Phylogeny of protostome worms derived from 18S rRNA sequences

The phylogenetic relationships of protostome worms were studied by comparing new complete 18S rRNA sequences of Vestimentifera, Pogonophora, Sipuncula, Echiura, Nemertea, and Annelida with existing 18S rRNA sequences of Mollusca, Arthropoda, Chordata, and Platyhelminthes. Phylogenetic trees were inferred via neighbor-joining and maximum parsimony analyses. These suggest that (1) Sipuncula and Echiura are not sister groups; (2) Nemertea are protostomes; (3) Vestimentifera and Pogonophora are protostomes that have a common ancestor with Echiura; and (4) Vestimentifera and Pogonophora are a monophyletic clade.      

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 position of Sipuncula derived from multi‐gene and phylogenomic data and its implication for the evolution of segmentation

The phylogenetic position of Sipuncula, a group of unsegmented marine worms, has been controversial for several decades: Especially based on morphological data, closer relationships to Mollusca or Annelida were among the most favoured hypotheses. Increasing amounts of molecular data in recent years have consistently placed Sipuncula either in close affinity to or even within Annelida, the segmented worms, and rejected a close relationship to Mollusca. Yet, it remained uncertain whether Sipuncula is the sister group of Annelida or an annelid subtaxon. Therefore, herein we gathered data for five nuclear genes, which have been rarely used regarding Annelida and Sipuncula, and combined these with data for six previously used genes to further elucidate the phylogenetic position of Sipuncula. We also compiled a data set for 78 ribosomal proteins from publicly available genomic data sets. These are the two largest data sets for annelids with more than 10 taxa to date. All analyses placed Sipuncula within Annelida. For the first time, topology tests significantly rejected the possibility that Sipuncula is sister to Annelida. Thus, our analyses revealed that Sipuncula had secondarily lost segmentation. Given that unsegmented Echiura is also an annelid subtaxon, segmentation, a key character of Annelida, is much more variable than previously thought. Yet, this conclusion does not support the hypothesis that the last common ancestor of Annelida, Arthropoda and Chordata was segmented, assuming several losses along the branches leading to them. As yet no traces of segmentation could be shown in taxa exhibiting serially organized organ systems such as certain Mollusca, while in Sipuncula and Echiura such traces could be demonstrated. An independent origin of segmentation in Annelida, Arthropoda and Chordata thus appears to be more plausible and parsimonious.     

Phylogenetic Relationships among Higher Nemertean (Nemertea) Taxa Inferred from 18S rDNA Sequences

We estimated the phylogenetic relationships of 15 nemertean (phylum Nemertea) species from the four subclasses Hoplo-, Hetero-, Palaeo-, and Bdellonemertea with 18S rDNA sequence data. Three outgroup taxa were used for rooting: Annelida, Platyhelminthes, and Mollusca. Parsimony and maximum-likelihood analyses supported the monophyletic status of the Heteronemertea and a taxon consisting of hoplonemerteans and Bdellonemertea, while indicating that Palaeonemertea is paraphyletic. The monophyletic status of the two nemertean classes Anopla and Enopla is not supported by the data. The unambiguous clades are well supported, as assessed by a randomization test (bootstrapping) and branch support values.     

Improvement of molecular phylogenetic inference and the phylogeny of Bilateria

Inferring the relationships among Bilateria has been an active and controversial research area since Haeckel. The lack of a sufficient number of phylogenetically reliable characters was the main limitation of traditional phylogenies based on morphology. With the advent of molecular data, this problem has been replaced by another one, statistical inconsistency, which stems from an erroneous interpretation of convergences induced by multiple changes. The analysis of alignments rich in both genes and species, combined with a probabilistic method (maximum likelihood or Bayesian) using sophisticated models of sequence evolution, should alleviate these two major limitations. We applied this approach to a dataset of 94 genes and 79 species using CAT, a previously developed model accounting for site-specific amino acid replacement patterns. The resulting tree is in good agreement with current knowledge: the monophyly of most major groups (e.g. Chordata, Arthropoda, Lophotrochozoa, Ecdysozoa, Protostomia) was recovered with high support. Two results are surprising and are discussed in an evo-devo framework: the sister-group relationship of Platyhelminthes and Annelida to the exclusion of Mollusca, contradicting the Neotrochozoa hypothesis, and, with a lower statistical support, the paraphyly of Deuterostomia. These results, in particular the status of deuterostomes, need further confirmation, both through increased taxonomic sampling, and future improvements of probabilistic models.     

