ИСТИНА

The investigation of metamorphosis sheds light on phylogeny and some steps of body plan formation. The metamorphosis of [Phoronopsis harmeri] was studied by light microscopy, TEM, SEM, and CLSM. Immediately before metamorphosis, paired upper and lower retractors contract strongly, exciting the pressure inside larval body and evertion of the metasomal sack. The preoral lobe macerates and separates on cellular bands, which are drown into the mouth and eaten. The epithelium of some areas of the oral field is peeled off and eaten. The places, which lost the epithelium, are covered by thick basal lamina that forms huge folds into the blastocoels of anterior body part. In the tentacles, the epithelium of postoral ciliated band is squeezed out and eaten. The telotroch retains about nine days and then disappears. Larval systems, which lose many elements, are following: muscular (esophageal and some tentacular muscles retain only), nervous (tentacular nerve ring, tentacular radial neurite bundles, and nervous system of digestive tract retain), and excretory (U-shaped channels retain). Larval systems, which do not lose any parts, are following: digestive, coelomic, and circulatory. Generally, the elements, which are lost in metamorphosis, provide special larval function. It does not concern the muscular system of tentacles, which has similar function and location in larva and in adult [Phoronopsis harmeri], but in metamorphosis forms [de novo]. In phylogenesis, in creeping phoronid ancestor, movable ventral protrusion had formed and allowed to dig into the substrate and avoid a danger.