Ultrastructure and formation of the hooked setae in

Ultrastructure and formation of the hooked setae in Owenia fusiformis delle Chiaje, 1842: implications for annelid phylogeny Can. J. Zool. Downloaded from www.nrcresearchpress.com by Guangzhou Jinan University on 06/04/13 For personal use only.

Karsten Meyer and Thomas Bartolomaeus

Abstract: Several members of the Annelida bear apically curved or hooked setae that are aligned in a transverse row inside the neuropodial rim. Based on the hypothesis that these specific setae characterize a monophyletic group within the Annelida, the structure and development of the hooked seta in Owenia fusiformis are analysed and compared with data from other annelids with such setae. The neuropodial hooks of 0 . fusiformis are arranged in multiple transversal rows or setal patches on each side of the body from the fourth setiger onwards. The setae are curved distally and consist of two identical spines lying side by side at the same level. Their tips generally point ventrofrontally. Within each patch, the setae lie inside a setal follicle that consists of a basal chaetoblast, at least one follicle cell, and varying numbers of epidermal cells. Each setal patch is basally surrounded by an extracellular matrix that is continuous with the subepidermal basal lamina. An additional discontinuous extracellular matrix lies between the epidermis and the follicle cells. It is of functional significance for the attachment of the epidermal cells and seems to be related to the special organization of the setal patches, because it is absent in juveniles; they have single neuropodial rows of hooked setae per segment. New setae are formed at the dorsal and caudal edges of each patch, whereas the degeneration of setae is observed at the frontal edge of each patch. Microvilli project from the apex of the chaetoblast into canals within the fully differentiated setae. These canals remain when the microvilli are withdrawn from the seta during formation. Each hook is formed by a single large microvillus. The results of the present paper substantiate the hypothesis of a homology of the hooked setae in the Oweniida and other Annelida. These results and data from the literature support the hypothesis that the Oweniida is the sister-group of a monophylum which consists of the Terebellida, Pogonophora, and Sabellida. RCsum6 : Plusieurs anndlides posskdent des soies recourbees 2 l'apex ou soies en crochet, disposees en une rangCe transversale 2 I'intCrieur de la bordure neuropodiale. En tenant compte de l'hypothkse selon laquelle ces soies spicifiques caracterisent un groupe monophylCtique au sein des Annelida, nous avons analysC la structure des soies en crochet chez Owenia fusiformis et compare les rCsultats 2 ceux d'analyses des m6mes soies chez d'autres annClides. Les crochets neuropodiaux d'O. fusiformis sont disposes en rangees transversales multiples ou en plages de soies de chaque c6tC du corps, B partir du quatrikme segment. Les soies sont recourbkes distalement et constitukes de deux Cpines identiques sises l'une 2 c6tC de l'autre au meme niveau. Leur extrdmite est gCnCralement orienttie ventro-anterieurement. Dans chaque plage, chaque soie repose 2 l'intdrieur d'une follicule constituC d'un chktoblaste basal, d'au moins une cellule folliculaire et d'un nombre variable de cellules Cpidermiques. Chaque plage de soies est entourtie 2 la base par une matrice extracellulaire en continuit6 avec la couche basale sous-Cpidermique. Une matrice extracellulaire additionnelle discontinue s'inskre entre 1'Cpiderme et les cellules folliculaires. Absente chez les juveniles, cette matrice a une importance fonctionnelle dans l'attachement des cellules Cpidermiques et semble reliCe 2 l'organisation particulikre des plages de soies; les juvCniles ne posskdent qu'une rangCe de soies en crochet par segment. Les nouvelles soies de forment le long des bordures dorsale et caudale de chaque plage, alors que la dCgCnCrescence des soies se produit 2 la bordure frontale. Des microvillositCs partent de l'apex du chdtoblaste et se projettent dans des canaux 2 I'intCrieur des soies totalement diffdrenciks. Ces canaux persistent meme aprks le retrait des microvillositCs au cours de la formation des soies. Chaque crochet est constituk d'une microvillositC de grande taille. Nos rCsultats corroborent l'hypothkse d'une homologie des soies en crochet chez les Oweniida et chez d'autres Annelida. Ces rCsultats et des donnCes de la 1ittCrature appuient l'hypothkse selon laquelle les Oweniida forment le groupe soeur d'un monophylum constituC des Terebellida, des Pogonophora et des Sabellida.

