Phoronis

The horseshoe worms live in tubes that are fully imbedded in the soil and are formed during digging or boring. The tubes are produced by the worm's epidermal glands. These tubes have an inner organic cylinder and an outer layer. The inner layer is composed of three parts: the inner film, middle film, and outer film. These thin films are composed of fibers from the worms that form nets that make up these tubes.[2]

These worms have a nervous system that consists of a dorsal ganglion, a tentacle nerve ring, an inner nerve ring, intertentacular groups of perikarya, and tentacle nerves. These tentacles nerves have three groups: abfrontal, frontal, and laterofrontal.[9] They have a U-shaped gut, meaning their anus is close in distance to their mouths and their digestive tract consists of three parts: the mouth, the stomach, and the anus.[10][11]

The lophophore structure has evolved from a simple, oval shape to the horseshoe shape from which the worms get their name, even into a complex spiral structure.[12] These worms can regenerate their lophophores when injured or even voluntarily drop their lophophores as Phoronis ovalis does when it is laying eggs.[10] The lophophore is used for both respiration and feeding. The tentacles of the lophophores have cilia on them which move water towards their mouth to filter out small particulates.[8][10][11] Through moving water over the tentacles of the lophophore, they act as an organ for gas exchange and allow oxygen into the blood as it passes through.[10][11]

Generally, Phoronis species have benthic adults with lecithotrophic larvae. These species incubate their embryos in various ways. Some species release their embryos into the water column and they develop there. In other species, embryos are incubated among the tentacles of the adult worms. Recently, it was discovered that some species even exhibit viviparity, where the embryos are incubated in the coelom of their mother's tube and the "hatch" to produce feeding actinotroch larvae. There is a correlation between egg size and development type: allowing embryos into the water column to develop takes less energy and can therefore produce more eggs. Incubating in the tube's coelom takes more energy so less eggs are produced.[2]

The lecithotrophic larvae, given the name actinotroch, are pelagic and float in the water column. This allows for wider dispersal of species. The larvae have also developed different morphologies over time such as differing pigmentation and arrangement of blood masses.[13]

When they are full grown, these worms typically have thin but long bodies that can grow up to 50 cm. The adults are typically colonial and can grow around each other in order to support each other.[10]