Bryozoa (moss animal)
Bryozoa are so named because some of their colonies resemble moss (a plant group known as "bryophytes"). Bryozoan individuals are tiny, and the lack respiratory and circulatory organs that most larger animals have. They have a U-shaped digestive tract with a mouth, esophagus, stomach, and anus. The ectoderm cells secrete a protective covering into which the animal may withdraw. The middle layer, or mesoderm, includes muscles, gonads (sexual organs), and the nervous system. The endoderm cells line the digestive tract and perform digestive functions.
The small individual animals are called zooids. Zooids live in individual compartments called zooecia, which each animal secretes. The animals asexually bud to form a colony called a zooarium. Sexual reproduction is carried on by hermaphroditic parents (each zooid functions as a male and female). New colonies can develop from buds of other colonies or by the budding of a sexually produced zooid.
Bryozoans are marine and fresh water animals that attach to objects such as rocks or shells. Some bryozoans form reefs in shallow sea water. The fossil remains are the mineralized (calcareous) secretions that protect the individuals and cement the colony together. Bryozoans appeared during the Early Ordovician (ca. 480 million years ago) and continue to the present. Reefs dominated by bryozoans with some corals occur in the northern Lake Champlain valley of northern New York and Vermont. These reefs are the oldest animal-produced reefs on earth.