The individual zooid each live in a box shaped or bud-shaped exoskeleton (zoecium) which can be mineralized, gelatinous or chitinous, and in some taxa may have an operculum over its little opening at the top. Typically suspension feeders, the zooid protracts through this opening a special feathery feeding organ called the lophophore, which is composed of a circle or horseshoe of tentacles. Cilia on the lophophore tentacles create water currents to carry appropriate sized food particles (including protists and invertebrate larvae) along food grooves on the lophophore which lead to the mouth.
Within a colony, individual zooids may be more or less connected to one another; many taxa have pores or a cord (funiculus) linking individuals in a colony, through which the individuals share coelomic fluids. In some kinds of colonies zooids function together to create more powerful water currents to bring in more food. All colonies contain autozooids, which feed and excrete wastes, some colonies also have non-feeding heterozooids, individuals specialized for gamete production, protection, or other functions and are supported with nutrients shared by surrounding zooids. Zooids may have spines on their zoecium, some that produce toxins, to ward off predators. Protective zooids may have their operculum modified into a protective structure, either an avicularium – a movable beak-like structure to rid the colony of pests, or a vibraculum – a long, movable setae-like structure thought to help in cleaning off the colony. Grazing by nudibranchs, snails, sea urchins and crustaceans is a common threat to bryozoans (Brusca and Brusca 2003; Kozloff 1990).
Bryozoans do not have nephridia or a circulatory system, instead gas exchange and nitrogenous excretions occurs passively by diffusion in the tiny zooids. When more complex wastes build up, the zooid forms a “brown body”, in which the soft tissue and lophophore (together called the polypide) degenerate within their casing (called the cystid). The cystid can then regenerate a new polypide, with the old brown body in its gut. The brown body in some taxa is then excreted through the anus (located near the mouth, but on the outside of the lophophore). In taxa with zooids arranged on stolons, the brown body simply falls off the shoot and a new zooid is regenerated. The nervous system in bryozoans is minimal, including a ganglion, nerve ring around the pharynx, and nerve net that extends into the tentacles and vicera. Sensory structures are limited to tactile cells on the lophophore (Brusca and Brusca 2003; Kozloff 1990).
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