The Entoprocta (= Kamptozoa) comprise a phylum of solitary and colonial sessile animals, one of the least-known groups of the Animal kingdom and whose phylogenetic position is unclear and controversial. About 180 species have been described worldwide. Small, transparent and of cryptic habits, the Entoprocta are usually inconspicuous, although they are relatively common on hard substrates such as rocks, pebbles, corals, shells, and algae. They live as commensals of many species of invertebrates such as sponges, polychaetes, and sipunculans, often being found inside the tubes and galleries of their hosts. Closely resembling some bryozoans and hydrozoans in the general appearance and habit, they consist mainly of a distal region, called calyx, which contains a crown of ciliated tentacles, and a stem or stalk, which sustain the calyx and attaches it to the substrate through a dilated structure, characteristic of solitary species, or by stolons in colonial species.