Ultrastructure of Nuchal Organs in Polychaetes (Annelida) — New Results and Review

Nuchal organs are epidermal sensory structures present in most polychaetes. They are situated at the posterior edge of the prostomium and may extend posteriorly onto the peristomium. Although there is considerable external variation, they all consist of ciliated supporting cells, bipolar primary sensory cells and retractor muscles. They are innervated directly from the brain by paired nerves. The sensory cells are usually monociliated; their sensory processes lie in subcuticular spaces, the olfactory chambers. Structural variability is to be observed in the location of the sensory cells, the course of the nuchal nerve, position of nuchal ganglia as well as in cytological features of sensory and supporting cells. These differences provide useful characters for phylogenetic considerations to establish supraspecific taxa within the phylogenetic system of the Annelida. Special emphasis is laid on the problem of whether the nuchal organs represent an autapomorphy of the Polychaeta or the Annelida and thus whether the lack of nuchal organs in Clitellata is primary or secondary. As is discussed, the probability of a loss of the nuchal organs in Clitellata is higher, which favours the second hypothesis: that nuchal organs are part of the ground pattern of the Annelida and very likely are an autapomorphy of this group.     

On the ground pattern of Annelida

Annelida, traditionally divided into Polychaeta and Clitellata, are characterized by serial division of their body into numerous similar structures, the segments. In addition, there is a non-segmental part at the front end, the prostomium, and one at the back, the pygidium. New segments develop in a prepygidial proliferation zone. Each segment contains four groups of chaetae made up of β-chitin, a pair of coelomic cavities separated by mesenteries, and septa. The nervous system is a rope-ladder-like ventral nerve cord with a dorsal brain in the prostomium. For the last stem species a trochophore larva and a benthic adult are commonly postulated. There are two conflicting hypotheses describing the systematization of Annelida: the first postulates a sister-group relationship of Polychaeta and Clitellata, the second sees Clitellata as a highly derived taxon forming a subordinate taxon within the polychaetes which, consequently, are regarded as paraphyletic. Depending on the hypothesis, different characters have to be postulated for the stem species of Annelida. Besides segmentation other characters such as nuchal organs, palps and antennae, body wall musculature, cuticle, parapodia as well as structure of the central nervous system and the foregut play an important role in this discussion. Here, the different characters and character states are critically reviewed and analyzed with respect to morphology and function. The consequences for systematization of their phylogenetic interpretation as autapomorphies, synapomorphies or plesiomorphies are outlined. The resulting hypotheses are compared with those relying on molecular data sets.     

Immunohistochemical (cLSM) and ultrastructural analysis of the central nervous system and sense organs in Aeolosoma hemprichi (Annelida, Aeolosomatidae)

The Aeolosomatidae are very small limnetic or terrestrial annelids of apparently simple organisation and uncertain phylogenetic position. They have been placed either at the base of the Clitellata, as a highly derived taxon within the Clitellata closely related to the Naididae, or as their sister group within the „Polychaeta”. A combined immunohistochemical (cLSM) and ultrastructural investigation of the central nervous system and the sense organs in Aeolosoma hemprichi was undertaken to look for characters which might support one of these theories. The position of the brain within the prostomium and the organisation of the ventral nerve cord, with its intraepithelial paired longitudinal nerves lying far apart from each other and the presence of a median longitudinal nerve, are atypical for the Clitellata and clearly differ from the situation found in Naididae. Moreover, the circumoesophageal connectives are bifurcated and enter the brain as dorsal and ventral roots; this arrangement is unknown in Clitellata, in which these connectives are unbranched. An ultrastructural analysis of the ciliated pits located laterally in the furrow between prostomium and peristomium in A. hemprichi and other Aeolosomatidae show that they are in fact nuchal organs. Such presumed chemosensory organs are typical of „Polychaeta” and absent in all Clitellata. Two pairs of ciliary sense organs are present in the prostomium in front of the brain of A. hemprichi. Although similarly organised sensory structures occur in many species of the Clitellata, they differ in position and certain ultrastructural features and are known from other Annelida as well. These results clearly support the exclusion of the Aeolosomatidae from the Clitellata and do not provide any evidence for a sister-group relationship between these two taxa. Accepted: 8 February 2000     

