Placozoans: Placozoa

Placozoans (PLACK-uh-zoh-uhns) are shape-shifting blobs up to one-sixteenth of an inch (2 millimeters) in diameter. There is only one species, Trichoplax adhaerens. The grayish white body is covered with tiny structures that look like hairs. Placozoans consist of several thousand cells that form two thin layers of tissue but not organs. There is only one known species of placozoan, and it is a tiny, flattened bag of cells that has a top and a bottom but no front or rear. A narrow space between the two layers of cells is filled with a gel-like substance.

Some scientists believe that placozoans may be the earliest form of animal life. Results of molecular studies suggest that placozoans are closely related to the group that includes corals and jellyfish. If confirmed, this finding would imply that placozoans are a simple form of more complex ancestors that had tissues and organs, including muscles and nerves.

Although the bodies of placozoans are made up of several thousand cells, there are only a few cell types. The top layer is thin and loosely constructed of cover cells that bear a single whiplike structure and contain droplets of fatty material. The bottom layer is made up of a thicker, denser layer of gland cells without a whip as well as other cells with a whip. The fluid-filled space between the top and bottom layers contains a network of loosely organized cells known as fiber cells. The fiber cells are connected to one another and to the top and bottom layers by branched extensions. The fiber cells are thought to function as both muscle and nerve cells. The fiber cells contain bacteria that live in placozoans.