Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about 0.5 millimetres (1⁄64 in) long, they have a special feeding structure called a lophophore, a "crown" of tentacles used for filter feeding. The bryozoans are classified as the marine bryozoans (Stenolaemata), freshwater bryozoans (Phylactolaemata), and mostly-marine bryozoans (Gymnolaemata), a few members of which prefer brackish water. Most marine bryozoans live in tropical waters, but a few are found in oceanic trenches and polar waters. 5,869 living species of bryozoa are known. Originally all of the crown group Bryozoa were colonial, but as an adaptation to a mesopsammal (interstitial spaces in marine sand) life or to deep-sea habitats, secondarily solitary forms have since evolved. Solitary species have been described in four genera: Aethozooides, Aethozoon, Franzenella, and Monobryozoon, the latter having a statocyst-like organ with a supposed excretory function.
The terms Polyzoa and Bryozoa were introduced in 1830 and 1831, respectively. Soon after it was named, another group of animals was discovered whose filtering mechanism looked similar, so it was included in Bryozoa until 1869, when the two groups were noted to be very different internally. The new group was given the name "Entoprocta", while the original Bryozoa were called "Ectoprocta". Disagreements about terminology persisted well into the 20th century, but "Bryozoa" is now the generally accepted term.