Cladistics and polychaetes

A series of cladistic analyses assesses the status and membership of the taxon Polychaeta. The available literature, and a review by Fauchald & Rouse (1997), on the 80 accepted families of the Polychaeta are used to develop characters and data matrices. As well as the polychaete families, non-polychaete taxa, such as the Echiura, Euarthropoda, Onychophora, Pogonophora (as Frenulata and Vestimentifera), Clitellata, Aeolosomatidae and Potamodrilidae, are included in the analyses. All trees are rooted using the Sipuncula as outgroup. Characters are based on features (where present) such as the prostomium, peristomium, antennae, palps, nuchal organs, parapodia, stomodaeum, segmental organ structure and distribution, circulation and chaetae. A number of analyses are performed, involving different ways of coding and weighting the characters, as well as the number of taxa included. Transformation series are provided for several of these analyses. One of the analyses is chosen to provide a new classification. The Annelida is found to be monophyletic, though weakly supported, and comprises the Clitellata and Polychaeta. The Polychaeta is monophyletic only if taxa such as the Pogonophora, Aeolosomatidae and Potamodrilidae are included and is also weakly supported. The Pogonophora is reduced to the rank of family within the Polychaeta and reverts to the name Siboglinidae Caullery, 1914. The new classification does not use Linnaean categories, and the Polychaeta comprises two clades, the Scolecida and Palpata. The Palpata has the clades Aciculata and Canalipalpata. The Aciculata contains the Phyllodocida and Eunicida. The Canalipalpata has three clades; the Sabellida (including the Siboglinidae) Spionida and Terebellida. The position of a number of families requires further investigation.     

Annelid phylogeny and the status of Sipuncula and Echiura

Background  

Annelida comprises an ancient and ecologically important animal phylum with over 16,500 described species and members are the dominant macrofauna of the deep sea. Traditionally, two major groups are distinguished: Clitellata (including earthworms, leeches) and "Polychaeta" (mostly marine worms). Recent analyses of molecular data suggest that Annelida may include other taxa once considered separate phyla (i.e., Echiura, and Sipuncula) and that Clitellata are derived annelids, thus rendering "Polychaeta" paraphyletic; however, this contradicts classification schemes of annelids developed from recent analyses of morphological characters. Given that deep-level evolutionary relationships of Annelida are poorly understood, we have analyzed comprehensive datasets based on nuclear and mitochondrial genes, and have applied rigorous testing of alternative hypotheses so that we can move towards the robust reconstruction of annelid history needed to interpret animal body plan evolution.     

The articulation of annelids

The aim of this paper is to assess the monophyly of the Annelida. Also, recent cladistic analyses of metazoan taxa, using a variety of data, have shown incongruities with regards to annelids and associated taxa that should be resolved. The Platyhelminthes is selected as the taxon to root our minimal length trees and polarise our characters in a parsimony analysis; ingroup taxa being Mollusca, Nemertea, Sipuncula, Echiura, Pogonophora, Vestimentifera, Euarthropoda, Onycho-phora, and the groups most commonly regarded as true ‘annelids’, the Clitellata and Polychaeta. We use 13 characters and a total of 33 states. This results in 18 minimal length trees of 23 steps. The consensus tree has the topology (Platyhelminthes (Nemertea (Sipuncula Mollusca (Echiura (Polychaeta (Vestimcntifera Pogonophora) Clitellata (Euarthropoda Onychophora)))))). The name Articulata is applied to the Clitellata, Euarthropoda, Onychophora, Pogonophora, Polychaeta, and Vestimentifera. The Vestimentifera is the sister group to, or more likely a clade within, the frenulate pogonophores, and the name Pogonophora is retained for this group. In half of the 18 minimal length trees, the traditionally formulated Annelida, i.e. Polychaeta and Clitellata, is paraphyletic if the Pogonophora are excluded. In the remaining minimal length trees, a monophyletic Annelida cannot be formulated. The name Annelida should not be used unless relationships within the Articulata are resolved to show it is a monophyletic taxon. The taxon name Articulata, originally formulated to include the Annelida and Arthropoda by Cuvier, is defined as the clade stemming from the first ancestor to show repetition of homologous body structures derived by teloblastic growth with a pygidial growth zone (segmentation) and longitudinal muscles broken into bands. The Articulata is considered, on current evidence, to consist of four monophyletic groups; the Arthropoda, Clitellata, Polychaeta, and Pogonophora, though the latter group may be a clade of polychaetes. If this is shown, the Pogonophora should revert to the original family name Lamellisabellidae Uschakov, 1933. An indented classification reflective of the cladistic pattern is provided. Other recent hypotheses about metazoan systematics arc analysed.     

