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.