Fossilized Bryozoa, Ordovician limestone, Batavia, Ohio

Bryozoans ("moss animals") are tiny colonial animals that generally build stony skeletons of calcium carbonate, superficially similar to coral. They generally prefer warm, tropical waters but even today are known to occur worldwide. Fossil bryozoans are found in rocks beginning in the Ordovician. There are about 5000 living species, with several times that number of fossil forms known.

Most species of Bryozoan live in marine environments, though there are about 50 species which inhabit freshwater. In their aquatic habitats, bryozoans may be found on all types of hard substrates: sand grains, rocks, shells, wood, blades of kelp, pipes and ships may be heavily encrusted with bryozoans. Some bryozoan colonies, however, do not grow on solid substrates, but form colonies on sediment. While some species have been found at depths of 8200 meters, most bryozoans inhabit much shallower water. Most bryozoans are sessile and immobile, but a few colonies are able to creep about, and a few species of non-colonial bryozoans live and move about in the spaces between sand grains. One remarkable species makes its living while floating in the Antarctic ocean.

Bryozoans are also colony-forming animals. A few to many millions of individuals may form one colony. The colonies range from millimeters to meters in size, but the individuals that make up the colonies are tiny, usually less than a millimeter long. In each colony, different individuals assume different functions. Some individuals gather up the food for the colony (autozooids), others depend on them (heterozooids). Some individuals are devoted to strengthening the colony (kenozooids), and still others to cleaning the colony (vibracula). There is only a single known solitary species, Monobryozoon ambulans, which does not form colonies.

Bryozoans skeletons grow in a variety of shapes and patterns: mound-shaped, lacy fans, branching twigs, and even cork screw-shaped. Their skeletons have numerous tiny openings, each of which is the home of a minute animal called a zooid. They also have a coelomate body with a "blind-ended" gut. This "gut" is a pouch-like intestinal sack where the same opening forms both the mouth and anus. They feed with a specialized, ciliated structure called a lophophore that is attached to tentacles surrounding the mouth. Their diet consists of small microorganisms, including diatoms and other unicellular algae. In turn, bryozoans are preyed on by grazing organisms such as sea urchins and fish. Bryozoans do not have any defined respiratory, circulatory or nerve systems due to their small size. However, they do have a hydrostatic skeletal system.

Because of their small size, Bryozoans have no need of a blood system. Gaseous exchange occurs across the entire surface of the body, but particularly through the tentacles of the lophophore.

Bryozoans can reproduce both sexually and asexually. All Bryozoans, as far as is known, are hermaphrodite (meaning they are both male and female). Asexual reproduction occurs by budding off new zooids as the colony grows, and is this the main way by which a colony expands in size. If a piece of a bryozoan colony breaks off, the piece can continue to grow and will form a new colony. A colony formed this way is composed entirely of clones (genetically identical individuals) of the first animal, which is called the ancestrula.

The phylum Bryozoa has been divided into several phyla that are likely unrelated: the Ectoprocta, the Entoprocta, and the Cycliophora. When speaking of Bryozoans, the first of these three is what is usually meant. Bryozoans can be further divided into two main classes: the Stenolaemata and the Gymnolaemata.

During the Mississippian (354 to 323 million years ago) bryozoans were so common that their broken skeletons form entire limestone beds. The Bryozoa are one of the few classical phyla from which no members have been found in the Cambrian. They seem to have evolved in the Ordovician.