&p.1:Abstract The pharynx of Gnathostomula paradoxaconsists of the partly syncytial pharyngeal musculature, apharyngeal epithelium, myoepitheliocytes, receptors,nerves, and three solid parts, called the jugum, the basalplate, and the jaw. Extended non-contractile regions ofboth pharyngeal and body wall musculature form the so-called parenchymatous tissue between the digestive tractand the body wall. The pharyngeal epithelium mediatesthe force from the pharyngeal musculature to the solidparts. The basal plate and jaw contain longitudinal cutic-ular rods which are elastic antagonists of the muscula-ture. There is no buccal ganglion in G. paradoxa. Thestudy supports the monophyly of the Gnathostomulidaand Gnathifera.&bdy:

A. Introduction

Gnathostomulida have been known since the investiga-tion of the marine interstitium by Remane in the 1920s(see Ax 1956). Gnathostomula paradoxaAx, 1956 andGnathostomaria lutheriAx, 1956 were the first two spe-cies to be described. Today round about 100 species areknown (Sterrer 1996). While the monophyly of Gnatho-stomulida was never disputed, their sister group re-mained questionable for a long time. Ax (1984, 1995) re-garded the Plathelminthes as the sister group and hegrouped both taxa togther as the Plathelminthomorpha.However, Reisinger (1961) discussed the Rotifera as apotential sister group. This hypothesis was supported byultrastructural data on the mastax of the Rotifera(Koehler and Hayes 1969a, b) and by unpublished find-ings of Lammert on the jaw of the Gnasthostomulida(see Rieger and Tyler 1995). One aim of the presentstudy is to describe the pharyngeal ultrastructure of G.

paradoxa. Gnathostomulida and Syndermata (=Roti-fera+Seison+Acanthocephala) were grouped together asGnathifera Ahlrichs, 1995 (Ahlrichs 1995; see Riegerand Tyler 1995). Recently Ahlrichs (1997) also includeda new unnamed taxon (“new group A”) in the Gnathif-era. Ahlrichs (1995) and Ehlers et al. (1996) discussedthe phylogenetic position of the Gnathifera within theBilateria. To test the validity of the Gnathifera is anotheraim of our study.

B. Materials and methods

Specimens of G. paradoxawere collected in April 1995 in thetidal flats of the so-called Königshafen off List/Sylt (Germany).Sediment from the burrows of Arenicola marina(Linné, 1758)was placed in Kautex bottles and taken to the laboratory of theBiologische Anstalt Helgoland. In the laboratory, the meiofaunawas extracted by the seawater-ice method (Pfannkuche and Thiel1988). The concentrated samples were sorted under 50× magnifi-cation.

The specimens so stunned were prefixed with ruthenium red-tinted 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer (pH7.2) for 1 h at 4° C. Rinsing in 0.1 M sodium cadocylate bufferwas followed by postfixation in 1% osmium tetroxide, in the samebuffer, for 1 h at 4° C. The specimens were dehydrated in a gradedacetone series and embedded in Araldite (intermedium: polypro-pylene). Ultrathin silver-interfering sections (ca. 70 nm) were cut(Reichert Ultracut-II, Diatome diamond knife). The sections wereput on Formvar-coated, one-slit grids. After automatic doublestaining with uranyl acetate and lead citrate (two times, LKB Ul-trastainer) the sections were viewed (Zeiss EM 900, at 50 kV).Section series from two cross-sectioned specimens and from onelongitudinally sectioned specimen were investigated. Results (mi-crophotographs and drawings) were taken from one cross-sec-tioned specimen.

C. Results

The digestive tract of G. paradoxacomprises the ventralmouth opening, the pharynx, and the blind intestine. Thepharynx consists of musculature, an epithelium, myo-epitheliocytes, receptors, nerves, and solid parts (the ju-gum, basal plate and jaw).

