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The phylum Mesozoa comprises a small group of parasitic animals that are related to flatworms—a widely dispersed group of free-living and parasitic organisms. Some 50 species have been identified in this phylum to date, all of which are exclusively marine in their lifestyle. Relatively few detailed studies have been conducted on the behavior and life cycles of these species and their taxonomic affiliations are also uncertain.

Mesozoans are all tiny organisms that have a very simple structure: the body wall consists of a thin outer membrane that is dotted with large numbers of small cilia. When they beat, they provide a means of locomotion through the host animal's body fluids. The vast bulk of the mesozoan body consists of egg and sperm cells. There are no specialized feeding organs, and nutrients and waste products pass directly across the cell wall.

Mesozoans are parasites of a wide range of marine invertebrates, including flatworms, roundworms, mollusks, and echinoderms. Two main groupings have been recognized: the order Orthonectida, which are free-living within their hosts, and the Dicyemida, which have specialized cells on the body wall that serve to attach them to the walls of the kidneys in species such as octopus and squid.

In most species, fertilized eggs are released inside the host organism and may either remain in the host system or pass out of the body with waste materials. The precise mechanisms of host detection and determination are not known. As free-swimming larvae, some juvenile mesozoans may enter other species such as fish or crustaceans prior to being ingested by their final hosts.

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