Data of each species and each class of neurons were tested for normal distribution with Shapiro–Wilk test. Since the data were normally distributed, parametric statistics (ANOVA) were used to test whether or not the size classes of each species differ significantly.

Data repository and voucher material

To allow data transparency, the aligned Azan-stained serial sections used for 3D modeling are freely available in MorphDBase: www.morphdbase.de (Grobe and Vogt 2008, 2014). Conspecific individuals of species studied (except A. albimaculosa, H. dubia and B. hemprichii) were collected at the same sampling site and deposited at the Institute of Evolutionary Biology and Zooecology of the University of Bonn as voucher material.

Hyperlinks

Lineus acutifrons: [https://www.morphdbase.de?P_Beckers_20140401-M-21.1]

Micrura purpurea: [https://www.morphdbase.de?P_Beckers_20140401-S-13.1]

Riseriellus occultus: [https://www.morphdbase.de?P_Beckers_20140401-S-7.1]

Ramphogordius sanguineus: [https://www.morphdbase.de?P_Beckers_20140401-S-8.1]

Lineus longissimus: [https://www.morphdbase.de?P_Beckers_20140401-S-6.1]

Ramphogordius lacteus: [https://www.morphdbase.de?P_Beckers_20140401-S-12.1]

Lineus ruber Müller: [https://www.morphdbase.de?P_Beckers_20140401-S-9.1]

Lineus bilineatus: [https://www.morphdbase.de?P_Beckers_20140401-S-4.1]

Notospermus geniculatus: [https://www.morphdbase.de?P_Beckers_20140401-S-14.1]

Cerebratulus fuscus: [https://www.morphdbase.de?P_Beckers_20140401-S-10.1]

Hubrechtella dubia: [https://www.morphdbase.de?P_Beckers_20140401-S-5.1]

Apatronemertes albimaculosa: [https://www.morphdbase.de?P_Beckers_20140401-S-11]

Baseodiscus hemprichii: [www.morphdbase.de/?P_Beckers_20140923-M-49.1]

Results

General remarks

The description of the morphological elements of the nemertean nervous system follows the definitions given by Beckers et al. (2013). All aligned slices of investigated specimen are freely available on Morph D Base (https://www.morphdbase.de). The respective hyperlinks are mentioned above. An overview of the elements present in the nervous system of Pilidiophora is given in Appendix Table 1.

The entire medullated nervous system may be surrounded by a prominent extracellular matrix (ecm) (outer neurilemma) (Fig. 2c). If the medullated nervous system or parts of it are located directly underneath the basal lamina of the epidermis, the outer neurilemma can basically not be discriminated from the latter. In these cases, the term outer neurilemma will only be used, if an ecm separates the outer face of the nervous system from the adjacent musculature, mesodermal tissue or blood vessel linings, in a way that the entire nervous system is surrounded by matrix. The brain of nemerteans consists of a central neuropil and peripheral layer of perikarya, called outer somata layer or outer layer of neuronal somata here. Both, central neuropil and peripheral layer of perikarya may be separated by an ecm, traditionally termed inner neurilemma (Figs. 2c, 5a, 6a, d).

Architecture of the nervous system

Heteronemertea

The central (medullated) nervous system (cns) (brain and lateral medullary cords) is located inside the musculature (Figs. 2, 3b–e, 4). The central neuropil is partly or completely surrounded by a layer of perikarya (Figs. 3c, 5a, 6, 7f). All lineid species except Apatronemerts albimaculosa possess long lateral cephalic slits extending from the tip of the animal to the brain region. Posteriorly, these slits terminate in a duct that passes the subepidermal muscle layer and enters the posterior section of the brain. This duct is lined by ciliated sensory cells and known as cerebral organ (Gibson 1972). In A. albimaculosa, the cephalic slits are extremely short and known as cephalic pits (Gibson 1972).

