The phylum of invertebrates comprising the mesozoans: The Mesozoa include the parasitic wormlike multicellular organisms sometimes considered to be intermediate in complexity between protozoans and metazoans.

The Mesozoa constitute the small phylum of ciliated worms, which are endoparasitic in marine invertebrates.

The small phylum of ciliated, multicellular animals are found as endoparasites (parasites living in the internal organs of animals) in various marine invertebrates.

They are characterized by having a solid, two-layered body lacking skeletal, muscular, nervous, digestive, and excretory elements.

They are all obligate parasites and their simple organization may be secondary, a result of parasitic adaptation and they are found on both sides of the Atlantic, and on the west coast of America.