Porifera, Placozoa, Ctenophora, Cnidaria, and Myxozoa are the earliest diverging animal groups (Figures 1 and 2). Available evidence supports Porifera as the basal-most of these taxa. However, in contrast to traditional ideas, the sponges may not form a clade, and the interrelationships of the other basal metazoans are unclear at the present time (Rokas et al., 2003a; Halanych, 2004).
Three main groups of living sponges are recognized: (1) Calcarea, or sponges with a skeleton of calcareous spicules; (2) Demospongiae (including most familiar sponges); and (3) Hexactinellida (the glass sponges, whose bodies are largely composed of syncytial tissues). Both demosponges and hexacti-nellids possess a skeleton of siliceous spicules.
The three groups of sponges share a unique adult body plan that is characterized by the possession of a water canal or aquiferous system, which consists of a complex of spaces inside the sponge body that are in open connection with the surrounding water. These spaces are lined by so-called collar units that are built from either single cells (choanocytes or collar cells) or syncytia (choanosome). These special cells bear a single flagellum that is encircled by a collar of microvilli. This microvillar collar assists in the capture of food particles from the water inflow created by the beating flagella. Sponges possess only a few general types of differentiated cell types, and the nerve cells, special sensory cells, and muscle cells that are characteristic of other metazoans are not developed in sponges (some hexactinellids do show the ability to conduct electrical impulses along their syncytia, and demosponges may possess contractile cells, sometimes referred to as myocytes).
Although traditionally considered to be a mono-phyletic group, recent molecular (SSU) phylogenetic analyses with a broad sampling of basal metazoans and nonmetazoan outgroups have instead found Porifera to be paraphyletic. Intriguingly, the calcar-eans were found to be the sister group to the remaining metazoans, which may be called Eumetazoa, with the demosponges and hexactinel-lids diverging earlier. The precise relationships of the demosponges and hexactinellids to each other and to the clade Calcarea plus Eumetazoa are not yet clear.
The paraphyly of the sponges (Figure 2) provides significant support for the hypothesis that the eume-tazoan ancestor was sponge-like. Sponges have generally not been considered a fertile substratum for the evolution of eumetazoan diversity. Their morphological simplicity on the one hand (e.g., lacking true germ layers, muscles, nervous and sensory systems, and generally, epithelia with basement membranes), and their morphological and functional specialization as epibenthic, sessile, and semipermeable water filters on the other hand, have led sponges to be regarded as a specialized dead end in evolution. Now, possible sponge para-phyly forces us to consider them more seriously as a starting point from which to derive eumetazoan diversity. The recent description of the carnivorous sponge Asbestopluma hypogea, which has completely lost the characteristic poriferan aquiferous system with choanocytes, and thus its ability to filter-feed, exemplifies that in principle the pori-feran body plan has the flexibility to transform into a fully functional macrophagous feeder, a transition that is thought to have taken place during the early evolution of the Metazoa (Vacelet and Duport, 2004).
Furthermore, molecular biological research has shown that the sponge genome contains a surprising array of genes that code for important structural and regulatory molecules present in more derived metazoans, such as the bilaterians (Muller, 2003). These molecules include a range of transmembrane receptor molecules, transcription factors, extracellular matrix proteins, and potential neurotransmitters and antibodies (neu-ronal-like receptors, homeobox genes, tyrosine kinases, and phosphatases, serotonin, crystallin, integrin, fibronectin, and immunoglobulin-like molecules). Extensive gene duplication is also indicated to have taken place before the divergence of the sponges and the remaining metazoans.
Only one species of placozoan has been described so far, Trichoplax adhaerens, and all placozoans known to date conform to the morphological description of T. adhaerens. However, new evidence from nuclear and mitochondrial markers in pla-cozoans collected from different localities around the world shows a remarkable degree of genetic divergence, with genetic distances being equal to those between different families of other marine nonbilaterians (Voigt et al., 2004). Placozoans are small (usually 1-2mm long), flat, ciliated, creeping organisms, without any fixed body axis. The body is composed of just four differentiated somatic cell types, organized into a ventral and a dorsal epithelium, and an enclosed space with so-called fiber cells. The presence of true epithelia in placozoans, with cells connected by belt desmosomes and septate-like junctions has been interpreted as a synapomorphy with all animals excluding sponges, which are generally thought to lack these features. The clade of Placozoa plus all nonsponge animals is therefore sometimes named Epitheliozoa (Figure 2). However, evidence for true epithelia exists in sponges as well, although it is frequently overlooked.
However, the consensus based strictly on morphological evidence that placozoans are the sister group to the Eumetazoa is now contested by accumulating molecular evidence (SSU). Molecular evidence suggests that placozoans may be less basal metazoans than previously thought, diverging from the remaining animals after the morphologically much more complex ctenophores or ctenophores plus cnidarians have branched off (Wallberg et al., 2004). This would imply that pla-cozoans have become morphologically extremely simplified, losing complex differentiated tissues and organs, including a nervous system, muscles, and sensory organs. Although there is some preliminary phylogenomic research that has indicated the possibility that Placozoa are in fact the sister group to all metazoans, including sponges, this finding has not yet been convincingly substantiated.