This paper is dedicated to Professor Dr. Peter Ax, Göttingen,on the occasion of his 70th birthday

H. Herlyn (✉) · U. EhlersII. Zoologisches Institut und Museum der Universität Göttingen,Berliner Strasse 28, D-37073 Göttingen, GermanyFax: 0049-511-5448; e-mail: hherlyn@gwdg.de&/fn-block:

Zoomorphology (1997) 117:135–145 © Springer-Verlag 1997

O R I G I N A L A RT I C L E

&roles:Holger Herlyn · Ulrich Ehlers

Ultrastructure and function of the pharynxof Gnathostomula paradoxa (Gnathostomulida)

&misc:Accepted: 4 April 1997

I. Pharyngeal musculature

Two unpaired and unbranched myocytes, 2 unpairedand branched myocytes, 8 paired and branched myo-cytes, and two paired and highly branched myosyncytiaform the pharyngeal musculature proximal to the basallamina. These 12 myocytes and two myosyncytia con-sist of non-contractile and contractile regions. The ex-tended non-contractile regions contain plenty of sarco-plasm, the nucleus, a diplosome, rough endoplasmic re-ticulum, free ribosomes, the Golgi apparatus, and mito-chondria (Figs. 2, 5, 6: bl, cr, ncr, rER). The contractileregions show cross-striated myofibrils pulling in differ-ent directions and little sarcoplasm. The cross-striationof the myofibrils is caused by bands of isolated z-dotslocated between the sarcomeres. The non-contractile re-gions of myocytes and myosyncytia are not entirelysurrounded by an extracellular matrix (Figs. 2–6: ecm,zd). The contractile regions from the muscle capsule(Figs. 2–6: mca), the paired lateral muscle pouches(Fig. 5: lp), and the unpaired caudal muscle pouch (Fig.6 cp). The muscle capsule is 58 µm long, up to 34 µmwide and up to 30 µm high. The non-contractile regionsform two dorsolateral tissue lobes and one ventral tis-sue lobe which exceed the muscle rostrally and caudal-ly respectively. The tissue lobes are not distinguishablefrom the surrounding non-contractile tissue at both thelight and electron microscopic levels (Figs. 2, 4, 5: dtl,vtl).

II. Pharyngeal epithelium

The unilayered cellular epithelium lies distally to thebasal lamina. The apical cytoplasm has free ribosomes, adiplosome, and different vesicles. The basal cytoplasmshows free ribosomes, the nucleus, much rough endo-

plasmic reticulum, the Golgi apparatus, and mitochon-dria. The epithelium has no ciliation and is differentiatedinto two types: a secretion epithelium with many vesiclesand long microvilli covered by a glycocalyx (Figs. 2–6,bl, gm, rER, ve), and a force-mediating epithelium char-acterized by tonofilaments running from desmosomesand hemidesmosomes to the solid parts (Figs. 2–5, 7A:ds, tf, hd).

III. Pharyngeal myoepitheliocytes

One pair of myoepitheliocytes is embedded in the phar-nygeal epithelium. These cells (length about 22 µm) runfrom the transition between the mouth cavity and esoph-agus to the caudal end of the caudal muscle pouch. Theapical cell membrane shows microvilli covered by a gly-cocalyx. A diplosome and some vesicles exist in the api-cal cytoplasm. The basal cytoplasm includes the nucleusand mitochondria. Contractile filaments run from theapical to the basal cell membrane. Bands of isolated z-dots located between the sarcomeres cause a cross-stria-tion of the myofibris. Anteriorly, both myoepitheliocytesappear in cross-sections as an erected U (Figs. 3–5: cp, e,gm, me, mc, pe, ve, zd).

IV. Pharyngeal solid parts

G. paradoxa has three separate pharyngeal solid parts,the jugum, the basal plate, and the jaw. The solid partsconsist of different types, turning at the margins into aglycocalyx. In the following, the jugum, situated at theanterior rim of the mouth opening, will not be consid-ered. The basal plate and the jaw share vertical rows oflongitudinal cuticular rods (diameter up to 0.4 µm) intheir construction. The cuticular rods consist of an elec-

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Fig. 1A, B Position of thecross-sections depicted in Figs.2–7. A Schematic reconstruc-tion of the basal plate of G.paradoxaon the basis of serialsections, dorsal view. B Sche-matic reconstruction of the jawof G. paradoxaon the basis ofserial sections, dorsal view. bpdbasal plate disc, ca cauda, dldorsal lamella, jh jaw half, rtrostral tooth row, rw rostralwing, sysymphysis, tr too-throws, vl ventral lamella&/fig.c:

tron-dense core containing cuticular material and a gly-cocalyx surrounded by an electron-lucent halo also ofcuticular material. The halo is interrupted by pores andslits. The cuticular material and the glycocalyx of thecore touch the cuticular material and glycocalyx outsidethe cuticular rods along these pores and slits (Figs. 3, 4,7A, B: cd, dc, gc, lh).

