Planulozoa

Planulozoa is a proposed grouping within the Metazoa that in some schemes unites the simple, multicellular phylum Placozoa with the cnidarians (Cnidaria) and, in certain analyses, also includes the Bilateria. The name is derived from the planula larva stage that is characteristic of many cnidarians, reflecting a suggested ancestral developmental mode shared by members of this clade.

The membership and even the validity of Planulozoa are subjects of ongoing debate. Several genome-scale analyses recover Planulozoa as a monophyletic group in which Placozoa is closely related to Cnidaria, and in some versions Bilateria is placed within Planulozoa. Other studies, however, place Placozoa as sister to Cnidaria without including Bilateria, or fail to recover Planulozoa as a natural clade at all. The controversy stems from differences in data sets, taxon sampling, and analytical methods, as well as the deep and rapid early diversification of animals that can obscure relationships.

As a hypothesis, Planulozoa aims to explain possible shared molecular features and the evolution of early eumetazoan development. Its acceptance influences hypotheses about the origin of certain developmental genes and life-history traits within a broad group that excludes the sponges (Porifera) but includes major animal lineages. Despite its influence on some phylogenetic models, Planulozoa remains one of several competing frameworks for early animal relationships.