Introduction The structure and arrangement of setae show remarkable variation in annelids. Setation is therefore regarded as an taxonomic character within this group Received January 18, 1996. Accepted June 17, 1996.

K. Meyer and T. Bartolomaeus. 11. Zoologisches Institut, Berliner StraRe 28, 37073 Gottingen, Germany (e-mail: tbartolagwdg .de) . Can. J. Zool. 74: 2143 -2153 (1996). Printed in Canada / ImprimC au Canada

1991; Fauchald 1977; Knight-Jones and Fordy 1979; Rosenfeldt 1982; Thomassin and Picard 1972). Annelid setae are unique in their symmetrical arrangement in paired dorsal (notoPodial) and ventral (neuropodial) groups in each segment. This trait is hypothesized to be an autapomorphy of the Annelida and strongly supports the monophyly of this taxon (Bartolomaeus 1994). In most polychaetous annelids, bundles of long setae typically originate from both notopodia and neuropodia, whereas certain other annelid taxa, the Arenicolida, Terebellida, and Oweniida, for example, differ in this

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Fig. 1. (A, B, D, and E) Adult Owenia fusiformis. (A) Patch of hooked setae in setigers 21 and 22 (arrow), capillary setae dorsal. (B) Setiger 6. Newly formed hooked setae at the caudal edge of the patch that have not yet pierced the cuticle (arrows). (C) Sagittal section of a single row of hooked setae in a juvenile (the arrows mark the setal basis). Coel, coelom; cu, cuticle; EC, epidermal cell; if, intermediate filaments. (D) Light microscopical section of the caudal edge of a setal patch. A second extracellular matrix (small arrows) separates the epidermis and follicles (the long arrow marks a developing seta and the short arrows bundles of intermediate filaments inside epidermal cells). (E) Longitudinal section of a degenerative site. A hooked seta is undergoing lysis inside a vesicle (arrow). ly, lysosome.

respect. The notopodia bear long, straight-shafted capillary setae, whereas the neuropodia consist of one or more transverse rows of apically toothed setae. These are called hooked setae because of their apically curved spines. Although other setae in certain annelid taxa can also be termed hooked setae, we do not wish to add to the existing confusion by creating a new term but rather present clear evidence for a unique evolution of the hooked setae in the taxa mentioned. In Owenia fisiformis, the hooked setae are arranged in semicircular patches, which are composed of multiple transverse rows (Nilsen and Holthe 1985; Thomassin and Picard 1972). In early developmental stages of this species, however, Thikbaut and Dauvin (1992) describe a segmental and paired arrangement of the single transverse rows. The phylogenetic relationship between the Oweniida and other taxa of the Annelida has been discussed by several authors with divergent points of view (Bubko 1973; Gardiner 1978; Liwanow and Porfirjewa 1967; Mileikowsky 1977; Rieger 1976; Smith et al. 1987). The most recent discussion is based on the observation that unspecialized monociliated cells are the dominant cell type of the tentacles (Gardiner 1978). Because monociliarity is considered to be plesiomorphic (Rieger 1976), the Oweniida are presumed to have a rather basal position within the annelids (Rieger 1986; Smith et al. 1987). Recently, Bartolomaeus (1995) hypothesized that the hooked setae of the Maldanida and Arenicolida and the uncini of the Terebellida, Pogonophora, and Sabellida are homologous structures. This hypothesis is sustained by the identical arrangement of the setae in transverse rows, a comparable process of setal formation, and a standard structure of the hooked setae. This consists of a single large hook called the rostrum, an apical group of small curved spines called the capitium, and the shaft or manubrium. The hooked setae of 0.fisiformis are known as bidentate because they have two curved spines at their tip (Gardiner 1992). To test whether these hooked setae can be homologized with those present in the above-mentioned annelid taxa, this paper analyses the ultrastructure and formation of the hooked setae of 0 . fisiformis. If this hypothesis of homology can be substantiated, it will imply that annelids bearing hooked setae shared a common ancestor in which these special setae evolved. Such an assumption will lead to an empirically testable hypothesis concerning the phylogenetic position of the Oweniida.

