June 2002, Volume 82, Issue 1, pp 295–314 | Cite as

The origin of metazoa and the main evolutionary lineages of the animal Kingdom: The gallertoid hypothesis in the light of modern research

Constructional Morphology and Evolution

A scenario for the evolutionary history of the Metazoa is presented, including the evolution of the Urmetazoa and the Ctenophora, Porifera, Coelenterata, and early Bilateria. The reasoning about evolutionary transformations is based on engineering morphology, and includes the results of comparative anatomy and of molecular research. According to this evolutionary model, the specific metazoan multicellularity, which is different of that of fungi and plants, evolved in multinucleate heterotroph unicellular organisms by the deposit of gelatinous/fibrous substance into the endoplasmatic reticulum. The multitude of cells, the syncytia, and the extracellular matrix with gelatinous properties and containing collagenous fibers, developed dependent of each other and simultaneously. The resulting ancestral metazoan, the ur-metazoan, was named ‘gallertoid’ (German: Gallerte = gelatine) for the main feature of its body support, viz., the gelatinous/fibrous material. By further internal differentiation of gallertoids several evolutionary lineages developed independently:Trichoplax, the Ctenophora, the Porifera, the Coelenterata, and the Bilateria (=Coelomata), which soon split into two lines, the Notoneuralia and the Gastroneuralia.
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