The evolutionary history of the placozoanTrichoplax adhaerens is reconstructed from a hypothetical ‘gallertoid’ ancestor. According to this approach the Placozoa derived from an early benthic gallertoid stage, the ‘placuloid’. The sequence of morphological transformations leading to the placozoan bauplan can be deduced from biomechanical and functional constraints. From the viewpoint of constructional morphology the only known placozoan speciesTrichoplax adhaerens represents a member of a basal metazoan stem line which evolved distinctly from other phyla and earlier than sponges and coelenterates. Since theTrichoplax bauplan is still similar to the hypothetical placuloid and gallertoid stages, the given reconstruction provides some specifications for the gallertoid model.