Molecular Phylogeny of Metazoan Intermediate Filament Proteins

We have cloned cytoplasmic intermediate filament (IF) proteins from a large number of invertebrate phyla using cDNA probes, the monoclonal antibody IFA, peptide sequence information, and various RT-PCR procedures. Novel IF protein sequences reported here include the urochordata and nine protostomic phyla, i.e., Annelida, Brachiopoda, Chaetognatha, Echiura, Nematomorpha, Nemertea, Platyhelminthes, Phoronida, and Sipuncula. Taken together with the wealth of data on IF proteins of vertebrates and the results on IF proteins of Cephalochordata, Mollusca, Annelida, and Nematoda, two IF prototypes emerge. The L-type, which includes 35 sequences from 11 protostomic phyla, shares with the nuclear lamins the long version of the coil 1b subdomain and, in most cases, a homology segment of some 120 residues in the carboxyterminal tail domain. The S-type, which includes all four subfamilies (types I to IV) of vertebrate IF proteins, lacks 42 residues in the coil 1b subdomain and the carboxyterminal lamin homology segment. Since IF proteins from all three phyla of the chordates have the 42-residue deletion, this deletion arose in a progenitor prior to the divergence of the chordates into the urochordate, cephalochordate, and vertebrate lineages, possibly already at the origin of the deuterostomic branch. Four phyla recently placed into the protostomia on grounds of their 18S rDNA sequences (Brachiopoda, Nemertea, Phoronida, and Platyhelminthes) show IF proteins of the L-type and fit by sequence identity criteria into the lophotrochozoic branch of the protostomia. Received: 2 April 1998 / Accepted: 19 June 1998     

Phylogeny of the Metazoa Based on Morphological and 18S Ribosomal DNA Evidence

Cladistic analysis of traditional (i.e. morphological, developmental, ultrastructural) and molecular (18S rDNA) data sets (276+501 informative characters) provides a hypothesis about relationships of all meta-zoan higher taxa. Monophyly of Metazoa, Epith-eliozoa (= -03non-Porifera), Triploblastica, Mesozoa, Eutriploblastica (=Rhabditophora+Catenulida+“higher triploblasts”=Neotriploblastica, including Xeno- turbellida and Gnathostomulida), Rhabditophora, Syndermata (=“Rotifera”+Acanthocephala), Neotrichozoa (=Gastrotricha+Gnathostomulida), Nematozoa (=Nematoda+Nematomorpha), Panarthropoda (=Onychophora+Tardigrada+ Arthropoda), Cephalorhyncha, Deuterostomia, Ambulacralia (=Hemichordata+Echinodermata), Chordata, Phoronozoa (=Phoronida+“Brachiopoda”), Bryozoa, Trochozoa (=Eutrochozoa+Entoprocta+ Cycliophora), Eutrochozoa, and Chaetifera (=Annelida+ Pogonophora+Echiura) is strongly supported. Cnidaria (including Myxozoa), Ecdysozoa (=Cepha- lorhyncha + Nematozoa + Chaetognatha + Panarthropoda), Eucoelomata (=Bryozoa+Phoronozoa+Deuterostomia+Trochozoa, possibly including also Xenoturbellida), and Deuterostomia+Phoronozoa probably are monophyletic. Most traditional “phyla” are monophyletic, except for Porifera, Cnidaria (excluding Myxozoa), Platyhelminthes, Brachiopoda, and Rotifera. Three “hot” regions of the tree remain quite unresolved: basal Epitheliozoa, basal Triploblastica, and basal Neotriploblastica. A new phylogenetic classification of the Metazoa including 35 formally recognized phyla (Silicispongea, Calcispongea, Placozoa, Cnidaria, Ctenophora, Acoela, Nemertodermatida, Orthonecta, Rhombozoa, Rhabditophora, Catenulida, Syndermata, Gnathostomulida, Gastrotricha, Cephalorhyncha, Chaetognatha, Nematoda, Nematomorpha, Onychophora, Tardigrada, Arthropoda, Echinodermata, Hemichordata, Chordata, Phoronozoa, Bryozoa s. str., Xenoturbellida, Entoprocta, Cycliophora, Nemertea, Mollusca, Sipuncula, Echiura, Pogonophora, and Annelida) and few i ncertae sedis g roups (e.g. Myzostomida and Lobatocerebromorpha) is proposed.     

Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida,Cycliophora, Plathelminthes,and Chaetognatha: a combined approach of 18S rDNA sequences and morphology

Triploblastic relationships were examined in the light of molecular and morphological evidence. Representatives for all triploblastic "phyla" (except Loricifera) were represented by both sources of phylogenetic data. The 18S ribosomal (rDNA) sequence data for 145 terminal taxa and 276 morphological characters coded for 36 supraspecific taxa were combined in a total evidence regime to determine the most consistent picture of triploblastic relationships for these data. Only triploblastic taxa are used to avoid rooting with distant outgroups, which seems to happen because of the extreme distance that separates diploblastic from triploblastic taxa according to the 18S rDNA data. Multiple phylogenetic analyses performed with variable analysis parameters yield largely inconsistent results for certain groups such as Chaetognatha, Acoela, and Nemertodermatida. A normalized incongruence length metric is used to assay the relative merit of the multiple analyses. The combined analysis having the least character incongruence yields the following scheme of relationships of four main clades: (1) Deuterostomia [((Echinodermata + Enteropneusta) (Cephalochordata (Urochordata + Vertebrata)))]; (2) Ecdysozoa [(((Priapulida + Kinorhyncha) (Nematoda + Nematomorpha)) ((Onychophora + Tardigrada) Arthropoda))]; (3) Trochozoa [((Phoronida + Brachiopoda) (Entoprocta (Nemertea (Sipuncula (Mollusca (Pogonophora (Echiura + Annelida)))))))]; and (4) Platyzoa [((Gnathostomulida (Cycliophora + Syndermata)) (Gastrotricha + Plathelminthes))]. Chaetognatha, Nemertodermatida, and Bryozoa cannot be assigned to any one of these four groups. For the first time, a data analysis recognizes a clade of acoelomates, the Platyzoa (sensu Cavalier-Smith, Biol. Rev. 73:203-266, 1998). Other relationships that corroborate some morphological analyses are the existence of a clade that groups Gnathostomulida + Syndermata (= Gnathifera), which is expanded to include the enigmatic phylum Cycliophora, as sister group to Syndermata.     

Phylogenetic study of the oxytocin-like immunoreactive system in invertebrates

1. A phylogenetic study of oxytocin (OXT)-like immunoreactive cells was performed by the PAP method in the central nervous system of invertebrates. 2. The immunoreactivity was detected in the nerve cells of Hydra magnipapillata of the Coelenterata; Neanthes japonica and Pheretima communissima of the Annelida; Oncidium verrucosum, Limax marginatus and Meretrix lamarckii of the Mollusca; and Baratha brassica of the Arthropoda. 3. No immunoreactive cells were found in Bipalium sp. of the Platyhelminthes; Pomacea canaliculata, Aplysia kurodai, Bradybaena similaris and Achatina fulica of the Mollusca; and Gnorimosphaeroma rayi, Procambarus clarkii, Hemigrapsus sanguineus, Helice tridens and Gryllus bimaculatus of the Arthropoda; Asterina pectinifera of the Echinodermata; and Halocynthia roretzi of the Protochordata. 4. These results demonstrate that an OXT-immunoreactive substance is widely present not only in vertebrates but also in invertebrates. 5. OXT seems to have been introduced into these invertebrates at an early stage of their phylogenetic history.     

The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes

Recent hypotheses on metazoan phylogeny have recognized three main clades of bilaterian animals: Deuterostomia, Ecdysozoa and Lophotrochozoa. The acoelomate and 'pseudocoelomate' metazoans, including the Platyhelminthes, long considered basal bilaterians, have been referred to positions within these clades by many authors. However, a recent study based on ribosomal DNA placed the flatworm group Acoela as the sister group of all other extant bilaterian lineages. Unexpectedly, the nemertodermatid flatworms, usually considered the sister group of the Acoela together forming the Acoelomorpha, were grouped separately from the Acoela with the rest of the Platyhelminthes (the Rhabditophora) within the Lophotrochozoa. To re-evaluate and clarify the phylogenetic position of the Nemertodermatida, new sequence data from 18S ribosomal DNA and mitochondrial genes of nemertodermatid and other bilaterian species were analysed with parsimony and maximum likelihood methods. The analyses strongly support a basal position within the Bilateria for the Nemertodermatida as a sister group to all other bilaterian taxa except the Acoela. Despite the basal position of both Nemertodermatida and Acoela, the clade Acoelomorpha was not retrieved. These results imply that the last common ancestor of bilaterian metazoans was a small, benthic, direct developer without segments, coelomic cavities, nephrida or a true brain. The name Nephrozoa is proposed for the ancestor of all bilaterians excluding the Nemertodermatida and the Acoela, and its descendants.     

