The phylum Phoronida comprises small, worm-like marine animals, usually no more than 200 mm in length found on the shore and sublittorally to about 400 m. Phoronids inhabit chitinous tubes to which sediment particles are often attached, and are found in sandy and muddy sediments, encrusting or burrowing into rocks and shells. The body is narrow, elongate and unsegmented, the anterior end being characterized by a lophophore, a group of ciliated tentacles surrounding the mouth. The tentacles are hollow and clothed with cilia and increase in number as the animal grows. They are used in suspension feeding, the cilia drawing in plankton and detritus which are trapped in mucus. As in the Bryozoa, the alimentary canal is U-shaped and the anus opens outside the lophophore. Either the sexes are separate or the animals are hermaphroditic, but in both cases internal cross-fertilization occurs. The eggs are either brooded in the parental tube or on the lophophore and have only a short pelagic life. In some cases the eggs are liberated directly into the sea. and develop as larvae, known as actinotrochs, with a pelagic life of several weeks. Asexual reproduction by budding also occurs. The animals have considerable powers of regeneration and can regenerate the anterior end in as little as two or three days.