solved issues in relation to the phylogenetic status of
To understand the early evolution of the Metazoa, it diploblasts are: whether all the metazoans including is necessary to determine the correct phylogenetic stadiploblasts and triploblasts are monophyletic, whether tus of diploblastic animals (poriferan, cnidaria, and the diplobasts are monophyletic or paraphyletic, and ctenophora). Despite cladistic studies of morphologihow the three diploblast phyla are related.
cal characters and recent molecular phylogenetic
From the morphological standpoint, several authors studies, it remains uncertain whether diploblasts are have supposed that metazoans are polyphyletic (Nurmonophyletic or paraphyletic, and how these three sall, 1962;Hanson, 1977;Inglis, 1985). Poriferans are phyla of diploblasts are phylogenetically related. To sometimes treated as Parazoa, which are independent obtain insight into these phylogenetic problems, we seof other multicellular animals (Eumetazoa) (Hadzi, quenced almost the entire nucleic acid sequence of 1963; Steinbo ¨ck, 1963). In contrast, Conway Morris elongation factor-1␣ from a sponge, two cnidarians, a (1993) proposed the extracellular matrix as a shared ctenophora, and a turbellarian. We then investigated derived feature that unites all multicellular animals as the phylogenetic status of the diploblasts. We coma monophyletic group. In addition, an acetylcholine/ pared the amino acid sequences, nucleotide sequences choline esterase system and flagellated sperm with conat the first and second codon positions, and those at densed chromatin and mitochondria have been prothe second positions. Phylogenetic trees were inferred posed as shared derived characteristics of all metazoby neighbor-joining, maximum likelihood, and maxians (Brusca and Brusca, 1990;Schram, 1991). The mum parsimony, and they supported the monophyly of molecular phylogenetic analyses based upon partial the Metazoa. However, the phylogenetic relationships sequences of 18S and 28S rRNAs failed to support of the diploblast groups were not significantly rethe notion of Metazoan monophyly (Field et al., 1987; solved, although the trees preferred the monophyly of the diploblasts.