Materials and methods Adult 0. fusiformis were collected between Zostera marina in the Bassin d6Arcachon, France, in September 1994 at low tide and at the Phoronis-Grund off the isle of Helgoland in August 1991. Juvenile and newly metamorphosed animals were collected from plankton samples taken off the isle of Helgoland in spring 1995. The animals were fixed in 2.5 % glutaraldehyde buffered in 0.1 M sodium cacodylate (pH 7.2, 4°C) for 60 min; ruthenium red was

added to the fixative. The animals were rinsed in the same buffer, postfixed in 1% OsO, buffered in 0.1 M sodium cacodylate for 60 min at 4OC, and dehydrated in an acetone series. Parts of the animals were embedded in Araldite and cut into a complete series of 65- to 75-nm sections with a diamond knife on a Reichert Ultracut microtome. The sections were kept on Formvar-coated singleslot copper grids and stained with uranyl acetate and lead citrate in an LKB Ultrostainer. Chaetogenesis was studied in three adult and one juvenile 0.fusiformis with Zeiss EM 10B and EM 900 electron microscopes. For scanning electron microscopy (SEM), samples were dehydrated in an alcohol series, dried in CO, with a Balzers critical-point dryer (using the critical-point method), sputtered with gold in a Balzers sputter coater, and examined with a Novascan scanning electron microscope. For light microscopical examination, adult 0. fusiformis were fixed in Bouin's fluid, dehydrated in an alcohol series followed by methyl benzoate and butanol, and embedded in Paraplast. Sections 10 pm thick were stained with Masson-Goldener's trichrome and examined in a Zeiss Axioskop. Individuals of 0.fusiformis collected in the Bassin d7Arcachon are deposited at the Zoological Museum, Gottingen.

Results General In 0 . fisiformis, the first three body segments bear only notopodial capillary setae. From setiger 4 onwards, the segments have dorsal bundles of capillary setae and ventrolaterally arranged multiple rows of hooked setae, which will be termed setal patches. In the anterior segments, these patches extend from the dorsolateral to the ventral side of the body. Towards the caudal end they become smaller and are restricted to the ventrolateral side (Fig. IA). Each hooked seta has two apical spines, which are curved anteriorly. These spines lie side by side almost at the same level, their tips generally pointing ventrofrontally (Fig. IB). The spines roof a broad, flat part of the setal shaft, the so-called subrostra1 process. This is continuous with the manubrium, which tapers basally. The whole seta has a slightly sigmoid shape because the apical section of the manubrium and the basal section meet at an angle of nearly 60" (Fig. 3B). Each follicle of the fully differentiated hooked setae in 0. fisiformis consists of one basally situated chaetoblast and at least one follicle cell. Some epidermal cells may surround the seta apically. Adjacent cells are connected by apical adhaerens junctions. All follicles of a setal patch lie side by side, i.e., there is no extracellular matrix (ECM) separating individual follicles. The whole patch is underlain by an ECM, which is continuous with the subepidermal basal matrix. Hemidesmosomes interconnect the epidermal cells proximally with an ECM, which seems to form a second layer between the epidermal and follicle cells when viewed in light microscopical sections (Fig. ID). Electron microscopy reveals that clusters of this ECM are interspersed between the epidermal and follicle cells and these faciliate

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Fig. 2. Adult Owenia fisiformis. (A) Sagittal section of an almost completely differentiated hooked seta. The arrow marks the canal left by the microvillus. (B) Longitudinal section of a hooked seta. Microvilli (mv) arise from the surface of the chaetoblast and extend into hollow canals in the seta. Intermediate filaments (if) of the chaetoblast attach the seta to the basal matrix (bm). CB, chaetoblast; cu, cuticle; EC, epidermal cell; ECM, extracellular matrix; FC, follicle cell; M, musculature.

