The actinotroch larvae are planktonic and eventually settle and undergo metamorphosis. The horseshoe worm can also reproduce asexually by transverse fission.
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The remaining species develop free-swimming actinotroch larvae, which feed on plankton. The actinotroch is an upright cylinder with the anus at the bottom and fringed with cilia. At the top is a lobe or hood, under which are: a ganglion, connected to a patch of cilia outside the apex of the hood; a pair of protonephridia (smaller and simpler than the metanephridia in the adult); the mouth; and feeding tentacles that encircle the mouth. After swimming for about 20 days, the actinotroch settles on the seabed and undergoes a catastrophic metamorphosis (radical change) in 30 minutes: the hood and larval tentacles are absorbed and the adult lophophore is created round the mouth, and both now points upward; the gut develops a U-bend so that the anus is just under and outside the lophophore.
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One species builds colonies by budding or by splitting into top and bottom sections, and all phoronids reproduce sexually from spring to autumn. The eggs of most species form free-swimming actinotroch larvae, which feed on plankton. An actinotroch settles to the seabed after about 20 days and then undergoes a radical change in 30 minutes: the larval tentacles are replaced by the adult lophophore; the anus moves from the bottom to just outside the lophophore; and this changes the gut from upright to a U-bend, with the stomach at the bottom of the body. One species forms a "slug-like" larva, and the larvae of a few species are not known.
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The actinotroch larva of Phoronis is similar to tornaria larva of Balanoglossus. In spite of these similarities following differences present between these two groups show that these groups are not closely related: :a) Absence of gill-slits in Phoronis. :b) Presence of lateral nerve cord in Phoronis. :c) Formation of coelom by splitting of the mesoderm in Phoronis (schizocoelic).
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Phoronids live for about one year. Some species live separately, in vertical tubes embedded in soft sediment, while others form tangled masses buried in or encrusting rocks and shells. In some habitats populations of phoronids reach tens of thousand of individuals per square meter. The actinotroch larvae are familiar among plankton, and sometimes account for a significant proportion of the zooplankton biomass.
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Mesoderm is formed from mesenchyme originating from the archenteron. The coelom is formed by schizocoely, and the blastopore (a dent in the embryo) becomes the mouth. Photo of an actinotroch larva The slug-like larva of Phoronis ovalis swims for about 4 days, creeps on the sea-bed for 3 to 4 days, then bores into a carbonate floor. Nothing is known about three species.
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When dispersed, they become amoeboid and make their way through the body wall of an adult worm before fertilising eggs in the metacoel. The resulting zygotes make their way out through the nephridiopores and become planktonic larvae. They may be brooded as an egg mass in the lophophoral cavity for a period. The larva is known as an actinotroch and was thought for a long time to be an adult organism in its own right and given the generic name Actinotrocha sabatieri.
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The actinotroch larvae are familiar among plankton, and sometimes account for a significant proportion of the zooplankton biomass. Phoronis australis bores into the wall of the tube of a cerianthid anemone, Ceriantheomorphe brasiliensis, and uses this as a foundation for building its own tube. One cerianthid can house up to 100 phoronids. In this unequal relationship, the anemone experiences no significant benefits nor harm, while the phoronid benefits from: a foundation for its tube; food (both animals are filter- feeders); and protection, as the cerianthid withdraws into its tube when danger threatens, and this alerts the phoronid to retract into its own tube.
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The actinotroch larva is transparent and has a ring of ciliated tentacles just behind the mouth, a straight gut and an anus at the posterior end. Metamorphosis is very rapid and in the course of half an hour, new tentacles appear and the body is transformed. The metasomal sac protrudes from the ventral surface and turns inside out, with the posterior end of the worm uniting with the anterior end while the previous ventral region becomes the new posterior end. In the process, the gut is bent double into the U-shape typical of this phylum.
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