ИСТИНА

It is well known that hydromedusae are normally formed from the polyp tissues. The subumrellar structures of the medusa bell arise from the so-called entocodon, which forms from the ectoderm and can be viewed as an analogue of bilaterian mesoderm. Aglantha digitale (Trachylina, Trachymedusae) has a holopelagic life cycle lacking the sessile polyp stage with the planula-larva developing directly into an adult medusa. The question is how does Aglantha form the medusa structures from the planula tissues? The Aglantha planula has only about 100 cells. The planula endoderm consists of highly vacuolated coin-shaped cells lined up along the planula OA axis. Synchronous division of these cells provides the development of the gastric cavity, then the first pair of tentacles appears, and the planula converts into an actinula-like larva. The larva gradually grows and the number of tentacles increases. At this stage we can distinguish a ring-like growth zone that is composed of i-cells and surrounds a small area of the aboral epithelium. The growth zone corresponds to the rim of the forming bell, and the aboral epithelium corresponds to the future exumbrella. The bell surface area rapidly increases due to proliferative activity of the growth zone and by the flattening of the aboral cells. The velum appears as a collar-shaped fold surrounding the manubrium. Another circular fold appears in between the velum and the tentacles. This folding provides the formation of the subumbrellar cavity. The fold has eight peaks, which grow inward and squeeze the gastric cavity that results in the formation of eight radial channels. We hypothesize that this fold can be considered as an analogue (if not a homologue) of the entocodon of other hydrozoans. Financial support: RFBR 15-04-08214-а.