ИСТИНА

The soft tissues of the colony modules in athecate and thecate hydroids predominantly display radial symmetry. However, in the case of the skeleton details, the organisation of the individual modules and the entire shoots of thecate hydroids testify for lack of any symmetry, or for predominance of combinations of bilateral symmetry and glide-reflection symmetry. Bilateral symmetry of stolon modules is evident for stolon growing tips that manifest anterior-posterior axis, “dorsal and ventral” sides in relation to the substrate (Marfenin et al., 2010). The coenosarc of the stolons between the shoots is physiologically polarised but morphologically displays radial symmetry. The individual morphogenetic modules in shoots of thecate hydroids mostly display bilateral symmetry. There is only one plain of symmetry: it is possible to recognise the left and right, abcauline and adcauline sides. Only the primary modules of the colony often lack the bilateral symmetry. Development of the bilateral symmetry of the morphogenetic modules in thecate colonial hydroids can be explained with the help of the model of growth and development regulation in solitary and colonial hydroids, proposed by Berking (2003, 2006, 2010). This model is based on the hypothesis of positional information (Wolpert, 1969, 1971, 2011; Wolpert et al., 1974) and “self-activation and lateral inhibition” (Gierer and Meinhardt, 1972; Meinhardt and Gierer, 2000). The primary module developing from the settled larva is bilaterally symmetrical. Order of its parts shaping depends on the positional information that increases within the growing tip during tip functioning. The new growing tip emerging upon the primary module turns out to be “functionally” bilateral: adcauline side has higher positional value compared to the abcauline side. Functional bilateral symmetry of the growing tip determines the bilateral organisation of all secondary modules of shoots and stolons within the colony. The main point is that functional bilaterity stabilises morphogenesis in colony shoots providing their regular spatial organisation being the by-product of mechanisms regulating the development of modular organisms based on the cyclic morphogenesis.