Brain

On each body side, the anterior part of the brain consists of two laterally bulging neuropils that are commissurized by a dorsal and a more prominent ventral commissural tract (Figs. 2, 3a, c, 5a, 7a). The commissures are on the same transverse level, so that the brain is ring-shaped in all species studied (Figs. 2a, c, d, 5a), except for Lineus acutifrons where the dorsal commissure is located posterior to the ventral commissure (Fig. 2b) and Apatronemertes albimaculosa where the dorsal commissure is anterior to the ventral one. Posterior to the dorsal and ventral commissures, the brain is separated into a left and right section. Here, the neuropil shows infoldings and furrows as well as lateral tracts that connect the dorsal and ventral portions of the neuropil. On either side of the brain, the dorsal portion is located more laterally and partly overlaps the ventral portion (Figs. 3c, 5b, 7f). Further posteriorly, the neuropils of ventral and dorsal portions of the brain are clearly separated by an own inner neurilemma but are initially still surrounded by a common outer neurilemma (Fig. 4d). Here, the duct of the cerebral organ originates in the cephalic slits (Fig. 4d). Where it turns medially to pass the muscle layers, the neuropil and neurons of both brain parts are also separated by an outer neurilemma to form the paired dorsal and ventral lobes of the brain. In Lineus bilineatus, neurites of the dorsal lobe branch off from the neuropil, pass the inner neurilemma and prominently extend into the dorsal section of the outer layer of perikarya (Fig. 5b). In all lineid species studied, the ventral lobes taper caudally and are confluent with the lateral medullary cords (Figs. 2, 3a, 4f), while the dorsal lobes bifurcate to form two branches, of which the superior one is smaller than the inferior one (Figs. 2, 3a, d). Initially, both dorsal lobes share a common outer neurilemma (Fig. 3d). Further posteriorly, the two branches are divided by their own outer neurilemma. The cerebral organ is now located between the dorsal and the ventral lobe of the brain next to the inferior neuropil of the dorsal lobe. An inner neurilemma initially separates both, cerebral duct and inferior neuropil, but disappears some micrometers posteriorly (Figs. 2, 8a). Here, neurons of the inferior neuropil are apposed to those of the cerebral organ. On the level of the posterior most margin of the superior branch, the neurons of cerebral organ fuse with the neuropil of the inferior branch (Fig. 8a). This fusion also marks the posterior margin of the inferior branch neuropil. The cerebral organ turns slightly laterally and a large group of presumably neurosecretory cells is now visible in the ventromedial section of the inferior branch (Figs. 3e, 8a, b). The outer neurilemma is now surrounded by muscles and the lining of the cerebral blood vessels. The cerebral organ ends within a mass of putative neurosecretory cells that mark the posterior margin of the inferior branch of the dorsal lobe. Lining cells of the blood vessels rest on the outer neurilemma.

In summary, the brain of lineids is anteriorly ring-shaped, splits into a left and a right section, differentiates a dorsal and a ventral lobe and finally, at its posterior margin, consists of six portions, isolated from each other by an outer neurilemma (Figs. 2, 3a, 8a). These six are a pair of ventral lobes and paired inferior and superior branches of the dorsal lobe. The ventral lobe is continuous with the medullary cords; the inferior branch of the dorsal lobe merges with the terminal section of the cerebral organ. The size of the brain as well as the volume of its lobes differ between the specimen studied, but their general anatomy is largely uniform (Figs. 2, 3a).

The brain of Apatronemertes albimaculosa differs slightly from the remaining heteronemerteans. Each dorsal lobe gives rise to a neuropil branch that extends anteriorly and blindly ends within the neuronal layer of the anterior section of the brain. In addition, the dorsal lobe neuropil is crescent-shaped prior to its separation into the inferior and superior branch neuropil (Fig. 4a). In contrast to the remaining lineids, the entire dorsal lobe extends into the cephalic blood vessels.

In Baseodiscus hemprichii, the dorsal commissure of the brain is only weakly developed and is located anterior to the ventral one. The neuropil of the dorsal lobe is only slightly bifurcated and the superior dorsal lobe terminates soon. The neuropil of the inferior branch, however, is again bifurcated into a dorsal and ventral part; the ventral part is associated with the cerebral organ (Fig. 8e).