attachment of the two types of cells (Fig. 2A). Therefore, the formation of a second ECM is necessary for the stabilization of multiple rows of hooked setae. Microvilli arise from the surface of the chaetoblast and extend into hollow canals in the setae. At least one of these canals runs apically into each spine. Apart from the nucleus, the chaetoblast contains small electron-lucent vesicles, mitochondria, and bundles of intermediate filaments that cross the cytoplasm. At one end, these filaments are attached to the apical microvilli; at the other end they adhere to hemidesmosomes on the matrix side of the chaetoblast, therefore they mechanically connect the seta to the perifollicular ECM and to the setal muscles, respectively (Fig. 2B). Almost one-third of the fully differentiated seta is surrounded by the chaetoblast, which is linked to the follicle cell by adhaerens junctions. More apically, the follicle cell covers the seta in the same manner as the chaetoblast and extends to the cuticule (Fig. 3B). Unlike the chaetoblast, no vesicles or organelles other than the nucleus are found in the electron-lucent follicle cell. Close to the surface, the follicle cell is connected with a certain number of epidermal cells by adhaerens junctions. Cuticular material is produced by these cells. The content of the epidermal cells is identical with that of the chaetoblast. Intermediate filaments cross the cytoplasm of the follicle cell and are linked to the basal matrix and the seta by hemidesmosomes. Chaetoblasts and follicle cells of adjacent setal follicles are in direct contact with each other without an ECM layer (Fig. 2A).

Development of the hooked setae In the adult stages, the formative site in each setal patch is located dorsally along its caudal edge (Figs. IB, 3A). The early stages of setal development, when the developing setae have not yet pierced the cuticle, are observed exclusively in this area. In the adults investigated, only a few such early stages were found. Therefore, it is presumed that chaetogenesis is a rather discontinuous process in older animals. Studying newly metamorphosed stages provided much more information. In these stages, hooked setae can be found in a single ventrolateral row extending from setiger 4 onwards (Fig. IC). The formation site lies midlaterally, and chaetogenesis seems to be a continuous process in this region. In 0 . fisiformis, the chaetoblast of the earliest Anlage secretes mainly extracellular material into the setal pouch, which is formed by the follicle cell and the chaetoblast (Fig. 4A). Short, thin chaetoblast microvilli extend into this amorphous matrix, which will later cover the developing seta. At the beginning of setal development, two stout microvilli extend from the chaetoblast into the setal pouch. Later, electron-dense setal material surrounds each microvillus and forms the apical spines of the seta (Figs. 4C, 4D). Subsequently, the orientation of the microvilli changes: they become curved and lie almost perpendicular to the basoapical axis of the chaetoblast . Several additional microvilli appear at the bases of the two large ones and determine the structure of the subrostral process (Fig. 4B). In the later stages of

development, bundles of microvilli converge to form the manubrium. It is generally assumed that the setal material is released by the chaetoblast and the follicle cell. Vesicles containing setal material fuse with the microvillar cell membrane and release material to enable the seta to elongate. During this process, the microvilli continuously withdraw from the newly formed seta. In the apical section of the seta, the remaining canals become filled with electron-dense material (Fig. 2A). When the seta has reached a specific length, the microvilli still extend into the canals at the setal base. The intermediate filaments of the chaetoblast attach the seta to the follicle (Fig. 2B).

Degeneration of the hooked setae Developing follicles have been found at the end of the patch opposite to the degenerating ones. At the light microscopical level, degeneration and development can hardly be distinguished because both are characterized by short or incomplete setae. Degeneration of the hooked setae occurs at the frontal edge of the setal patch, diametrically opposed to the formative site (Fig. 3A). Numerous lysosomes within certain cells characterize this area. These cells rest on a basal matrix and are thought to be former chaetoblasts. Each cell contains one degenerating seta inside a large vesicle (Fig. IE). Both types of lysosomes are found: primary, electron-dense lysosomes, which have not yet been used for intracellular digestion, and secondary lysosomes, which result from merging vesicles. The secondary lysosomes may contain fragments and whorls of membranes. Occasionally, deep incisions inside the setae reveal the process of degeneration.

