Moss animals are colonies of very small independent individuals that are only about 1 mm long. There are over 300 species of moss animals (bryozoans) that live in the coastal waters of the Pacific Northwest. These colonies can have a flat shape that cling to rock surfaces, or a branching upright structure, and may also be flexible in nature. The individual zooids that make up a colony each secrete calcium carbonate making a protective structure in which they live. The zooid partially extends out of its home revealing a crown of tentacles to the surrounding water for feeding.
Moss animals (bryozoans) can be mistaken for hydroids and vice versa. Moss animals are colonies of single individuals living together in a structure, whereas hydroids have specialized cells for feeding or reproduction. Under microscopic examination bryozoan tentacle crowns are lined with cilia, unlike hydroid tentacles where the cilia are absent.