Medullary cords

The paired ventrolateral medullary cords run posteriorly between the longitudinal muscle layer and the inner circular muscle layer. The neuropil of the medullary cords is capped by a c-shaped layer of nuclei of type 1 neurons, so that the nuclei are located dorsally, laterally and ventrally. An inner neurilemma separates the layer of nuclei and the neuropil (Fig. 4f); the medullary cords are surrounded by an outer neurilemma.

Minor nerves and nerve plexus

Numerous cephalic nerves originate from the lateral parts of the anterior face of the brain, taper toward the anterior end and are apposed to the head lacuna (Figs. 2, 3a, b, 7a). A dorsal nerve arises from the mid-posterior face of the dorsal commissural tract (Figs. 2, 3a, 4e, f). Posteriorly, the nerve ascends dorsally to run posteriorly outside the inner circular muscle layer. In Lineus longissimus, Lineus acutifrons and Riseriellus occultus, an additional dorso-median cephalic nerve originates on the same level from the anterior margin of the dorsal commissure where the dorsal nerve originates from its posterior margin. Whether or not the anterior nerve is an anterior branch of the dorsal nerve could not be seen in terms of the course of its neurites (Figs. 2a, b, d, 3a). In these three species, the dorsal nerve is connected to the medullary cords by a prominent commissure that overarches the dorsal part of the rhynchocoel (Figs. 2a, b, d, 4e, f). The paired esophageal nerves originate from the ventral lobes, posterior to their separation from the dorsal lobes (Figs. 2, 3a, d, 4f). These nerves run in parallel toward the anterior margin of the mouth where they are connected by a small commissure. The nerves encircle the mouth (pharynx) and continue posteriorly ventrolaterally on either side of the esophagus. Along their entire length alongside the foregut, the esophageal nerves give rise to posteriorly directed branches that surround the gut. In R. occultus, R. sanguineus and R. lacteus, where the brain is located far anterior to the mouth opening, the esophageal nerves are elongated accordingly. In these three species, both esophageal nerves are connected by several commissures prior to surrounding the mouth (pharynx) (Fig. 2a, b, d). In L. ruber and L. longissimus, the esophageal nerves are also connected to the lateral medullary cords in front of the mouth opening (Fig. 2c, d). A ventral nerve is not present. The paired proboscidial nerves arise from the dorso-median portion of the ventral commissural tract (Figs. 2, 3b). The nerves are interconnected by a proboscidial plexus.

Several minor nerves originate in the ventral and dorsal lobes of the brain that run toward epidermis in all investigated heteronemerteans (Fig. 3d). These nerves pass the longitudinal muscle layer and innervate the dorsal or the ventral parts of the animal. Nerves running toward the dorsal side of the animal may originate in the ventral part of the brain and vice versa. While some of these nerves directly run to the respective face of the animal, some of them are branches of a single nerve that originates from one of the brain lobes (Fig. 3d).

Several nerve plexus are revealed by immunohistochemistry in Lineus ruber and Lineus bilineatus (Fig. 7b). Staining with the antibodies of FMRF, serotonin and tubulin reveals that the neurites of the intraepidermal nerve plexus are arranged in a regular, ladder-like manner. The neurites of the subepidermal, commissural plexus and the suprastomatogastric nerve plexus are horizontally arranged. The neurites of the intrastomatogastric nerve plexus are arranged in an irregular, net-like manner. The suprastomatogastric and the intrastomatogastric as well as the subepidermal nerve plexus are interconnected (Fig. 7b). There is a spherical concentration of neurites in the subepidermal plexus at the points where the connections to the suprastomatogastric plexus branch off (Fig. 7b).

Sensory structures

Frontal organ

The frontal organ of investigated lineid heteronemerteans is located at the very anterior tip of the animals. It consists of one dorsal and two lateral pits that are open to the exterior in front of the proboscis pore. Each pit tapers into a small, densely ciliated canal which runs underneath the epidermal basal lamina and terminates in a layer of neurons in front of the head lacuna (Fig. 8c). Neither the canals nor their lining cells are in contact; a connection of the lining cells to the brain was not found.

