Phoronids

Phoronids are a small group of tube-dwelling marine worms that comprise the phylum Phoronidae. Some 15 species have so far been described. All phoronids are exclusively marine-dwelling and live in shallow waters up to a depth of about 195 ft (60 m) in both tropical and temperate oceans. They are thought to be related to moss animals (phylum Bryozoa) and lamp shells (phylum Brachiopoda). They may occur either individually or in clusters.

Phoronids are recognized by their tube-like body, which averages 8 in (20 cm) in length. The head region of the body is dominated by a crown of tentacles each covered with tiny hair-like cilia that are used for collecting food. When threatened, the tentacles may be withdrawn within the tube. Each phoronid may have from 18-500 tentacles depending on age and the species. Beneath the crown is a slender body that is mostly cylindrical apart from a broadened base on which the animal rests. The muscular trunk contains a U-shaped coelom, which serves as the digestive tract. These soft-bodied animals live within a hardened tube that they develop around themselves for protection. In addition, many species bury themselves partly in soft substrate, while others are firmly attached to rocks, shells, and other firm supports. Only the head of the animal emerges from the protective tube.

When feeding, the tentacles are opened outwards and tiny cilia that line the tentacles beat downwards, drawing the water current and food particles towards the mouth region. Here food items are trapped on a mucus-coated organ called the lophophore—a horseshoe-shaped fold of the body wall that encircles the mouth. Plankton and other suspended matter are trapped on the mucus lining, which is then passed down towards the mouth for ingestion. The mouth develops into a long tubular esophagus and a greatly enlarged stomach at the base of the animal.

Phoronids exhibit a range of breeding strategies: some species have separate male and female organs, while others are hermaphrodites. It is also thought that phoronids can reproduce by asexual means, either by budding off small replicas of the parent animal or by a process of fission. In its sexual reproduction, the male and female gametes are usually released for external fertilization, although a few species are known to brood their young offspring for a short period. The larvae are microscopic, and after several weeks of a free-living existence they settle, either individually or collectively, and begin to secrete their own protective coating.