Phoronida

Phoronida, commonly known as horseshoe worms, is a small phylum of marine, worm-like invertebrates that live in protective secreted tubes. They are lophophorates, a group united by a crown of ciliated tentacles called the lophophore, used for feeding and gas exchange. The body typically consists of a short, retractable trunk and a longer preoral region, with the lophophore surrounding the mouth. The gut is generally U-shaped, and the coelom is limited, with most exchange occurring across the body surface and through the lophophore.

Habitat and ecology: Phoronids inhabit a range of marine environments from shallow to deep waters, often embedded in sandy or muddy substrates or attached to shells and rocks. They construct and inhabit chitinous or proteinaceous tubes in which the animal resides, extending the lophophore from an opening to filter-feed on suspended particles.

Life cycle and reproduction: Reproduction is sexual, with both dioecious and hermaphroditic species described. Fertilization is typically external, producing free-swimming actinotroch larvae, a distinctive larval form for phoronids and related groups. The larva metamorphoses into a juvenile that grows a tube and joins the adult population.

Systematics and evolution: Phoronida is a monophyletic phylum within the larger grouping of Lophotrochozoa. They are closely related to brachiopods and bryozoans, sharing the lophophore, though the precise phylogenetic position of Phoronida remains a topic of discussion. Today, only a small number of species are recognized, primarily in temperate and polar seas.