The phylum Entoprocta (=Kamptozoa) includes around 180 known species (Iseto and Hirose 2010) and the actual world fauna likely exceeds 500 species (Nielsen 1989). Entoprocts are small, transparent, sessile (i.e., fixed to the substrate), solitary or colonial animals that superficially resemble cnidarian hydroids and ectoprocts ( = bryozoans in some usage, see below). They are largely radially symmetrical. All but two known species of entoprocts are marine. Marine entoprocts are found along the coasts of every continent, including Antarctica. One of the two freshwater species, Urnatella gracilis, is known from every continent except Antarctica and, in contrast to other entoprocts, forms massive growths that may be quite conspicuous to the naked eye (Nielsen 1989).
Colonial entoprocts (around a quarter of known species, Nielsen 2010) may live attached to algae, shells, rocks, or other living or non-living substrates. Solitary species may be commensal on a variety of hosts, such as sponges, ectoprocts, polychaetes, sipunculans, and ascidians and are typically associated with just one or a few host species. They are often found inside the burrows or tubes of hosts that produce ventilating currents. Nearly all members of the genus Loxosoma live in association with polychaetes (Nielsen 1996). Some solitary entoprocts are found on non-living substrates.
Entoprocts are found mainly in shallow water such as in the intertidal zone, but some species may occur to depths as great as 500 meters. Individual entoproct zooids consist of a cuplike "calyx" supported by a stalk. A whorl of 6 to 36 ciliated tentacles (depending on age and species) surrounds the calyx (unlike either ectoprocts or cnidarians, entoprocts fold their tentacles when disturbed; they do not withdraw them into their bodies). The calyx has a circular sphincter that controls its size and small bundles of muscle fibers along the stalk cause a characteristic "nodding" behavior. Both the mouth and anus are located on the surface of the calyx and are surrounded by tentacles. The anus is elevated on a distinct structure called the anal cone. The entire animal, including tentacles, calyx, and stalk, may extend to 10 mm.
(Brusca and Brusca 2003; Fuchs et al. 2010; Margulis and Chapman 2010)
Entoprocts are suspension feeders, extracting food particles (mainly phytoplankton) from currents produced by the lateral cilia on the tentacles. The dorsal suface of these animals is attached to the stalk and the ventral side faces upward. Water currents pass from dorsal to ventral, flowing between the tentacles. Food is trapped by the lateral cilia and moved in a sheet of mucus to the frontal cilia, which move the mucus and food to food grooves at the base of the ring of tentacles. (Brusca and Brusca 2003)
Colony growth occurs by budding. Solitary forms also reproduce by budding. Sexual reproduction occurs as well. Colonial forms may have hermaphroditic or (more commonly) dioecious (i.e., separate sex) zooids, and a colony may contain just one or both sexes. Sperm apparently are released into the water and enter the female reproductive tract, with fertilization occurring in the ovaries or oviducts. In most species, embryos develop within a brood chamber, though in others the embryos are retained within the ovary. Larvae are typically free-swimming and planktotrophic (i.e., feeding in the water column), although a few species produce lecithotrophic (non-feeding) or benthic crawling larvae. Following a brief swimming phase, larvae of most species settle, affix to a substrate, and metamorphose into adults (larvae of a few species do not metamorphose but bud instead). (Brusca and Brusca 2003; Fuchs et al. 2010)
During most of the 18th and 19th centuries, entoprocts and ectoprocts were included together in the phylum Bryozoa. For most of the 20th century, however, the Entoprocta have been treated as a distinct phylum that may not even be closely related to the ectoprocts (with the ectoprocts being referred to as either Ectoprocta or Bryozoa). The main anatomical and developmental differences between entoprocts and ectoprocts are the position of the anus (inside vs. outside the tentacle crown), the cleavage pattern during development (spiral vs. radial), and the body cavities (acoelomate vs. coelomate) (Fuchs et al. 2010). Beginning in the 1970s, some authors have been arguing that the entoprocts and ectoprocts may indeed be very closely related (Nielsen 2001 and references therein). Recently, some molecular data have supported this position (Hausdorf et al. 2007; Hejnol et al. 2009), although it appears that the sister group to Ectoprocta may be (Entoprocta + Cycliophora), as was originally suggested by Funch and Kristensen (1995). However, some other analyses have suggested that Ectoprocta may not be very close to (Entoprocta + Cycliophora) (Passamaneck & Halanych 2006; Hejnol et al. 2009; Fuchs et al. 2010; Mallatt et al. 2010). The putative clade consisting of Ectoprocta, Entoprocta, and Cycliophora has been termed Polyzoa (Cavalier-Smith 1998).
The biology of the Entoprocta was reviewed by Wasson (2002). Nielsen (1989) also reviewed entoproct biology and provided a dichotomous key to all species known from northwestern Europe, with the caveat that the possibility of encountering undescribed species is not unlikely in many areas. Wasson (1997) reviewed and revised the systematics of the colonial entoprocts of the Pacific coast of North America.
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