Phylogenetic analysis of the Amphitritinae (Polycnaeta: Terebellidae)

The phylogenetic relationships of the Amphitritinae (Polychaeta: Terebellidae) were studied using parsimony analysis of 22 external morphological characters. To choose outgroups to polarize the characters, I carried out a preliminary analysis of the relationships of the four terebellid subfamilies and the Trichobranchidae. The single most parsimonious tree from the analysis supports monophyly of the Terebellidae by the presence of ventral glandular shields. However, this character is homoplasious within the Terebellomorpha, and further evaluation of the Terebellidae is recommended. Artacama and Thelepus were chosen as outgroups for the analysis of amphitritine genera. The generic level analysis yielded seven equally parsimonious trees, which are consistent in their topologies except for the relationships among seven genera in one large clade. In all trees, Artacama is the sister taxon to a large clade within the Amphitritinae; the Artacaminae is therefore synonymized with the Amphitritinae, which is diagnosed by the presence of double rows of uncini. Within the Amphitritinae, the status of several monotypic genera is questioned; plesiomorphic character states indicated by the analysis are discussed. The results presented are offered as working hypotheses of the relationships among amphitritine genera. The large number of homoplasies indicated by the analysis emphasizes the need to further evaluate these hypotheses using additional characters. With a robust phylogenetic hypothesis of amphitritine relationships, a re-classification of the group based on apomorphic character states can be undertaken, and questions regarding the evolution of morphological characters, reproductive modes, or biogeographical patterns can be properly addressed.     

A reevaluation of Wiwaxia and the polychaetes of the Burgess Shale

Wiwaxia corrugata and the indisputable polychaetes of the Middle Cambrian Burgess Shale, particularly Canadia spinosa, have figured prominently in recent hypotheses about the early evolution of polychaete annelids. Based on similarities between the sclerites of Wiwaxia and the notochaetae of Canadia with the broad notochaetae (paleae) of Recent chrysopetalid polychaetes, these two fossil taxa have been variously treated as closely related to the most highly derived stem forms of the polychaete (and annelid) crown group or as members of a specific, Recent subgroup within Polychaeta, the order Phyllodocida. Chrysopetalidae is a member of Phyllodocida, which is part of the major polychaete clade Aciculata; the latter two taxa are distinguished by four and six well defined autapomorphic characters, respectively. The best preserved or otherwise appropriate fossils of Wiwaxia corrugata, Canadia spinosa and the other polychaetes of the Burgess Shale have been studied in detail in order to determine whether they possess any characters that could support the homology of wiwaxiid sclerites, canadiid notochaetae and chrysopetalid paleae. Most of these fossil taxa have significant autapomorphies but the specific characters of the Aciculata and Phyllodocida are entirely absent. It is demonstrated that constraining cladograms in such a manner that wiwaxiid sclerites, canadiid notochaetae and chrysopetalid paleae are homologous leads to results that are markedly unparsimonious. Furthermore, Canadia and the other polychaetes of the Burgess Shale cannot be referred to any extant subgroup within the Polychaeta and cannot be used to polarize character evolution within the annelid crown group. Apart from its dubious sclerites, Wiwaxia has no characters that could indicate any close relationship with Polychaeta or Annelida.     