Conservation and diversification of Msx protein in metazoan evolution

Msx (/msh) family genes encode homeodomain (HD) proteins that control ontogeny in many animal species. We compared the structures of Msx genes from a wide range of Metazoa (Porifera, Cnidaria, Nematoda, Arthropoda, Tardigrada, Platyhelminthes, Mollusca, Brachiopoda, Annelida, Echiura, Echinodermata, Hemichordata, and Chordata) to gain an understanding of the role of these genes in phylogeny. Exon-intron boundary analysis suggested that the position of the intron located N-terminally to the HDs was widely conserved in all the genes examined, including those of cnidarians. Amino acid (aa) sequence comparison revealed 3 new evolutionarily conserved domains, as well as very strong conservation of the HDs. Two of the three domains were associated with Groucho-like protein binding in both a vertebrate and a cnidarian Msx homolog, suggesting that the interaction between Groucho-like proteins and Msx proteins was established in eumetazoan ancestors. Pairwise comparison among the collected HDs and their C-flanking aa sequences revealed that the degree of sequence conservation varied depending on the animal taxa from which the sequences were derived. Highly conserved Msx genes were identified in the Vertebrata, Cephalochordata, Hemichordata, Echinodermata, Mollusca, Brachiopoda, and Anthozoa. The wide distribution of the conserved sequences in the animal phylogenetic tree suggested that metazoan ancestors had already acquired a set of conserved domains of the current Msx family genes. Interestingly, although strongly conserved sequences were recovered from the Vertebrata, Cephalochordata, and Anthozoa, the sequences from the Urochordata and Hydrozoa showed weak conservation. Because the Vertebrata-Cephalochordata-Urochordata and Anthozoa-Hydrozoa represent sister groups in the Chordata and Cnidaria, respectively, Msx sequence diversification may have occurred differentially in the course of evolution. We speculate that selective loss of the conserved domains in Msx family proteins contributed to the diversification of animal body organization.     

Larval Planktotrophy—A Primitive Trait in the Bilateria?

The concept of Gösta Jägersten of a primary biphasic metazoan life-cycle, consisting of a planktotrophic larva and a benthic adult, forms the basis for several theories on metazoan phylogeny. In this paper the assumed planktotrophic life-style of the larva is critically analyzed and reconsidered. It is shown, in particular for the Mollusca, that a biphasic life-cycle with a lecithotrophic larva is probably the plesiomorphic condition. Character distribution and structural data suggest a parallel evolution of the downstream collecting system used in planktotrophic larvae or filter-feeding adults of gastropods, bivalves and other spiralian or aschelminth taxa. In the basic metazoans (Parazoa, Placozoa, coelenterates) direct or lecithotrophic development dominates by far. For the acoelomate (Platyhelminthes, Gnathostomulida) and pseudocoelomate taxa direct development is probably the plesiomorphic condition. The structural similarities of the upstream collecting system in tentaculate and deuterostome phyla may also be explained by parallel events of heterochrony out of an ancestor with adult filter-feeding. The main conclusion of this survey is that larval planktotrophy is likely to be secondary and not a plesiomorphic condition among the Bilateria. Accordingly, theories which are based on the assumed plesiomorphy of larval planktotrophy of the Bilateria, need careful reevaluation.     

Phylogenetic study of the arginine-vasotocin/arginine-vasopressin-like immunoreactive system in invertebrates

1. A phylogenetic study of arg-vasotocin (AVT)/arg-vasopressin (AVP)-like immunoreactive cells was performed by the PAP method in the central nervous system of invertebrates. 2. The immunoreactivity was detected in the nerve cells of Hydra magnipapillata of the Coelenterata; Neanthes japonica and Pheretima communissima of the Annelida; Pomacea canaliculata, Aplysia kurodai, Oncidium verrucosum, Bradybaena similaris, Achatina fulica, Limax marginatus and Meretrix lamarckii of the Mollusca; Gnorimosphaeroma rayi, Hemigrapsus sanguineus, Gryllus bimaculatus and Baratha brassicae of the Arthropoda; Asterina pectinifera of the Echinodermata; and Halocynthia roretzi of the Protochordata. 3. No immunoreactivity was detected in Bipalium sp. of the Platyhelminthes, or in Procambarus clarkii and Helice tridens of the Arthropoda. 4. From these results, it appears that AVT/AVP is a phylogenetically ancient peptide which is present in a wide variety of invertebrates. 5. The actions of AVT/AVP and its presence in invertebrates are discussed.     