The basal plate is divided anteriorposteriorly into thefollowing components (lengths in parentheses) (Fig.1A): paired rostral wings (ca. 9 µm), a rostral tooth rowlocated between the rostral wings (ca. 7 µm) and a bas-al plate disc (ca. 12 µm). In total the basal plate mea-

sures ca. 20 µm in length, maximally ca. 24 µm inwidth measured along the rostral wings, and maximallyca. 4 µm in height measured along the basal plate disc.Anteriorly the basal plate separates the mouth cavityfrom the mouth opening (Fig. 2). Posteriorly it lies onthe ventral pharyngeal epithelium. Altogether, 14 cutic-ular rods are arranged in paired vertical arcs at the me-dian margins of the rostral wings (Fig. 7B: bpd, cd, mc,mo, pe, rt, rw).

The longitudinal jaw has a horizontal resting positionwithin the mouth cavity. It consists, from anterior to pos-terior, of the following parts (lengths in parentheses)

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Fig. 2 Cross-section of thepharnynx of G. paradoxa.Non-contractile regions (ncr) of thepharyngeal musculature formthe dorsolateral tissue lobes(dtl). The pharyngeal epitheli-um (pe) includes much roughendoplasmic reticulum (rER)and many vesicles (ve). Thebasal plate, consisting of rostralwings (rw) and rostral toothrow (rt), separates the mouthcavity (mc) from the ventralmouth opening (mo). Both jawhalves (jh) show two toothrows (tr). bl basal lamina, crcontractile regions, dsdesmo-somes, ecmextracellular ma-trix, gmglycocalyx-coveredmicrovilli, hshemidesmo-somes, mcamuscle capsule, mfmyofibrils, zdz-dots&/fig.c:

(Fig. 1B): paired jaw halves (ca. 24 µm) armed withthree horizontal tooth rows (ca. 9 µm), an unpaired sym-physis (ca. 9 µm) resulting from the joining of the innerwalls of the jaw halves, paired dorsal (ca. 1 µm) andpaired ventral lamellae (ca. 8 µm) resulting from the dor-sal and ventral parts of the jaw halves, and an unpairedcauda (ca. 11 µm) following the symphysis. The sym-physis and cauda overlap for ca. 2 µm. Altogether thejaw measures ca. 34 µm in length, maximally ca. 8 µm inwidth measured along the cauda, and maximally ca. 10µm in height measured along the jaw halves (Figs. 2–5:ca, dl, iw, jh, mc, sy, tr, vl). In all, the jaw contains 81cuticular rods arranged in five vertical rows. The paired

138

Fig. 3 Cross-section of the pharynx of G. paradoxa.Beginning ofthe esophagus (e) and of myoepitheliocytes (me). Division of thejaw halves into inner walls (iw), dorsal lamellae (dl), and ventrallamellae (vl). Note the distinct tonofilaments (tf) running fromdesmosomes (ds) to hemidesmosomes (hs). bl basal lamina, crcontractile regions, ecm extracellular matrix, gc glycocalyx, gmglycocalyx-covered microvilli, ir inner row, mcmouth cavity, mcamuscle capsule, mf myofibrils, pe pharyngeal epithelium, rERrough endoplasmic reticulum, vevesicles, zdz-dots&/fig.c:

inner rows (ca. 18 µm) start the tooth rows of the jawhalves and extend into the symphysis. They consist of 60cuticular rods and reach maximally ca. 5 µm in height.The outer rows (ca. 2 µm) are also paired, contain 18 cu-ticular rods, and reach maximally ca. 2 µm in height.They run through the symphysis. The unpaired medianrow (ca. 5 µm) with a maximal height of ca. 1 µm alsoruns through the symphysis. It consists of 3 cuticularrods. The longest cuticular rods are located halfway be-tween the dorsal and ventral edge of every row. Dorsallyand ventrally to these the cuticular rods get shorter.

