ENTOPROCT

any member of the phylum Entoprocta, a group of aquatic invertebrate animals, composed of more than 100 species. Entoprocts occur throughout the world, primarily in marine habitats, although one genus, Urnatella, is a freshwater form. Entoprocts may either exist singly or form colonies of communicating members, called zooids, by budding. The zooids measure only about 0.4 to 5 mm (0.016 to 0.2 inch) in height. Each of them has a stalk (peduncle), which may be jointed, that attaches to shells, seaweed, or to other animals, such as sponges, bryozoans, hydroids, and segmented worms, that produce water currents. The body of an entoproct consists of a globular head (calyx) that houses a U-shaped gut and the nervous, excretory, and reproductive systems. A crown of ciliated tentacles creates a water current that draws food particles toward a central mouth. The calyx often is discarded and then regenerated in colonial species. The solitary species develop daughter buds, which detach themselves when mature. Entoprocts also have free-swimming larvae that settle and develop into new colonies or individuals. Although the larva and the pattern of its early development indicate that the entoprocts may be linked remotely with the annelids and their related phyla, there is no evidence of a close link with any phylum. First described in 1774, entoprocts were classified with the moss animals (bryozoans), in which filter-feeding, tentaculated zooids also are present. The similarities between the two groups, however, are superficial. Their methods of food capture, for example, differ fundamentally; in addition, bryozoan zooids are coelomate, and the anus opens outside the tentacles. The name Entoprocta was introduced in 1869, and the group was first recognized as a phylum in 1888. Alternative names (Calyssozoa and Kamptozoa) were proposed in 1921 and 1929. The phylum Entoprocta is small and has no known fossil members. The subdivision of the phylum into three families does not accord fully with the considerable differences between colonial and noncolonial forms. It might be more accurate if two orders were recognized; the first order would include the families Pedicellinidae (about 30 species classified into six genera according to stalk structure) and Urnatellidae (2 species), and the second order would contain only the Loxosomatidae (about 100 species).