A multigene framework for polychaete phylogenetic studies

Producing a robust phylogenetic reconstruction for Polychaeta using either morphological or molecular data sets has proven very difficult. There remain many conflicts between morphological analyses and hypotheses based on DNA data, the latter principally derived from 18S rRNA sequences. For the present study a data set covering a broad range of polychaete diversity was assembled, including 38 new sequences from 21 species. Besides available 18S rRNA data, five additional gene segments were examined: the D1 and D9-10 expansion regions of 28S rRNA, histone H3, snU2 RNA and cytochrome c oxidase subunit I. Maximum parsimony, maximum likelihood and Bayesian analyses were conducted.Annelida and Mollusca were reciprocally monophyletic in maximum likelihood analyses, but Polychaeta included a cephalopod in maximum parsimony analyses, and a patellogastropod in Bayesian analyses. When rooted on the Mollusca, optimal topologies from maximum likelihood analyses showed a recognisable basal group of taxa, including Oweniidae, Chaetopteridae and Amphinomidae. The six studied phyllodocidan families plus Orbiniidae (as the sister group of the scale-worms) formed the next most basal group. All analyses support the inclusion of Echiura, Clitellata and Siboglinidae within polychaetes. Bayesian analyses show Echiura as the sister group of Capitellidae, in agreement with previous 18S rRNA results, In contrast, Echiura formed the sister group to Trichobranchidae in maximum likelihood and maximum parsimony analyses.Supra-familial groupings consistently recovered within Polychaeta in the analyses are: (i) Terebellida without Ampharetidae; (ii) Scolecida (excepting Orbiniidae); (iii) Eunicidae, Lumbrineridae and Clitellata; and (iv) “Cirratuliformia” (including Sternaspidae) plus Sabellidae, Serpulidae and Spionidae.     

The phylogenetic position of the Aeolosomatidae and Parergodrilidae, two enigmatic oligochaete-like taxa of the 'Polychaeta', based on molecular data from 18S rDNA sequences

The Aeolosomatidae and the Parergodrilidae are meiofaunal Annelida showing different combinations of clitellate‐like and non‐clitellate character states. Their phylogenetic positions and their systematic status within the Annelida are still in debate. Here we attempt to infer their systematic position using 18S rDNA sequences of the aeolosomatid Aeolosoma sp. and the parergodrilid Stygocapitella subterranea and several other meiofaunal taxa such as the Dinophilidae, Polygordiidae and Saccocirridae. The data matrix was complemented by sequences from several annelid, arthropod and molluscan species. After evaluation of the phylogenetic signal the data set was analysed with maximum‐parsimony, distance and maximum‐likelihood algorithms. Sequences from selected arthropods or molluscs were chosen for outgroup comparison. The resolution of the resulting phylogenies is discussed in comparison to previous studies. The results do not unequivocally support a sister‐group relationship of Aeolosoma sp. and the Clitellata. Instead, depending on the algorithms applied, Aeolosoma clusters in various clades within the polychaetes, for instance, together with eunicidan species, the Dinophilidae, Harmothoë impar or Nereis limbata. The position of Aeolosoma sp. thus cannot be resolved on the basis of the data available. S. subterranea always falls close to a cluster comprising Scoloplos armiger, Questa paucibranchiata and Magelona mirabilis, all of which were resolved as not closely related to both Aeolosoma sp. and the Clitellata. Therefore, convergent evolution of clitellate‐like characters in S. subterranea and hence in the Parergodrilidae is suggested by our phylogenetic analysis. Moreover, the Clitellata form a monophyletic clade within the paraphyletic polychaetes.     