Discussion The Oweniida is generally considered to consist of the five genera: Owenia delle Chiaje, 1842, Myriowenia Hartman, 1960, Myrioglobula Hartman, 1967, Myriochele Malmgren, 1867, and Galathowenia Kirkegaard, 1959 (Fauchald 1974; Gardiner 1979). However, Galathowenia africana is not included in this discussion because it has been regarded as poorly separated from Myriochele species (Blake and Dean 1977; Nilsen and Holthe 1985). The monophyletic origin of the Oweniida is substantiated by the formation of neuropodia1 patches of hooked setae, each consisting of a long manubrium and two identical hooks (Nilsen and Holthe 1985). Further autapomorphies are the formation of a second layer of ECM, the mitraria larva, specialized parapodial glands, and the reduction of the nephridia, except for a single pair in the sixth segment (Gilson 1894, 1895; Liwanow and Porfirjewa 1967).

Structure and formation of hooked setae In 0 . fisiformis, each spine of the bidentate hook is formed by a single large microvillus. Later in development, additional microvilli appear on the surface of the chaetoblast and determine the structure of the subrostral process and manubrium. In the course of setal formation, these microvilli are

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Fig. 4. Formation of the hooked setae in Owenia fusiformis. (A) Adult. Earliest Anlage of a hooked seta. An amorphous matrix (arrow) covers the developing seta. (B, C, and D) Juvenile. Series of sagittal sections of a developing hooked seta. (B) One microvillus extends into each spine of the capitium (arrow). (C) Electron-dense setal material (arrow) surrounds the microvilli. (D) Microvilli determine the manubrium. Note the large microvilli of the spines. bm, basal matrix; CB, chaetoblast; FC, follicle cell: mv. microvilli. Distances between sections: B-C, 500 nm; C-D, 1.5 pm. Fig. 3. Owenia fusiformis. (A) Sketch of a section of the setal patch in adults, indicating the formative and degenerative sites. (B) Scheme of a longitudinally sectioned hooked seta of an adult (redrawn from a series of electron micrographs). bm, basal matrix; CB, chaetoblast; cu, cuticle; EC, epidermal cells; ECM, extracellular matrix; FC, follicle cell; hd, hemidesmosomes; if, intermediate filaments.

continuously withdrawn from the setae. The canals inside the setae reflect this process (Fig. 5). At the end of chaetogenesis, microvilli can still be observed in fully differentiated hooked setae. Thus, both spines of the hooked setae in 0. fusiformis are formed in a manner identical with the spines of the capitium in hooked setae of arenicolids and in the uncini of terebellidans and sabellidans (Bartolomaeus and Meyerl ; Bartolomaeus 1995). The rostrum of hooked setae, T. Bartolomaeus and K. Meyer. Morphogenesis and phylogenetic significance of hooked setae in Arenicolida (Annelida). Submitted for publication.

which has been studied in maldanids and arenicolids is always formed by a bundle of microvilli (Bartolomaeus and Meyer, see footnote I). We therefore conclude that both spines of the hooked seta of 0.fusiformis are homologous to the capitium of the hooked setae in other annelids and that the rostrum must have been reduced in this species. This conclusion leads to a interpretation different from that given by Nilsen and Holthe (1985). On the basis of comparative SEM investigations of different oweniid species, these authors conclude that one of the two hooks represents the rostrum and the other a single spine of the capitium, but provided that the spines of hooked setae in other oweniids are formed in

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Fig. 5. Schematic drawing of the formational process of a hooked seta in Owenia fusiformis. (A) Formation of the spine. (B) Formation of the subrostral process. (C) Formation of the manubrium. CB, chaetoblast; FC, follicle cell.

