ИСТИНА

Trachylina, the sister group of all other Hydrozoa, is important for understanding the hydrozoan origin and evolution but is poorly investigated. The characteristic feature of Trachylina is a minute polyp, which is totally absent in Trachymedusae and Narcomedusae. When the polyp stage is lost, the medusa is formed directly from the planula-larva. As medusae develop normally from the polyp tissues, it is interesting to compare this unusual developmental pathway with the formation of medusa buds in the ‘typical’ hydrozoans. Aglantha digitale (Trachylina, Trachymedusae) is a common White Sea species. Its minute planktonic larva consists of about 100 cells. Multiciliary ectodermal cells cover highly vacuolated cells of the endoderm, which are arranged linearly. The planula-larva transforms to actinula-like larva by forming the gastric cavity, tentacles and manubrium. At this stage we can distinguish a ring-like ectodermal growth zone that is composed of i-cells and surrounds the aboral-most area. The growth zone corresponds to the rim of the forming bell, and the aboral epithelium corresponds to the future exumbrella. The bell surface area increases due to divisions of cells of the growth zone and by the flattening of the aboral cells. Subumbrellar cavity is formed as a circular fold in between the tentacles and manubrium. This fold moves inward squeezing the gastric cavity that results in the formation of eight radial canals. The most part of medusa bell tissues originates from the activity of the growth zone, which moves orally in the course of development. Financial support: RFBR-15-04-08214-a.