Lavrov, Dennis V. Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, Iowa.
Last reviewed:August 2019
- Germ layer inversion
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The name, literally translated as “beside the animals,” coined by William Johnson Sollas (1849–1936) for a phylum (later subkingdom) of animals comprising sponges, which, he believed, evolved independently of other multicellular animals (Enterozoa or Eumetazoa). Erection of a separate subkingdom, the Parazoa, for the sponges was based on the perceived uniqueness of the sponge body plan—the lack of differentiated tissues, nervous system, and terminal differentiation of cells—and the remarkable similarity between sponge choanocytes (collar cells) and unicellular eukaryotes called choanoflagellates. In addition, much importance was given to the hypothesis of “germ layer inversion.” See also: Animal kingdom; Choanoflagellida; Metazoa; Porifera
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