Phylum Phoronida

There are only about a dozen phoronid species found anywhere from the intertidal zone to 400 meters down in the marine environment. These unsegmented, bilaterally symmetric worm-shaped animals live inside an open-ended chitinous tube embedded in either soft or hard calcareous substrates. They're usually only 5 to 25 centimeters in length, and their anterior is crowned by horseshoe-shaped lophophore of ten to several hundred hollow tentacles, the origin of the name--horseshoe worms. Phoronids have a tripartite coelom. The prosoma, the most anterior coelom forms a flap, the epistome, next to the mouth; the mesosome, thelophophore; and the metasome, the rest of the trunk, which includes an enlarged end bulb, or ampulla, at the posterior end that anchors the animal in its burrow. The body wall is covered in densely ciliated epidermis that includes gland cells that secrete mucus and the materials that build their tube. A thin layer of circular muscles covers a much thicker layer of longitudinal muscles, and these combine with the coelomic fluids of the metacoel to form a hydrostatic skeleton used for movement. Phoronids don't leave their burrow but can change position inside it and will quickly retract if there is any predator that might feed on their delicate lophophore. The nervous system extends throughout the body wall and connects to sensory neurons on the surface. Each tentacle is supplied with a nerve, and all of this connects with a simple nerve ring at the base of the lophophore.

The main functions of the lophophore are in gas exchange and feeding. Red corpuscles with hemoglobin enhance the oxygen-carrying capacity of the blood that flows in two large longitudinal vessels connected by the blind-ended vessels of the vascular system in the lophophore, where it is oxygenated, and the stomach, where it picks up nutrients as it flows through lacunar spaces, instead of capillaries, in the stomach wall. A pair of metanephridia filters the coelomic fluid of the metacoel, although how nitrogenous wastes accumulate there from the blood and how they work isn't well understood.

The gut is U-shaped with the mouth located in the center and at the base of the lophophore. The esophagus leads from the mouth to the stomach inside the end bulb and from there an intestine rises back up the animal to the anus, located next to the mouth but outside the ring of tentacles. Phoronids are filter feeders, although some may absorb dissolved organic matter directly, and capture food from water pumped across the lophophore by cilia that pull water into the center of the lophophore and push it out at the base of the tentacles. Once captured, the food moves along ciliated grooves on the tentacles to the mouth.

Phoronids include both monoecious and dioecious species and gonads form seasonally on the wall of the metacoel next to the stomach. Gametes are released into the metacoel and leave through the metanephridia. In some species the sperm is contained in a spermatophore trapped by another phoronid. Once trapped, amoeboid sperm migrate into the metacoel and fertilize the eggs. Although in most species fertilization occurs externally, some brood their eggs next to the lophophore. Eggs hatch into an actinotroch larvae that swims, settles, and undergoes its metamorphosis into the adult form. Phoronids have distinct deuterostome characteristics, including: tripartite coelom and radial cleavage, although the blastopore becomes the mouth and they synthesize chitin--both protostome traits. Traditional morphological interpretations place phoronids with the deuterostomes. Some authors feel they should be combined with the Brachiopoda and Bryozoa, who also have lophophores, in a single deuterostome phylum, the Lophophorata. But it becomes even more confusing. Molecular data place the phoronids and their lophophore friends this in with the protostomes, confirming the importance of the protostomes, rather than the deuterostome, traits.

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