ИСТИНА

Colonial hydrozoans (Cnidaria, Hydrozoa) are good model objects for evolutionary morphology investigation. These animals have modular organization. Most of hydrozoans from subclass Hydroidolina belong to the two main groups – athecate and thecate hydroids. These groups differ in certain features of hydranth morphology and hydranth development. In present work we analysed the process of hydranth development (from hydranth rudiment to tentacles formation) in typical thecate species Gonothyraea loveni (Allman, 1859), typical athecate Sarsia tubulosa (M. Sars, 1835) and atypical thecate Halecium halecinum (Linnaeus, 1758). The latter species is characterised by reduced hydrotheca, so the hydranth can’t retract into it. This morphological feature makes it similar to athecate species. The development of Halecium tentacles during hydranth formation display some differences from other thecate species. Obtained data showed, that the hydranth formation of thecate and athecate hydrozoans has similar features, regardless of the hydrotheca presence or absence. But, in Halecium hydranth development differs from that in both groups, and these differences are greater than differences between typical thecates and typical athecates.