The Bryozoa are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about 0.5 millimetres long, they have a special feeding structure called a lophophore, a "crown" of tentacles used for filter feeding. The bryozoa are classified as the marine bryozoa (Stenolaemata), freshwater bryozoa (Phylactolaemata), and mostly-marine bryozoa (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 life or to deep-sea habitats, secondarily solitary forms have since evolved. Five genera are solitary: Aethozooides, Aethozoon, Franzenella, Solella, which together make up the family Aethozoidae, and Monobryozoon, which has a statocyst-like organ with a supposed excretory function.