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Phoronida

Phoronida (fərŏn´ədə), small phylum of slender, wormlike marine tube-dwellers, typically found in temperate, shallow seas. About 10 species are known. Although the body is free in the tube, the organisms extend only a crown of ciliated tentacles (the lophophore) to capture food. Water currents generated by the lophophore cilia sweep food particles against mucus secreted at the base of the tentacles and ciliated grooves propel the food to the mouth. Phoronids have a U-shaped digestive tract, a blood-vascular system containing hemoglobin, and excretory organs called metanephridia. The coelom, or body cavity, is divided into compartments resembling those seen in the Ectoprocta and the Brachiopoda, which are phyla related to the Phoronida; the compartments also resemble those of the Echinodermata. Phoronids are ancient, and some zoologists have suggested they are the ancestors of the brachiopods because of similarities in embryology. Tubes seen in early paleozoic sandstones appear to be identical with modern phoronid tubes, but little is known of their history.

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Phoronida

Phoronida (horseshoe worms) A phylum of marine, worm-like animals whose common name refers to their horseshoe-shaped, filter-feeding organ (a lophophore). They are sessile suspension-feeders, most living in chitinous burrows. The gut is U-shaped and the anus opens to the exterior on the dorsal surface above the lophophore. Horseshoe worms have a vascular system and blood containing red blood corpuscles containing haemoglobin (a feature that distinguishes them from Bryozoa). It is widely believed that they are ancestral to the Brachiopoda and Bryozoa, although they first appear in rocks of Devonian age (i.e. after the other two phyla). There are two extant genera, with 10 species.

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