ИСТИНА

The investigation of development, organization, and metamorphosis of muscular system allow to realize some steps of phylogeny and body plan formation, peculiarities of larval life and behavior, and to trace the fate of all muscular elements. We have studied all these events by light, electron, and confocal microscopy. In phoronids, the muscular system originates from mesoderm cells, which immigrate from the anterior wall of archenteron in early gastrula. First strong muscles appear around the esophagus and in the preoral lobe. Simultaneously transversal muscles appear on the ventral side under the mouth. With age, myofibrils form cross-striated structure, the number of muscular fibers increases; they fuse each other and form large muscles. Ultrastructure of some components of the muscular system differs in different phoronid larvae. In late larva, there are several main muscles. For the first time, paired supinators and pronators of telotroch and telotroch ring muscle were found in all larvae. The last is formed by epidermal cells, which contain strong myofibrils in apical cytoplasm. In metamorphosis, the main roles play two groups of paired muscles: trunk retractors and preoral lobe retractors. These muscles fasten to circular tentacular muscle and can be recognized in juvenile during long time. In metamorphosis, most elements of muscular system are eaten or destroyed (preoral lobe muscles, circular tentacular muscle, tentacles elevators and depressors, two pairs of retractors), nevertheless some muscles retain and are involved in definitive muscular system (additional muscles of tentacles, esophageal muscles).