Phoronida is a small taxon of sessile, vermiform (worm-shaped), marine invertebrates that, like brachiopods and bryozoans, are suspension feeders, using a specialized organ called the “lophophore.” Phoronids generally live in the shallow waters of continental shelves, with the chitinous tubes they secrete either buried in sand or attached to rocks or the shells of other invertebrates (some species can bore into rock and shell). The diameter of phoronids is only about 1 – 3 mm, but they range in length from 2 – 20 cm. The body-plan is fairly simple, consisting of the anterior lophophore – whose ciliated tentacles can be organized in a ring, a horseshoe, or a spiral around the mouth – the trunk, and the ampulla, the swollen posterior end. In addition to its use as a feeding organ, the lophophore is used for gas exchange. Species with larger body-size have lophophores with proportionately more tentacles and, therefore, more complex overall lophophore shape. Phoronids have specialized blood vessels containing hemocytes with hemoglobin, which is unusual in invertebrates. Most phoronids reproduce clonally and sexually and most are hermaphrodites. Packages of sperm are released into the surrounding water where they’re collected by the lophophores of other individuals. Eggs are fertilized internally, and zygotes are either released into the plankton or brooded in the lophophore.
(Ruppert et al. 2004)