The Origin and Main Trends in the Evolution of Bilaterally Symmetrical Animals

This paper has been written from the standpoint of the phylogenetic tetrad method which includes comparative anatomy, evolutionary developmental biology (“evo–devo”), paleontology, and molecular phylogenomics. The Bilateria consists of two sister groups such as Cnidaria and triploblastic Bilateria. Their common ancestor was a bilaterally symmetrical diploblastic organism with two circles of tentacles, labial and marginal, around a slit-like mouth. Anthozoa retains a primary bilateral symmetry. Medusozoa develops a secondary radial symmetry as a result of adaptation to a sedentary lifestyle. The common ancestor of the triploblastic Bilateria had a through gut formed by amphistomy, a coelom, segmentation, and metameric limbs. The marginal circle of tentacles gave rise to metameric limbs, the labial circle of tentacles gave rise to perioral ciliated tentacles, ventral ciliated sole (neurotroch), and telotroch. The homology of the ciliated tentacle apparatus in Trochozoa, Lophophorata, and Deuterostomia is substantiated. The ciliary armament of the triploblastic Bilateria larvae follows the general structural plan and includes the adoral ciliary field (a homologue of the ciliary tentacles of adult forms), neurotroch, and telotroch. The homology of pre-oral and oral (=tentacular) segments in triploblastic Bilateria is substantiated. The origin of Ecdysozoa as primarily semi-aquatic organisms is discussed. The chitinous cuticle of Ecdysozoa is homologous to the chitinous theca attached to the epidermis. The primary biramous limbs of Ecdysozoa resulted from the integration of tentacles of the marginal and labial circle of the common bilaterian ancestors. Tentacles of the marginal circle gave rise to exopods, and those of the labial circle gave rise to endopods. Examples of a primary biramous limb in Cambrian Ecdysozoa and recent Chelicerata are considered. The secondary biramous limb of Crustacea is a result from the secondary splitting of the endopod of the primary biramous limb into exopodite and endopodite. The problem of Hexapoda wings homology is discussed. Lobopoda is an Ecdysozoa lineage which adapted to life in loose substrate and acquired a worm-like body shape; the primary endopods became lobopods, while the primary exopods gave rise to metameric dorsolateral spines. Habitation in the sediment (Palaeoscolecida and Cephalorhyncha) caused a complete reduction of limbs and segmentation. The role of progenesis in the nematode origin is discussed.