More than a century ago, the simplest of all metazoans was discovered and described as Trichoplax adhaerens. These tiny, flattened animals lack symmetry, mouth, gut, nervous system, and extra-cellular matrix and constitute the apparently monotypic phylum Placozoa. Placozoans diverged early in metazoan history [2–7], making them important organisms for evolutionary research [2,3,8]. Placozoans can be found in warm, shallow, marine environments around the world  and all observed individuals fit the general morphological description of T. adhaerens. Our analyses, however, show that the phylum Placozoa is significantly more diverse than previously thought.
Voigt, O., Collins, A. G., Pearse, V. B., Pearse, J. S., Ender, A., Hadrys, H., & Schierwater, B. (2004, November 23). Placozoa - No longer a phylum of one . Current Biology. Cell Press. https://doi.org/10.1016/j.cub.2004.10.036