ИСТИНА

Phoronids have remarkable metamorphosis, in which some parts of larval body are consumed by the juvenile and larval body plan completely changes. The investigation of metamorphosis sheds light on phylogeny and some steps of body plan formation. Only one idea about phylogenetic formation of phoronid body plan was suggested: a hypothetical ancestor of phoronids inhabited a U-shaped burrow in soft sediment, where it drew the anterior and posterior parts of the body together and eventually fused them. We have inspected this hypothesis and studied the metamorphosis of Phoronopsis harmeri with light microscopy, TEM, SEM, and CLSM. During metamorphosis the larval hood is engulfed by juvenile; the epidermis of postroral ciliated band is squeezed from the tentacular epidermis and then engulfed; the larval telotroch undergoes the cell death and disappears; the juvenile body forms from the metasomal sack of larva. Dorsal side of larva becomes very short, whereas the ventral side expands enormously. Juvenile inherits the protocoel, mesocoel, and metacoel of larva. Lateral mesenteries of juvenile arise in nine days by proliferation of trunk coelom lining. Lateral mesenteries bear nephridial funnels, which also arise de novo. Lateral mesenteries are situated at a right angle to the short anterior–posterior axis. According to our hypothesis, the phoronid ancestor was worm-like animal that possessed not only preoral and tentacular coelomes but also two coelomic compartments in the trunk. The paired nephridial funnels opened on a dissepiment between these coelomic compartments. This worm-like ancestor buried itself in soft sediment by means of the ventral protrusion to which the loop of the intestine and dissepiment were drawn. This work was supported by RFBR 11-04-00690, 12-04-33045.