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Fig. 4 Cross-section of the pharynx of G. paradoxa. Myofibrils(mf) of the pharyngeal musculature insert broughtly in the sym-physis (sy). Both myoepitheliocytes (me) appear as an erected U.The paired inner rows (ir ), the paired outer rows (or), and the un-paired median row (mr) are visible within the symphysis. bl basallamina, ca cauda, cr contractile regions, ds desmosomes, e esoph-agus, ecmextracellular matrix, gm glycocalyx-covered microvilli,hs hemidesmosomes, mcamuscle capsule, ncr non-contractile re-gions, pe pharyngeal epithelium, rER rough endoplasmic reticu-lum, tf tonofilaments, vevesicles, vl ventral lamella, vtl ventral tis-sue lobe, zdz-dots&/fig.c:

Thus, the maximal number of cuticular rods can befound halfway between the anterior and posterior tips ofeach row and the number of cuticular rods decreases to-ward the anterior and posterior tips (Figs. 2–5, 7A: cd,ir , mr, or).

D. Discussion

I. Assessment of the features

The following discussion is based on the phylogeneticsystem of the Bilateria as established by Ahlrichs (1995)and Ehlers et al. (1996). Within the Gnathostomulida thediscussion partly follows Sterrer′s (1972) recommenda-tion. As all species of the Acanthocephala lack a diges-tive tract they remain unconsidered here.

1. Pharyngeal musculature

The location of the musculature (proximally to the basallamina) suggests its mesodermal origin (Lammert 1991).

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Fig. 5 Cross-section of the pharynx of G. paradoxa.Extendednon-contractile regions (ncr) of the pharyngeal musculature arevisible within the contractile regions (cr). The lateral musclepouches (lp) are filled with large mitochondria. The myo-epitheliocytes (me) are rounded off. The cauda (ca) consists ofthree lobes. bl basal lamina, e esophagus, ecmextracellular matrix,gmglycocalyx-covered microvilli, mcamuscle capsule, mf myofi-brils, pe pharyngeal epithelium, rER rough endoplasmic reticu-lum, vevesicles, vtl ventral tissue lobe, zdz-dots&/fig.c:

Ax (1956), however, discusses mesodermal as well as ec-todermal components for the pharyngeal musculature.Since data derived from ontogenesis are not available(Müller and Ax 1971), the origin of the musculature re-mains obscure. Isolated z-dots and cross-striated muscu-

lature represent autapomorphies of the Gnathifera (seeAhlrichs 1995) as indicated in the diagram of relation-ships (Fig. 8: 1). Within the Gnathostomulida, myo-syncytia are described only for G. paradoxaand Rast-rognathia macrostoma(Kristensen and Nørrevang, 1977,both species of the Bursovaginoidea (see Kristensen andNørrevang 1977). Species of the Filospermoidea, Roti-fera, and Seison lack myosyncytia (Ahlrichs 1995;Koehler and Hayes (1969b). Thus myosyncytia are auta-pomorphic for the Bursovaginoidea (Fig. 8: 4), whereasthe exclusive existence of myocytes in the other taxa rep-resents the plesiomorphic alternative, inherited from thestem species of the Acrosomata (see Ax 1989, 1995).

