The Bryozoa, also known as Polyzoa, Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about 0.5 millimetres long, they are filter feeders that sieve food particles out of the water using a retractable "crown" of tentacles lined with cilia. Most marine species live in tropical waters, but a few occur in oceanic trenches, and others are found in polar waters. One class lives only in a variety of freshwater environments, and a few members of a mostly marine class prefer brackish water. One genus is solitary and the rest are colonial.

Predators of marine bryozoans include sea slugs, fish, sea urchins, crustaceans, mites and starfish. Freshwater bryozoans are preyed on by snails, insects, and fish.

Bryozoans are among the three dominant groups of Paleozoic fossils. Marine fossils from the Paleozoic era, which ended 251 million years ago, are mainly of erect forms, those from the Mesozoic are fairly equally divided by erect and encrusting forms, and more recent ones are predominantly encrusting.

The oldest species with a mineralized skeleton occurs in the early Ordovician. It is likely that the first bryozoans appeared much earlier and were entirely soft-bodied, and the Ordovician fossils record the appearance of mineralized skeletons in this phylum. The Early Ordovician fossils may also represent forms that had already become significantly different from the original members of the phylum. Other types of filter feeders appeared during the Devonian, which suggests that some change made the environment more favourable for this lifestyle.