Evolution of habitat preference in Clitellata (Annelida)

Clitellata (earthworms, leeches, and allies) is a clade of segmented annelid worms that comprise more than 5000 species found worldwide in many aquatic and terrestrial habitats. According to current views, the first clitellates were either aquatic (marine or freshwater) or terrestrial. To address this question further, we assessed the phylogenetic relationships among clitellates using parsimony, maximum likelihood and Bayesian analyses of 175 annelid 18S ribosomal DNA sequences. We then defined two ecological characters (Habitat and Aquatic‐environment preferences) and mapped those characters on the trees from the three analyses, using parsimony character‐state reconstruction (i.e. Fitch optimization). We accommodated phylogenetic uncertainty in the character mapping by reconstructing character evolution on all the trees resulting from parsimony and maximum likelihood bootstrap analyses and, in the Bayesian inference, on the trees sampled using the Markov chain Monte Carlo algorithm. Our analyses revealed that an ‘aquatic’ ancestral state for clitellates is a robust result. By using alterations of coding characters and constrained analyses, we also demonstrated that the hypothesis for a terrestrial origin of clitellates is not supported. Our analyses also suggest that the most recent ancestor of clitellates originated from a freshwater environment. However, we stress the importance of adding sequences of some rare marine taxa to more rigorously assess the freshwater origin of Clitellata. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 447–464.     

Unsegmented annelids? Possible origins of four lophotrochozoan worm taxa

In traditional classification schemes, the Annelida consists of the Polychaeta and the Clitellata (the latter including the Oligochaeta and Hirudinida). However, recent analyses suggest that annelids are much more diverse than traditionally believed, and that polychaetes are paraphyletic. Specifically, some lesser-known taxa (previously regarded as separate phyla) appear to fall within the annelid radiation. Abundant molecular, developmental, and morphological data show that the Siboglinidae, which includes the formerly recognized Pogonophora and Vestimentifera, are derived annelids; recent data from the Elongation Factor-1α (EF-1α) gene also suggest that echiurids are of annelid ancestry. Further, the phylogenetic origins of two other lesser-known groups of marine worms, the Myzostomida and Sipuncula, have recently been called into question. Whereas some authors advocate annelid affinities, others argue that these taxa do not fall within the annelid radiation. With advances in our understanding of annelid phylogeny, our perceptions of body plan evolution within the Metazoa are changing. The evolution of segmentation probably is more plastic than traditionally believed. However, as our understanding of organismal evolution is being revised, we are also forced to reconsider the specific characters being examined. Should segmentation be considered a developmental process or an ontological endpoint?     

Sense organs and central nervous system in an enigmatic terrestrial polychaete, Hrabeiella perighndulata (Annelida)–implications for annelid evolution

Abstract. The terrestrial polychaete Hrabeiella periglandulata has many features in common with the Clitellata and the polychaete taxon Parergodrilidae. An ultrastructural investigation of the central nervous system and the sense organs of H. periglandulata individuals was undertaken to look for structural similarities with these taxa as well as to elucidate whether these structures might exhibit adaptive characters typical of terrestrial annelids in general. The central nervous system of H. periglandulata is subepidermal and consists of a brain situated in the first achaetigerous segment. The circumoesophageal connectives are without dorsal and ventral roots, and the ventral nerve cord has closely associated connectives and ill-defined ganglia. In contrast to clitellates and the terrestrial parergodrilid Parergodrilus heideri , nuchal organs are present. They are internal and highly modified compared with those of marine polychaetes but are similar to those of the intertidal parergodrilid Stygocapitella subterranea . A pair of ciliary sense organs is present inside the brain, resembling similar structures in many microdrile oligochaetes. These observations indicate that there are, in fact, structural similarities between the nervous system and the sense organs of clitellates, parergodrilids, and Hrabeiella individuals. These similarities may very likely be the result of convergent evolution in adaptation to the terrestrial environment.     

