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In the phylum Placozoa, only two species have been described, Trichoplax adhaerens Schulze (1883) and Treptoplax reptans Monticelli (1893). Whereas the former has been reported from tropical and subtropical localities around the world, the latter has not been observed since Monticelli's original description, and its validity is generally doubted. Thus, the high level of genetic diversity in our samples from a relatively small number of localities is quite surprising. Four separate molecular markers (nuclear 18S, ITS1-5.8S-ITS2, and 28S, and mitochondrial 16S)indicate that Placozoa comprises at least four distinct, deeply divided clades, all of which fit the gross morphological description of T. adhaerens. Whereas molecular differences within the four groups are modest, differences between the four clades appear large (for example, partial 16S lengths range from 544 to 689 bp). The deep divisions between the clades are comparable to those seen between different families or orders of cnidarians, suggesting a similarly long history. So far, no straightforward relationship has emerged between placozoan clades and biogeography or habitat. For instance, multiple groups are present on both the Pacific and Caribbean sides of Panama, in two cases at the same site, but a single group dominates our samples from both regions. Continued sampling from diverse localities and habitats from around the world promises to provide a more accurate picture of biodiversity in this supposedly monotypic phylum, and scrutiny of the ultrastructure of placozoans in the four clades may reveal diagnostic differences in morphology. Indeed, the discovery of a large complex of nested placozoan taxa now seems inevitable.

The placozoan Trichoplax adhaerens displays the s mplest morphology of any free-living animal, can be easily maintained in the laboratory, and possesses the smallest genome of any animal yet measured (50 Mb). A consortium of marine invertebrate biologists, geneticists and genomic scientists has been assembled to begin the task of genome sequencing, annotation and gene analysis of the Trichoplax genome. We will report on biological, genomic and informatic resources now under development for this system. A geographically diverse collection of laboratory strains has been established. The Trichoplax Genome Project has completed a confirmation of the genome size, gene density estimates, AT:GC content. The composition of several large contiguous genomic regions and the mitochondrial genome sequence and content. An arrayed fosmid library and small-insert genomic library has been finished and a full-length cDNA library is currently under construction for EST and microarray investigations. Collectively these efforts are expected to collectively generate the suite of resources expected in a modern experimental model.

A more than a century old debate has been focusing on the origin of the Metazoa and the hypothetical urmetazoan bauplan. A basal position for the morphologically most simply organized metazoan animal, the placozoan Trichoplax adhaerens, was widely accepted for almost a century. The limited number of only four somatic cell types, the smallest metazoan genome, and the lack of any foot or head structures, of any anterior-posterior organization, any kind of organs, and of both a basal lamina and an extracellular matrix (ECM) placed Trichoplax in a basal and isolated position relative to all other metazoan phyla. Butschlis placula hypothesis of the urmetazoon was based on this view and provided a hypothetical scenario for both, the evolution of diploblast and triploblast bauplans. We have been analysing (i) the sum of morphological data, (ii) the molecular sequence data from two mitochondrial genes, COII and COIII, (iii) the molecular sequence data from two regulatory nuclear genes, Cnox-2 and the PaxB gene, (iv) the secondary structure of the 16S rRNA molecule, and (v) the putative cluster organization of Hox genes. The evidence strongly supports the view that Placozoa are basal relative to all other recent diploblast phyla. Furthermore, the comparison of Hox/ ParaHox gene expression patterns provide an intriguing hypopthetical scenario for Butschlis original placula hypothesis and thus the hypothetical urmetazoan bauplan.

Cnidaria has been the subject of several analyses of molecular data that attempt to resolve evolutionary relationships among and within its component groups. These studies have renewed interest in cnidarian character evolution and may be partly responsible for spurring the numerous investigations of cnidarian relationships based on morphological and life history features. This bounty of analyses aimed at achieving the single best phylogenetic hypothesis has produced an unprecedented number of alternative ideas about the evolutionary history of Cnidaria. Great uncertainty would appear to exist when only a short time ago we enjoyed unprecedented optimism concerning the tractability of phylogenetic questions. However, independently proposed phylogenetic trees based on diverse data, when looked at as a set of hypotheses, show strong congruence of results. We further clarify the emerging view of cnidarian phylogeny by using nearly complete 28S rDNA sequences (roughly 3,300 bp). Diverse data now indicate that Hydrozoa and Cubozoa are monophyletic, while Scyphozoa is not. The sessile members of Stauromedusae are likely to be the sister group of all other medusozoans. Thus, the ancestral medusozoan was probably solitary, biradial in its early ontogeny, with four intramesogleal muscles of ectodermal origin associated with peristomial pits surrounding the mouth. From this state, hydrozoan and cubozoan polyps are probably simplified. The ancestral hydrozoan probably produced medusae with a velum via lateral budding from polyps and development involving an entocodon. Gastric filaments, hollow sensory structures derived ontogenetically from tentacles, a circular coronal muscle, and septa of the medusa probably were lost in the ancestry of Hydrozoa, perhaps indicating (though not confirming) independent origins of the medusa.