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The common ancestor of Ambulacraria was a mobile bilaterally symmetrical organism. Its body was subdivided into a preoral (proboscis) region, a perioral (collar) region with ciliated tentacles, and a trunk region with a metameric coelom and metameric gill slits. Like in other Bilateria, the preoral and tentacular regions of Ambulacraria are free of Hox gene expression, which begins in the area of the first pair of gill slits in compliance with the colinearity. The axial complex is a synapomorphy of the Ambulacraria clade. A bilaterally symmetrical ancestor of Echinodermata lay on its dorsal side, so its anus was shifted to the ventral side as in present-day echinoderm larvae. This stage of evolution corresponds to the Early Paleozoic bilaterally symmetrical forms, such as Protocinctus, Ctenocystis, and Ctenoimbricata. The common ancestor of echinoderms had a symmetrical tentacular apparatus consisting of five ciliated tentacles on each side of a collar. At the next stage, the ancestors of echinoderms lay on the right side that resulted in the reduction of the tentacles on the right side and the right hydrocoel. This evolution stage includes various Early Paleozoic forms (Cothurnocystis, Dendrocystoides, Syringocrinus, Castericystis, Coleicarpus, Rhenocystis, etc.). The next stage is related to the sedentary lifestyle. During this stage, the mouth and tentacles occupied an apical position, the anus has shifted up, and thus an intestinal loop was formed. The five primary tentacles of the left side of the ancestor of Ambulacraria predestined the formation of pentaradial symmetry of echinoderms. The primary tentacles remained only in Holothuroidea. The secondary ciliary grooves were formed between the primary tentacles. These grooves were accompanied by hydrocoelic canals, which gave rise to the water-vascular (ambulacral) system. The present-day echinoderms are characterized by several metameric rings formed by the derivatives of the left somatocoel. They derive from the coelomic segments of the left side of the trunk of the ambulacrarian ancestor and, thus, the echinoderms retain coelomic metamerism. The attachment of the crinoid larvae by the preoral lobe reflects the ancient method of locomotion of deuterostomes using the proboscis, but, in reality, the Pelmatozoa stalk is homologous to the Pterobranchia stalk, i.e., the posterior end of the body, because the right somatocoel grows into it. During metamorphosis, the internal coelomic complex of the larva is inverted by 180°. The significance of this inversion for the translocation of the anterior genes of the Hox cluster is discussed. The ancestors of Eleutherozoa began to crawl on the oral surface, which led to a shift of the anus to the aboral side. Thus, the ontogeny and phylogeny of echinoderms exhibits a change from bilateral symmetry to dissymmetry, and then the development of pentameric symmetry. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Within all major taxa of Bilateria, there are forms with coelomic metamerism. This suggests that coelomic metamerism was characteristic of the common ancestor of Bilateria. Among deuterostomes, metamerism is clearly expressed in chordates, and elements of metamerism are present in hemichordates. Do echinoderms have remnants of coelomic metamerism that was inherited from the common ancestor of Bilateria? The coelomic system of echinoderms includes several metameric coelomic rings located along the oral-aboral axis, namely: the axocoelomic ring, the hydrocoelomic ring, 2 to 6 coelomic rings originating from the left somatocoel, and one epigastric ring originating from the right somatocoel. Thus, in echinoderms, there is a dissymmetrical metamerism, derived from the original metamerism of the common ancestors of Deuterostomia and, possibly, the common ancestors of Bilateria. The problem of dexiothetism as the cause for the formation of coelomic dissymmetry in echinoderms is discussed. [ABSTRACT FROM AUTHOR]