The Mesozoa are parasitic worm-like animals. It is unclear as to whether they are degenerate platyhelminthes or truly primitive metazoans. Generally, they consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. At one time they were classified as a phylum, but are now generally believed to be unrelated to one another, so the term mesozoan is applied descriptively rather than taxonomically. Historically Mesozoa was the sole phylum of the subkingdom Agnotozoa. A parasite is an organism that lives in or on the living tissue of a host organism at the expense of it. ... Phyla Placozoa (trichoplax) Orthonectida (orthonectids) Rhombozoa (dicyemids) Subregnum Parazoa Porifera (sponges) Subregnum Eumetazoa Radiata (unranked) (radial symmetry) Ctenophora (comb jellies) Cnidaria (coral, jellyfish, anemones) Bilateria (unranked) (bilateral symmetry) Acoelomorpha (basal) Myxozoa (slime animals) Superphylum Deuterostomia (blastopore becomes anus) Chordata (vertebrates, etc. ... Classes Monogenea Trematoda Cestoda Turbellaria The flatworms (Platyhelminthes, Greek platy: flat; helminth: worm) are a phylum of relatively simple soft-bodied invertebrate animals. ... Phylum (plural: phyla) is a taxon used in the classification of animals, adopted from the Greek phylai the clan-based voting groups in Greek city-states. ... Phyla Placozoa Orthonectida Rhombozoa Agnotozoa is a sub-kingdom of three small phyla of simple animals without organs. ...

The two main mesozoan groups are the Rhombozoa and Orthonectida. Rhombozoa, or Dicyemida, is a phylum of tiny parasites that live in the renal appendages of cephalopods. ... Orthonectida is a small phylum of poorly-known parasites of marine invertebrates that are among the simplest of multi-cellular organisms. ...

Rhombozoa

Rhombozoa, or dicyemid mesozoans, are found in the kidneys of cephalopods. They range from a few millimeters long with twenty to thirty cells that include anterior attachment cells and a long central reproductive cell called an axial cell. This axial cell may develop asexually into vermiform juveniles or it may produce eggs and sperm that self-fertilize to produce a ciliated infusiform larva. Rhombozoa, or Dicyemida, is a phylum of tiny parasites that live in the renal appendages of cephalopods. ... Kidneys viewed from behind with spine removed The kidneys are bean-shaped excretory organs in vertebrates. ... Orders Sepiida Sepiolida Spirulida Teuthida Octopoda Vampyromorphida Nautilida The Cephalopods (head-foot) are the mollusc class Cephalopoda characterized by bilateral body symmetry, a prominent head, and a modification of the mollusc foot into the form of arms or tentacles. ... Vermiform is an adjective meaning worm-like. ...

Orthonectida

Orthonectida are found in the body spaces of various marine invertebrates. Orthonectida is a small phylum of poorly-known parasites of marine invertebrates that are among the simplest of multi-cellular organisms. ...

Other groups included with the mesozoa are the Placozoa and the Monoblastozoa.