Phoronids

One of the smallest and least studied types of animals - it contains about 20 modern species belonging to the genera Actinotrocha , Phoronis and Phoronopsis . These animals are found in all seas and oceans (including the circumpolar White Sea, where Phoronis ovalis was first discovered in 2000). Most species of phoronids are cosmopolitan . There are 5 species in Russia ^[3] . Phoronids are quite common in shallow marine waters, from the tidal zone to 400 m in depth, although they are most often found at a depth of up to 70 m. The length of the phoronids is from 6 mm to 50 cm.

The worm-shaped body of an adult phoronid, from 0.6 cm to 45 cm long, is enclosed in a chitin tube secreted by it, which is immersed in the silt or sand with its lower end. From the upper end, the head section protrudes outward, carrying a lophophore - a bunch of tentacles covered with cilia, arranged in a circle, horseshoe-shaped or in a spiral. Between the tentacles is a slit-like mouth. The phoronids feed on small organisms, which are deposited on the tentacles and then delivered to the mouth along with the mucus by the cilia of the tentacles. The intestines are loop-shaped, the anus is on the dorsal side, near the mouth. The secondary body cavity is represented by 3 pairs of coelomic sacs. Excretory organs - a pair of metanephridia , which open with two holes near the anus. There is a closed circulatory system. Nervous system - with the spinal "brain", a ganglion between the mouth and anus. Lead a sedentary lifestyle. Some phoronids are very beautiful; the surface of their tubes is usually inlaid with adhering grains of sand, fragments of shells , etc. , giving them a peculiar appearance.

Dioecious and hermaphrodites. Reproduction continues from spring to autumn. Fertilization in most species is probably external-internal (sperm are swept out into the water and penetrate into the body of the female, where fertilization occurs). A planktonic larva - actinotroha - leaves the egg . The larva develops in the water column for about 20 days, then settles to the bottom and in just 30 minutes turns into a young phoronid. The life expectancy of phoronids is about a year.

Part of the species described asexual reproduction by transverse division. Under unfavorable conditions and when eggs are laid, Phoronis ovalis undergoes autotomy of the anterior end of the body with lophophore, which subsequently regenerates. Separated lophophore floats in the water column, then settles and releases the tube. Perhaps he is also able to give a new individual ^[4] .

There are no known fossils of the phoronid bodies themselves. Characteristic minks carried out by representatives of the Talpina genus in such substrates as limestones, mollusk shells, echinoderms and belemnite rosters have been known since the end of the Devonian .

• Zezina O.N. , Temereva E.N. Biota of the Russian waters of the Sea of ​​Japan. T. 3 Brachiopods and phoronids . - Vladivostok: Dalnauka, 2005 .-- 138 p. - ISBN 5-8044-0552-7 .
• Malakhov V.V. , Temereva E.N. Who are the phoronids? // Nature (journal) : Journal. - 2008. - No. 12 . - S. 29-38 . - ISSN 0032-874X .
• Temereva E.N. Synopsis of the world fauna phoronids (Phoronida). Scientific report . - M .: Moscow State University, 2017 .-- 72 p. - ISBN 978-5-906906-14-4 .