Sense organs in polychaetes (Annelida)

Polychaetes possess a wide range of sensory structures. These form sense organs of several kinds, including the appendages of the head region (palps, antennae, tentacular cirri), the appendages of the trunk region and pygidium (parapodial and pygidial cirri), the nuchal organs, the dorsal organs, the lateral organs, the eyes, the photoreceptor-like sense organs, the statocysts, various kinds of pharyngeal papillae as well as structurally peculiar sensory organs of still unknown function and the apical organs of trochophore larvae. Moreover, isolated or clustered sensory cells not obviously associated with other cell types are distributed all over the body. Whereas nuchal organs are typical for polychaetes and are lacking only in a few species, all other kinds of sensory organs are restricted to certain groups of taxa or species. Some have only been described in single species till now. Sensory cells are generally bipolar sensory cells and their cell bodies are either located peripherally within the epidermis or within the central nervous system. These sensory cells are usually ciliated and different types can be disinguished. Structure, function and phylogenetic importance of the sensory structures observed in polychaetes so far are reviewed. For evaluation of the relationships of the higher taxa in Annelida palps, nuchal organs and pigmented ocelli appear to be of special importance.     

Phylogenetic inference regarding Parergodrilidae and Hrabeiella periglandulata ('Polychaeta', Annelida) based on 18S rDNA, 28S rDNA and COI sequences

Parergodrilidae and Hrabeiella periglandulata are Annelida showing different combinations of clitellate-like and aclitellate characters. Similarities between both of these taxa and Clitellata have widely been regarded as the result of convergent evolution due to similar selection pressures. The position of the three taxa in the phylogenetic system of Annelida is still in debate. However, in analyses based on 18S rDNA sequences a close relationship of Parergodrilidae with Orbiniidae and Questidae was suggested. To infer their phylogeny the sequences of the 28S rDNA and of the cytochrome oxidase I (COI) gene of Stygocapitella subterranea , Parergodrilus heideri and H. periglandulata were determined. The data were extended by sequences of various species including species from Clitellata and Orbiniidae. Prior to tree reconstruction the dataset was analysed in detail for phylogenetic content by applying a sliding window analysis, a likelihood mapping and Modeltest V.3.04. Subsequently, generalized parsimony and maximum likelihood methods were employed. Clade robustness was estimated by bootstrapping. In addition, combined analyses of the sequences of 18S rDNA and 28S rDNA as well as of 18S rDNA, 28S rDNA and COI were performed. The combination of the data of the two structure genes and a mitochondrial gene improved the resolution obtained with the single datasets slightly. These analyses support a close relationship of Parergodrilidae and Orbiniidae but cannot resolve the position of H. periglandulata . In every analysis Clitellata cluster within 'Polychaeta', confirming previous investigations.     

Phylogeny of Annelida (Lophotrochozoa): total-evidence analysis of morphology and six genes

Background  

Annelida is one of the major protostome phyla, whose deep phylogeny is very poorly understood. Recent molecular phylogenies show that Annelida may include groups once considered separate phyla (Pogonophora, Echiurida, and Sipunculida) and that Clitellata are derived polychaetes. SThe "total-evidence" analyses combining morphological and molecular characters have been published for a few annelid taxa. No attempt has yet been made to analyse simultaneously morphological and molecular information concerning the Annelida as a whole.     

Spermatogenesis and Spermatozoa in <Emphasis Type="Italic">Stygocapitella subterranea</Emphasis> (Annelida,Parergodrilidae), an Enigmatic Supralittoral Polychaete

The intertidal polychaete species Stygocapitella subterranea (Parergodrilidae) is characterised by extraordinary biology and morphology, resembling those of clitellates and Hrabeiella periglandulata, a terrestrial species of Annelida. An ultrastructural study of the spermatogenesis and spermatozoa was undertaken to elucidate whether these similarities might exhibit adaptive characters typical of annelids with highly derived reproductive modes. A second goal was to find out whether there are some common apomorphic features between S. subterranea and its sister taxon, Parergodrilus heideri, instead of the differences observed on the light-microscopic level, as well as to look for potential synapomorphies to support a suggested relationship to Orbiniidae and Questidae. Spermatogenesis conforms to the general pattern typical of Annelida. Spermatids develop on large cytophores comprising at least 128 cells. The spermatozoa are extremely thread-like and, with a length of about 320 μm, are among the longest spermatozoa known for annelids. The acrosome is elongated and consists only of an acrosomal vesicle with a large subacrosomal space. A conspicuous feature is the incomplete chromatin condensation, resembling late spermatids. In the long midpiece, there is a single ring-shaped mitochondrial derivative, which develops by fusion out of a multiple array of eight mitochondria surrounding the axoneme. There is a distinct annulus between midpiece and tail. The proximal part of the tail is immobile; the axoneme is surrounded by a thick layer of cytoplasm and bears a velum-like extension. In addition to characters apomorphic for S. subterranea, these latter three features exhibit certain similarities to P. heideri that are likely to be synapomorphic. Unfortunately, a relationship of Parergodrilidae to an orbiniid/questid clade does not receive additional support from spermatozoal characters. Similarities with either Clitellata or H. periglandulata are likely to be primarily related to corresponding features of their reproductive biology rather than to phylogenetic relationship.     