Metagenomic analysis of the marine coastal invertebrates of South Korea as assessed by Ilumina MiSeq

Metagenomic analysis was carried out for the first time on the marine coastal invertebrates of South Korea. Samples collected from coastal areas of Korea were filtered through a 63 µm mesh, then their 18S rDNA V4 regions were amplified. High-throughput sequencing of PCR amplicons using Illumina MiSeq and BLAST against the SILVA database showed that a total of 319 eukaryotic Operational Taxonomic Units were identified at the species level. Annelida, Arthropoda, Mollusca, Nematoda, and Platyhelminthes and for 92.23% of the total 103 metazoan species belonging to 101 genera, 75 families, and 10 phyla. Of these, several taxa previously unreported to exist in Korea were detected at the family level compared with the integrated database from three main Korean biodiversity DBs (MABIK, KOMBIS, and MRBR). Analysis of beta diversity of the community structure of invertebrates indicated that the composition of marine invertebrates is likely to be affected by habitat type rather than geographical distance. The present study showed that metagenomic high-throughput technology can be used to unravel species diversity and for various studies regarding marine invertebrate community structure.     

Remarkably preserved benthic organisms and their traces from a Middle Triassic (Muschelkalk) mud flat

Knaust, D. 2010: Remarkably preserved benthic organisms and their traces from a Middle Triassic (Muschelkalk) mud flat. Lethaia, Vol. 43, pp. 344–356. A new Fossil‐Lagerstätte is reported from the Middle Triassic of Germany, preserving Foraminifera, Nematoda, Platyhelminthes, Nemertea, Annelida and a range of Arthropoda together with their traces. This is the oldest fossil record of free‐living nematodes and turbellarians, and the first occurrence of nemerteans in the Mesozoic. The rare preservation of the benthic associations together with their traces is unique; especially the abundant occurrence of different phyla as meiofauna (organisms with shortest dimension between 0.06 and 1 mm) provides an insight into the palaeoecological conditions of a 240‐Ma‐old muddy tidal flat. The preservation of benthic animals at the termination of their traces offers the exclusive opportunity to assign producers to the trace fossils. The results show that the discussed phyla were already established in the early Mesozoic in a similar diversity and composition as in modern analogues. The new Fossil‐Lagerstätte has the potential to prove a number of soft‐bodied taxa immediately after the end‐Permian mass extinction, which has wide‐ranging implications for phylogenetic interpretations. □Benthos, soft‐bodied organisms, Fossil‐Lagerstätte, Muschelkalk, trace fossils, Triassic.     

Distribution of pelagic larvae of benthic marine invertebrates in the Bellingshausen Sea

During November and December 1992, plankton samples were collected using a ring net of mesh size 200 m vertically hauled through a 600 m water column, at five stations along a transect running north from the Allison Peninsula in the Bellingshausen Sea. Three stations were located over the continental shelf; two of these were ice bound, whilst the third was at the ice edge. Two other stations were in deeper, ice-free water. Sixteen different larval and juvenile types were found representing seven phyla: Echinodermata, Nemertea, Coelenterata, Mollusca, Annelida, Arthropoda and Bryozoa, of which the first two were the most abundant. Larval numbers and types decreased with distance offshore and away from permanent sea ice. The presence of many stages of nemertean larval development within a short time scale, in an area where developmental tends to be slow, suggests that reproduction occurs over an extended period and that the larvae have a long planktonic phase. The increased size of later developmental stages of the nemertean larvae indicates they obtain nutrition within the water column during winter, when little particulate food is present.     

Occurrence,abundance and distribution of benthic macroinvertebrates in the Nyando River catchment,Kenya

A baseline study was conducted of the occurrence of macroinvertebrates at 26 sites in the Nyando River catchment in 2005–2006. A total of 13 orders and 16 families of Arthropoda, Mollusca, Platyhelminthes and Annelida were collected, with the order Ephemeroptera being most abundant in the up- and mid-stream reaches, followed by Hemiptera and Plecoptera respectively. The downstream sections of the river were dominated by Hirudinea and tubificids, as the water quality deteriorated mainly due to local land use, raw sewage effluent discharge and annual floods. Insects and annelids were the main invertebrates found and the extent of pollution increased from mid-section (Site 15) downwards as the river flowed into the Winam Gulf. Stringent management measures are required to safeguard the environment and ecosystems of Lake Victoria.     