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Fig. 6 Cross-section of the pharynx of G. paradoxa.Non-contrac-tile regions (ncr) separate the muscle capsule (mca) from the cau-dal muscle pouch (cp) and form the extended ventral tissue lobe(vtl). Basal lamina (bl) surrounds the whole pharyngeal epithelium(pe) and its derivates (nerves and receptors). There is no distinctbuccal ganglion separated by a basal lamina. The first gastrocytes(ga) indicate the beginning of the blind intestine. cr contractile re-gions, ecmextracellular matrix&/fig.c:

Extended non-contractile regions are found outside G.paradoxawithin species of the Filospermoidea, Rotifera,and Seison(see Ahlrichs 1995). Outside the Grathiferasuch large non-contractile regions are uncommon. Theexistence of extended non-contractile regions in the Gna-thifera ground pattern might represent an autapomorphy

of this taxon (Fig. 8: 1). A parenchyme or mesenchymelocated between the digestive tract and body wall is re-ported by several authors (e.g. Balsamo 1992). But thepresent study indicates that species of the Gnathostomu-lida do not have a distinct parenchyme. Instead, the tis-sue between the digestive tract and body wall consists

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Fig. 7A, B Cross-section ofthe pharynx of G. paradoxa. AThe inner rows (ir ) are visiblealong the inner walls (iw) ofthe jaw halves. Note that everycuticular rod (cd) consists of anelectron-dense core (dc) and anelectron-lucent halo (lh). Tono-filaments (tf) run from hemi-desmosomes (hs) to desmoso-mes (not visible here). gc gly-cocalyx, gmglycocalyx-cov-ered microvilli, mcmouth cavi-ty. B Both rostral wings (rw) ofthe basal plate form scatteredarcs of altogether 14 cuticularrods (cd) at their median mar-gins&/fig.c:

mostly of the non-contractile regions of the pharyngealmusculature and of the body wall musculature.

Besides G. paradoxa, a muscle capsule and a caudalmuscle pouch are described for many other Gnathostomu-lida (Ax 1956, 1964; Farris 1973; Kristensen and Nørrev-ang 1977, 1978; Sterrer and Farris 1975). A muscle cap-sule and a caudal muscle pouch do not exist outside theGnathostomulida. The muscle capsule and caudal musclepouch thus constitute autapomorphies in the ground patternof the Gnathostomulida (Fig. 8: 2). Paired lateral musclepouches are realized only within a nameless subtaxon ofthe Bursovaginoidea including Rastrognathia macrostoma,the Onychognathiidae, the Gnathostomulidae, and the Au-strognathiidae (Ax 1956; Ehlers and Ehlers 1973; Farris1973; Kristensen and Nørrevang 1977, 1978; Sterrer andFarris 1975). Paired lateral muscle pouches are absent inGnathostomaria lutheri(see Ax 1956) and the Filospermo-idea, as well as in the other taxa of the Gnathifera. Lateralmuscle pouches represent an autapomorphy in the groundpattern of this unnamed taxon.

2. Pharyngeal epithelium

Species of the Gnathostomulida (Kristensen and Nørrev-ang 1977; Lammert 1991; Sterrer 1971), the Plathelmin-thes (Ahlrichs 1995; Ax 1984; Doe 1981; Ehlers 1985,1986, 1995, Tyler 1984), and the Euspiralia (Ax 1995)have a pharyngeal epithelium with microvilli covered bya glycocalyx. However, species of the Rotifera (Ahlrichs1995; Koehler and Hayes 1969b) and Seison(see Ahl-richs 1995) show a pharyngeal cuticle. The existence of apharyngeal cuticle is an autapomorphy of the Syndermata(Fig. 8: 3). An epithelium without any ciliation is a fea-ture shared by species of the Gnathostomulida (Ahlrichs1995; Ax 1956; Lammert 1991; Sterrer et al. 1985), Roti-

fera (Ahlrichs 1995; Lorenzen 1996), and Seison(seeAhlrichs 1995). Species of the Plathelminthes (Ax 1984;Doe 1981; Ehlers 1985, 1986, 1995) and Euspiralia (Ax1995; Purschke and Tzetlin 1996) show the plesiomor-phic alternative with a ciliated pharyngeal epithelium.The absence of pharyngeal ciliation is an autapomorphyin the ground pattern of the Gnathifera (Fig. 8: 1).