The frontal organ of Baseodiscus hemprichii is composed of one ventrally located pit at the tip of the head. This pit is located anterior to the proboscis pore and leads to a ciliated duct which terminates in a ciliated cavity. Posterior to the proboscis pore there are paired additional pits which lead into short ciliated canals.

Cerebral organ

The cerebral organ is an epidermal duct that originates at the posterior margin of the cephalic slits or the cephalic pits. This part of the cerebral organ will be called distal section of the cerebral organ in the following description. The duct pierces the basal lamina of the epidermis and runs posteriorly perpendicular to the longitudinal body axis toward the posterior part of the dorsal lobe of the brain (Figs. 2, 3a, 8a–d). This section of the cerebral organ will be termed proximal duct in the following description. The extension of the cephalic slits is species- or genus-specific, but most impressive in Notospermus geniculatus, where it causes a rectangular outline of transverse sections of the tip of the head (Fig. 4d). A distal, epidermal section of the duct can morphologically be discriminated from a subepidermal proximal section. A small pore connects the duct to the exterior. Underneath this pore, the distal duct widens to form a larger compartment with an intraepidermal dorsal and a ventral pouch (Figs. 3d, 8a, d). In A. albimaculosa, the duct directly leads to the cells of the cerebral organ; there is neither a compartment nor are there pouches.

In Lineus ruber, the distal section is densely lined with ciliated cells, most of them are sensory cells, since there is a direct connection to the inferior dorsal lobe, as revealed by immunostaining against alpha tubulin (Fig. 7c, d). The cilia of the part of the duct are longer than the cilia of the epidermis and can easily be discriminated from the latter (Figs. 4c, 8a). Toward the inner of the animal, the compartment tapers and gives rise to the proximal duct, which pierces the epidermal basal lamina and runs toward the brain. Within the inferior branch of the dorsal brain lobe, it runs posteriorly for several micrometers and is convoluted or laterally bending, depending on the species. Only in Lineus acutifrons this part of the proximal duct is straight (Fig. 2b).

The proximal duct terminates in a layer of neurons and voluminous glandular cells within the inferior branch of the dorsal lobe (Figs. 4d, 8a, 7c, d). The neurons and the glandular cells are in close contact to the blood vessels only separated from the latter by an ecm (Figs. 4e, 8b).

The neurons of the cerebral organ are of type 1 (S1) described for the brain (Fig. 8a, b) and show no immunoreactivity against FMRF in Lineus ruber (Fig. 7c, d). The cerebral organ of N. geniculatus is located ventrolateral of the dorsal lobe of the brain. It terminates close to the nephridial system.

The architecture of the cerebral organ of Baseodiscus hemprichii differs from that of the lineids. The proximal duct originates ventrolaterally from a pouch formed by a ventral epidermal fold (Fig. 8d). The course of the duct is straight dorsally (Fig. 8e), and first at its posterior end, it turns perpendicular toward the inside of the animal and terminates in a layer of neurons and glandular cells, close to the blood vessel. The glandular cell layer is not as developed as in lineids (Fig. 8e). The superior dorsal lobe is very short and does not rest on the cells of the cerebral organ (Fig. 8e). The neuropil of the inferior dorsal lobe is again bifurcated, and the ventral part is connected to the cells of the cerebral organ (Fig. 8e).

Eyes

Numerous eyes are present in all investigated heteronemerteans except L. acutifrons. The pigment cup eyes are arranged on the margins of the head (Figs. 1f, 2). Prominent nervous connections between the eyes and the brain (optical nerves), as found in Hoplonemertea (pers. observation), were not detected.

Hubrechtella dubia (Hubrechtidae)

The central (medullated) nervous system (cns) of H. dubia is located directly below the epidermal basal lamina (subepidermally) (Fig. 9). An outer neurilemma as defined above is not visible (Fig. 9b).