[Molecular evidence of regression in evolution of metazoa]

Molecular data permit to construct phylogenetic trees independently of morphological characters. It allows to consider their evolution without the frames of a priori hypothesis of regularities of morphological evolution and independently of palaeontological data. Cladistic analysis of elements of secondary structure of varible areas V7 and V2 in 18S rRNA with different Protozoa as "external" groups shows that Bilateria + Cnidaria are monophyletic, Ctenophora and Porifera are early derivatives of Metazoa, Trichoplax (Placozoa) is a form related to Cnidaria, while Rhombozoa, Orthonectida and Myxozoa were branched within Bilateria. Morphological reduction with losses of any organs and tissues took place many times in early evolution of Metazoa and Bilateria not only in parasitic species. It occurred both at early and late stages of embryonic development and differentiation. Two alternative scenario of morphological degeneration in Trichoplax and the way of their testing are suggested. The similarity of Ctenophora and Calcarea is discussed. Meridional or oblique position of the third cleavage furrow of ovule can be considered as an evidence of their origin from common ancestor.