Pharyngeal myoepitheliocytia

Paired pharyngeal myoepitheliocytia were also found inPterognathia meixneriSterrer 1969 and HaplognathiaroseaSterrer, 1968 (Filospermoidea) (Lammert, unpub-lished observation). Interestingly, the myoepitheliocytiaof these two species show an inverse orientation. Anteri-orly, both myoepitheliocytia appear in cross-section notas an erected U, but as a tipped over one. Nevertheless,the myoepitheliocytia of the Filsospermoidea and of G.paradoxa can be homologized. Outside the Gnatho-stomulida, pharyngeal myoepitheliocytia are unknownfor species of the Rotifer, Seison, Plathelminthes, andEuspiralia. Within the Nemathelminthes, pharyngealmyoepitheliocytia are known for the Gastrotricha and theNematoda. But the pharynges of Gastrotricha and Nema-toda species display more than two myoepitheliocytia. Ahomologization of the myoepitheliocytia of the Gnatho-stomulida with those of the Gastrotricha and the Nema-toda is not possible. Thus paired epithelially embeddedpharyngeal myoepitheliocytia can be judged as an aut-apomorphy of the Gnathostomulida (Fig. 8: 2).

4. Pharyngeal solid parts

A pharyngeal jaw in a horizontal resting position andwith cuticular rods belongs to the ground pattern of the

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Fig. 8 Diagram of the relation-ships within the Gnathifera (seeAhlrichs 1997). The blacksquares1–4 mark autapomorp-hies&/fig.c:

Gnathostomulida (see also Ahlrichs 1995; Ax 1995;Sterrer 1996). The jaw halves and symphysis of thegnathostomulid jaw can be homologized with the manu-bria and fulcrum of the Rotifera and Seisonchewer (seealso Ahlrichs 1995, Koehler and Hayes 1969a, b). As hy-pothesized by Ahlrichs (1995), a jaw or chewer, respec-tively, represents an autapomorphy of the Gnathifera(Fig. 8: 1). However, a basal plate is an autapomorphy ofthe Gnathostomulida (see Ax 1995) (Fig. 8: 2).

5. Buccal ganglion and buccal nerves

A buccal ganglion and paired buccal nerves were de-scribed for Rastrognathia macrostomaby Kristensen andNørrevang (1977). This study shows the absence of abuccal ganglion and of buccal nerves in G. paradoxa(Fig. 6). A reinterpretation of the former study leads tothe following conclusion. An accumulation of epithelialnuclei as well as pharyngeal nerves were misinterpretedas a buccal ganglion and buccal nerves, respectively. Nobasal lamina restricted to this accumulation exists. Thus,the Gnathostomulida lack any buccal ganglion and buc-cal nerves.

6. Conclusions

The assessment of the features discussed in this papersupports the monophyly of the Gnathifera, as well as ofthe Gnathostomulida, Syndermata, and Bursovaginoidea(Fig. 8).

II. Functional aspects

The pharyngeal solid parts of G. paradoxaare moved bycontraction of the pharyngeal musculature. The force re-sulting from the muscle contraction is mediated to thesolid parts by the force-mediating epithelium character-ized by desmosomes, tonofilaments, and hemidesmo-somes. The muscle contraction causes a deformation aswell as a delocation of the solid parts. After relaxation ofthe musculature, the solid parts passively return to theiroriginal shape by releasing the kinetic energy stored inthe cuticular rods. The basal plate moves vertically bycontraction of the whole muscle capsule. When slidingforward, the basal plate grasps diatoms, bacteria, or de-tritus from sediment grains and piles them up. The foodis transferred from the mouth opening to the esophagusby the jaw. The jaw halves protrude out of the mouthopening by contraction of the muscle capsule and a si-multaneous contraction of the lateral muscle pouchescauses the jaw halves to open and surround the food. Re-laxation of the lateral muscle pouches closes the rigidjaw halves and then relaxation of the muscle capsule andsimultaneous contraction of the caudal muscle pouchmove the jaw dorsally, transferring the food to the esoph-agus. The jaw halves are opened again by contraction of

the lateral muscle pouches and the food is released intothe esophagus which has been opened by the contractionof myoepitheliocytes.

&p.2:Acknowledgements We are indebted to the collaborators of theII. Zoological Institute for their kind help. This work was support-ed by the Akademie der Wissenschaften und Literatur, Mainz, andby the Friedrich-Ebert-Stiftung, Bonn.

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