Protonephridia in the larvae of the paleonemertean species Carinoma mutabilis (Carinomidae,Nemertea) and Cephalothrix (Procephalothrix) filiformis (Cephalothricidae,Nemertea)

During spiralian development, the first pair of nephridia forms anterior to the mouth. Each organ consists of a few cells, which is characteristic for spiralian larvae. In nemerteans, one of the unambiguously spiralian taxa, so far protonephridia, has been reported only in advanced pilidium larvae, where they likely persist as juvenile and adult nephridia. These organs have not been recorded in larvae of the basally branching nemertean taxa. In search for these organs, we examined the ultrastructure of pelagic planuliform larvae of the palaeonemerteans Carinoma mutabilis and Cephalothrix (Procephalothrix) filiformis. In both species, a pair of protonephridia is located at the level of the stomodaeum. Each protonephridium of C. mutabilis consists of two terminal cells, two duct cells and one nephropore cell, while that of C. filiformis consists of three terminal cells, three duct cells and one nephropore cell. In C. mutabilis and in C. filiformis, all terminal cells contribute to forming a compound filtration structure. In both species, the protonephridia seem to develop subepidermally, since in C. filiformis, the nephropore cells pierce the larval epidermis and in C. mutabilis, the nephropores are initially covered by the binucleated multiciliated trophoblast cells. On the fifth day, these cells degenerate, so that the protonephridium becomes functional. The occurrence of protonephridia in the larvae of both paleonemertean species is in accordance with the hypothesis that a common ancestor of Nemertea and Trochozoa had a larval stage with a pair of protonephridia. This does not contradict previous hypotheses on placing the Nemertea as an ingroup of the Trochozoa or Spiralia (= Lophotrochozoa). Whether these protonephridia are restricted to the larval phase or whether they are transformed into the adult protonephridia, like those of the pilidium larva, remains to be answered.     

Polychaete phylogeny based on morphological data – a comparison of current attempts

Annelid phylogeny is one of the largest unresolved problems within the Metazoa. This is due to the enormous age of this taxon and also strongly influenced by the current discussion on the position of the Arthropoda, which traditionally is hypothesized to be the annelid sister taxon. Within the framework of recent discussions on the position of the Annelida, the ground pattern of this taxon is either a clitellate-like, parapodia-less dwelling organism or an organisms that resembles errant polychaetes in having parapodia and gills and probably being a predator. To solve this problem different attempts have been made in the past, cladistic analysis, scenario based plausibility considerations and a successive search for sister taxa base on isolated characters. These attempts are presented and critically discussed. There is at least strong support for the Annelida as wells as for several of its taxa above the level of traditional families; the monophyly of the Polychaeta, however, remains questionable. The term taxon is used here in the sense of group of things that share certain characteristics. Biological taxa are not necessarily monophyletic, although many of them turned out to be. In terms of phylogenetic systematics taxa should be monophyletic.     

Bilaterian Phylogeny Based on Analyses of a Region of the Sodium–Potassium ATPase β-Subunit Gene

Molecular investigations of deep-level relationships within and among the animal phyla have been hampered by a lack of slowly evolving genes that are amenable to study by molecular systematists. To provide new data for use in deep-level metazoan phylogenetic studies, primers were developed to amplify a 1.3-kb region of the subunit of the nuclear-encoded sodium–potassium ATPase gene from 31 bilaterians representing several phyla. Maximum parsimony, maximum likelihood, and Bayesian analyses of these sequences (combined with ATPase sequences for 23 taxa downloaded from GenBank) yield congruent trees that corroborate recent findings based on analyses of other data sets (e.g., the 18S ribosomal RNA gene). The ATPase-based trees support monophyly for several clades (including Lophotrochozoa, a form of Ecdysozoa, Vertebrata, Mollusca, Bivalvia, Gastropoda, Arachnida, Hexapoda, Coleoptera, and Diptera) but do not support monophyly for Deuterostomia, Arthropoda, or Nemertea. Parametric bootstrapping tests reject monophyly for Arthropoda and Nemertea but are unable to reject deuterostome monophyly. Overall, the sodium–potassium ATPase -subunit gene appears to be useful for deep-level studies of metazoan phylogeny.