In addition, efforts are under way to sequence the whole genome of Trichoplax, and genetic studies as well as the first analyses of expression patterns of developmental regulatory genes have started to yield new insights into the biology of Placozoa, such as the presence of genetic signatures of the occurrence of sex in placozoans (placozoan sex has never been observed), and hints indicating that Trichoplax may exhibit a greater degree of histological differentiation than previously thought (Jakob et al., 2004; Signorovitch et al., 2005).
Living ctenophores are very delicate, transparent, mostly pelagic, biradial animals, and they exhibit a passing resemblance to cnidarian jellyfish (hence their vernacular name comb jellies). However, they can be clearly distinguished from medusae by the possession of eight rows of comb plates that run the length of the body. Comb plates consist of closely apposed cilia of several aligned cells. Ctenophores also possess a complex aboral apical organ that is the main sensory center of the animal. An additional unique feature found in many ctenophores is the colloblast, a specialized epidermal cell type located on the tentacles of tentaculate ctenophores that is used for catching prey. Ctenophores possess nerve cells organized in a nerve net at the base of the epidermis and in the mesodermal derivatives. They also possess subepidermal, nonepithelial muscle cells, which are also widespread in bilater-ians, but are primitively absent from the other nonbilaterians, including cnidarians. In common with bilaterians, ctenophore muscles are derived from endomesoderm, although genuine epithelial germ layers are not formed during ctenophore development.
The possession of mesodermally derived non-epithelial muscles has been taken as the main character to indicate a sister group relationship between ctenophores and bilaterians. Several workers defend the union of ctenophores and cnidarians in a clade Coelenterata on the basis of similarities in the early embryonic development of these groups, but molecular evidence (SSU and LSU) does not support either this or the previous hypothesis.
Instead, these data indicate that ctenophores are very basal metazoans, grouping either with calcar-ean sponges, or as a sister group to calcareans and the remaining metazoans. Yet, a recent comprehensively sampled phylogenetic analysis of SSU sequence data dismissed these results as being artifacts due to insufficient taxon sampling, and instead found strong support for a sister group relationship between ctenophores and (Cnidaria plus Bilateria) (Wallberg et al., 2004).
The new basal phylogenetic position of cteno-phores or their close juxtaposition to the much simpler calcarean sponges has interesting implications for our understanding of the course of character evolution if proved correct. It may imply that the morphological features shared between bilaterians, cnidarians, and ctenophores, including well-developed nervous systems, muscles, sensory organs, and digestive system, are either convergent on some level or have been lost altogether in calcar-ean sponges.
Cnidaria is a well-supported monophyletic group, comprising the Anthozoa (including sea anemones and corals), Hydrozoa (including the familiar laboratory animal Hydra and the colonial siphono-phores), Scyphozoa (including most familiar jellyfishes), and Cubozoa (including the notorious sea-wasps). Cnidarians exhibit approximately the same morphological grade of organization as the ctenophores, including a nervous system in the form of a diffuse nerve net (localized nerve concentrations may be found, for example, around the mouth of polyps, and the bell of medusae), epithe-lio-muscle cells (nonepithelial muscle cells in certain groups are thought to have evolved convergently to those in ctenophores and bilaterians), and an alimentary canal.
Cnidarians are definitely more closely related to the bilaterians than the sponges, and although this is a tentative conclusion, they may represent the sister taxon to the bilaterians (Figure 2; Halanych, 2004; Wallberg et al., 2004). This is supported by riboso-mal gene sequences, information from Hox genes, and SSU secondary structure. However, their relationship to placozoans, ctenophores, and myxozoans remains contentious. Significantly, researchers are increasingly focusing on the cnidar-ians to understand the origin of evolutionary novelties at the base of the Bilateria, including the molecular genetic underpinnings of the establishment of the main body axes and bilateral symmetry, the mesodermal germ layer, and sensory organs. As mentioned above for sponges, the genome of cnidarians contains a set of genes involved in the elaboration of complex morphologies in bilaterians, but without forming these complex structures themselves. Studying the functions of these genes in basal metazoans could yield precious insights into the origin of novelties during animal evolution. In contrast, cnidarian genomes also seem to have retained a set of genes from nonmetazoan ancestors, which have been lost in the bilaterians (Technau et al., 2005). This hints at the role of gene loss in animal evolution and underscores the lack of correlation between phenotypic and genomic complexity in metazoans.
The last group of nonbilaterians to be discussed are the myxozoans. Until very recently they were considered to be parasitic protists. Previously invisible to most zoologists, this diverse group of obligate parasites of a host of invertebrates and vertebrates is of substantial economic importance because of the diseases they can cause in commercial fish hosts (Kent et al., 2001). New morphological and molecular evidence has indicated their metazoan affinities. The myxozoan's polar capsules show striking morphological and functional similarities to cnidarian stinging capsules or nematocysts, and myxozoan spores are multicellular. It is currently unclear whether myxozoans are derived cnidarians, or basal bilaterians (Figure 2).
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