the same way as in 0.fisiformis, chaetogenesis provides strong evidence that a reduction of the rostrum and a conversion of the capitium into two adjacent spines had already evolved in the stem lineage of the Oweniida. In juvenile 0.fisiformis, the hooked setae are aligned in single transverse rows at first, and these become multiple setae during further development. The formative site lies midventrally in these stages. Additional rows are formed during further development (ThiCbaut and Dauvin 1992), so that in older animals, which already possess large semicircular patches composed of multiple transverse setal rows, the formation of additional setae can also be observed along .the caudal edge of each patch. In terebellidans and sabellidans the formative site also lies midlaterally, so that the oldest uncini are found ventrally (Bartolomaeus 1994, 1995; Bhaud 1988), whereas it secondarily lies in a ventral position in arenicolidans and maldanidans (Ashworth 1912; Bartolomaeus and Meyer, see footnote 1; Gamble and Ashworth 1900).

Homology of hooked setae The Oweniida, Maldanida, Arenicolida, and Psammodrilida all have transverse rows of apically toothed setae, which are situated inside the neuropodial rim (Gamble and Ashworth 1900; Ashworth 1912; Hartmann-Schroder 1971; Kristensen and Norrevang 1982). The uncini of the Terebellida, Sabellida, and Pogonophora lie side by side inside the neuropod.ia1 rim and have a rather short manubrium, from which the microvilli are withdrawn at the end of formation (Bartolomaeus 1995; George and Southward 1973; Norrevang 1970;

Knight-Jones 1981; Holthe 1986). The uncini and hooked setae are presumed to be homologous because both are composed of a strong, anteriorly curved rostrum, a capitium consisting of several small, curved spines, and a manubrium, which comprises the shaft of these setae. The rostrum is formed by a bundle of small microvilli, whereas each spine of the capitium is formed by a single, large microvillus. The rostrum may have been reduced in certain terebellidan, sabellidan, and pogonophoran taxa. During further development the longitudinal axis of the setal Anlage shifts almost perpendicularly, so that the axis of the rostrum and of the manubrium form an angle of almost 90"; all microvilli converge to form the manubrium (Bartolomaeus 1995; Bartolomaeus and Meyer, see footnote 1). Owing to the reorientation of the longitudinal axis during setal formation, the apical spines of hooked setae are always curved anteriorly. When formation is complete, the microvilli either are withdrawn (uncini) or remain in the seta (hooked seta) and a system of intermediate filaments, which attaches the seta to the follicle, is generated inside the chaetoblast. This hypothesis of homology forces us to assume that hooked setae must have been evolved in a common ancestor of the annelid taxa mentioned, and restricts the search for the sister-group of the Oweniida to those taxa of the Annelida that bear such setae.

Phylogenetic implications In oweniidans, hooked setae are arranged ventrolaterally in setal patches that consist of several transverse rows (Day 1967; Fauchald 1974; Thomassin and Picard 1972) and are

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Fig. 6. Proposed phylogenetic relationship between the Oweniida and the monophylum consisting of Terebellida, Pogonophora, and

Sabellida (after Bartolomaeus 1995). The Psammodrilida, Arenicolida, and Maldanida are the closest relatives of this monophylum. Autapomorphic characters are as follows: 1, a tranverse row of hooked setae inside the neuropodial rim of each segment, and the formation site in a lateral position, prolonged manubrium, tori, intraepidermal nervous system without ganglia; 2, lack of hooked setae in the first setiger; 3, formation of neuropodial patches of hooked setae, capitium with two hooks situated side by side and reduction of the rostrum, second layer of ECM, mitraria larva, specialized parapodial glands, a single pair of nephridia in the sixth segment; 4, uncini with manubrial length reduced, anteriorly curved spines aligned in transverse rows; 5, nonretractile perioral feeding tentacles; 6, one pair of nephridia draining the first segment, nephropori dorsal; 7, trophosome, prolonged second segment, multicellular glands; 8, setal inversion, perioral funnel-like tentacular crown consisting of a pair of semicircular lobes, each lobe with three bipinnate filaments, nephridia with a common dorsomedian nephridiopore, larval protonephridia with multiciliated flame cells. nn, nomen nominator.

