Agnotozoa is the name of a taxon of simple animals. The name first appeared in an invertebrate paleontology book as one of the "branches" of the subkingdom Metazoa. The branch contained only one group: Mesozoa.
More recently, some have used the name to refer to a sub-kingdom of three small phyla of simple animals without organs. These phyla are Placozoa, Orthonectida, and Rhombozoa. They still have differentiated tissue, but that tissue is only organized in simple ways; for example, by being layered. The Orthonectida and Rhombozoa are also grouped into the Mesozoa.
Biologists today generally do not use the taxon Agnotozoa. There are a few likely reasons for this.
- There is doubt that placozoans are closely related to mesozoans.
- There is doubt even that the two mesozoan groups (orthonectids and rhombozoans) are related to each other.
- Even if the two mesozoan groups are related, there is little need for another name in addition to mesozoa.
Wikispecies places the mesozoans in kingdom protista.