Identification of apomorphies and the role of groundpatterns in molecular systematics

Putative apomorphic character states are the only relevant phylogenetic signal contained in sets of sequence data. Using the sequence position as a character, a way to identify putative apomorphies prior to phylogenetic analysis is proposed. It is shown that distance-matrix methods use trivial characters. The concept of the asymmetrical split is presented for determination of character polarity. It is furthermore argued that groundpatterns (node sequences) should be reconstructed prior to the study of relationships between taxa of high phylogenetic age. The 'evolutionary noise'contained in groundpatterns can be illustrated with a network of distances using a split-decomposition analysis.     

Phylogeny of oligochaetous Clitellata

Clitellata, with more than one third of all annelid species described, is briefly introduced, and an overview of the hypotheses of phylogenetic relationships among the groups traditionally referred to as oligochaetes is given. The presentation is placed in a historical context and describes the trend to move from intuitive, narrative approaches to more formal analyses of character patterns. Monophyly of the earthworms (the megadriles, or Metagynophora sensu Jamieson), or at least a major part of them (Crassiclitellata sensu Jamieson), and paraphyly of the ‘microdrile’ largely aquatic, groups are supported by both morphological and molecular data. Further, DNA sequences as well as spermatozoal ultrastructure corroborate that all leech-like taxa (Hirudinida, Acanthobdellida and Branchiobdellida) constitute a clade derived within ‘Oligochaeta’, closely related to the family Lumbriculidae. Molecular systematic studies also support relationships already identified on the basis of morphological data, e.g., the position of Naididae within Tubificidae, the position of Phreodrilidae close to, but outside, the same family, and the putative sistergroup relationship between the newly discovered Capilloventridae and the rest of Clitellata. A recent study using 18S rDNA suggests that Enchytraeidae is closely related to Metagynophora, and that these two taxa, which contain all terrestrial oligochaetous clitellates, form a clade derived from aquatic ‘microdriles’ This refutes a recent hypothesis proposing that the ancestor of Clitellata was terrestrial. To a great extent, however, the basal resolution of the oligochaetous clitellates remains unclear.     

The role of character loss in phylogenetic reconstruction as exemplified for the Annelida

Annelid relationships are controversial, and molecular and morphological analyses provide incongruent estimates. Character loss is identified as a major confounding factor for phylogenetic analyses based on morphological data. A direct approach and an indirect approach for the identification of character loss are discussed. Character loss can frequently be found within annelids and examples of the loss of typical annelid characters, like chaetae, nuchal organs, coelomic cavities and other features, are given. A loss of segmentation is suggested for Sipuncula and Echiura; both are supported as annelid ingroups in molecular phylogenetic analyses. Moreover, character loss can be caused by some modes of heterochronic evolution (paedomorphosis) and, as shown for orbiniid and arenicolid polychaetes, paedomorphic taxa might be misplaced in phylogenies derived from morphology. Different approaches for dealing with character loss in cladistic analyses are discussed. Application of asymmetrical character state transformation costs or usage of a dynamic homology framework represents promising approaches. Identifying character loss prior to a phylogenetic analysis will help to refine morphological data matrices and improve phylogenetic analyses of annelid relationships.