\

TEREBELLIDA

n.n.

/

POGONOPHORA

\

SABELLIDA

PSAMMODRILIDA ARENlCOLlDA MALDANIDA

restricted to certain segments. These patches of hooked setae are present from setiger 4 onwards in Myriochele oculata Zaks, 1923 and Myriochele heeri Malmgren, 1867, Myriowenia californiensis Hartman, 1960, Myriowenia gosnoldi Hartman, 1960, and Owenia fusiformis. In contrast, the first three setigers bear only bundles of notopodial capillary setae. In Myrioglobula antarctica, neuropodial hooked setae are found from the second segment onwards. The same condition occurs in sabellidans and terebellidans, where ventrolateral rows of uncini are found on setiger 2 or start to appear more caudally, but are always absent on setiger 1 (HartmannSchroeder 1971; Fauchald 1977; Fitzhugh 1989). It is therefore assumed that paired ventrolateral semicircular patches of hooked setae in setiger 4 and all following setigers represent an apomorphic condition within the Oweniida and that the setation found in M. antarctica as well as in certain sabellidans and in terebellidans represents the primary condition. Compared with maldanidans and arenicolidans, which have ventrolateral rows of hooked setae from setiger 1 onwards, it is more parsimonious to assume that a ventrolateral row of hooked setae has only been reduced once in setiger 1 in a common ancestor of oweniidans, sabellidans, terebellidans,

and pogonophorans (No. 2 in Fig. 6) than to suppose that the rows of hooked setae secondarily occupied a more frontal position in certain species of the taxa mentioned. Multiple transverse rows of setae have also been described for the perviate Pogonophora (Southward 1993) and the obturate Pogonophora (Gardiner and Jones 1993). In the past, attempts have been made to link the Oweniida with the perviate Pogonophora. Liwanow and Porfirjewa (1967) presented the similarity in structure of the neuropodium and the neuropodial hooked setae. Nevertheless these similarities and other characteristics do not necessarily point to a close phylogenetic relationship between the two taxa; the similarities may be based on a pattern present in a shared ancestor (George and Southward 1973). More recently, a rather basal position for the Oweniida within the Annelida has been substantiated by the monociliarity of some Oweniida as well as the similarity of the nephridia in the mitraria larva and deuterostomia larva and the myoepithelial lining of the coelom (Gardiner 1979; Rieger 1986; Smith et al. 1987). These findings might imply a sister-group relationship between the Oweniida and the other Annelida. Based on the presumed homology of the

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Can. J. Zool. Vol. 74, 1996 hooked setae, however, such a relationship seems unlikely, because the existence of hooked setae in the ground pattern of the annelids would then have to be presumed and they must have been reduced several times within this taxon. In this paper, the hypothesis of a sister-group relationship between the Oweniida and the monophylum consisting of the Terebellida, Sabellida, and Pogonophora is favoured. It is supported by ,the absence of hooked setae in the first setiger (No. 2 in Fig. 6). As a consequence of this hypothesis, a midlaterally located site of formation of hooked setae, an elongated manubrium , tori, neuropodial hooked setae placed in transverse rows, paired nuchal organs, and a nervous system without ganglia are presumed to be plesiomorphic features of the Oweniida. Nevertheless, this hypothesis does not necessarily contradict Gardiner (1979), Rieger ( 1986), and Smith et al. (1987), because according to the hypothesis favoured here, oweniids could have retained several primary characters from the ground pattern of annelids.

.

Acknowledgements W e thank Heide Riisch and Marc Buffet for translating the French abstract and Sebastian K. Herzog for assistance with English. Harald Hausen kindly printed the cladogram. Our thanks are also extended to two anonymous reviewers for their kind and helpful comments on the manuscript. This investigation was financially supported by the Akademie der Wissenschaften und der Literatur, Mainz.

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