ИСТИНА

The probable ancestor of Ambulacraria was a bilaterally symmetric coelomic organism. It has a preoral region of the body, a perioral region with hunting ciliated tentacles, and a metameric trunk region with metameric gill slits and gill pores. The coelomic cavity of this organism was divided into metameric sacs: paired protocoels, paired mesocoels, and paired trunk coeloms (metacoels). As in other Bilateria, the preoral and tentacle (perioral) regions of the body were free from the expression of Hox genes. Expression of the Hox genes began in the area of the first pair of gill slits in compliance with the principle of colinearity as in modern Enteropneusta. Development of the axial complex of organs is a synapomorphy of the clade Ambulacraria. The common ancestor of Ambulacraria lived in bottom soils and used the developed preoral body region for locomotion. The ancestors of Ambulacraria had the pelagobenthic life cycle with the planktotrophic larva, which possessed the preoral and postoral ciliary bands (surrounding the adoral ciliary zone), neurotroch, and telotroch. The ancestors of echinoderms buried themselves into the soft soils by the posterior body end. This led to the displacement of the anus to the ventral side. This evolutionary stage is reflected in the structure of echinoderm larvae, whose anus is shifted on the ventral side and it caused the reduction of neurotroch and telotroch. On the second stage, the ancestors of echinoderms began to lie on the right side of the body on the soft soils. It caused the complete reduction and disappearance of the right half of tentacle apparatus and right mesocoel. Herewith, the anus shifted on the dorsal side and the loop of intestine was formed. The fossil ancestors of echinoderms, which lied on the right side, are represented by the subphylum Carpozoa, who gave a wide diversity of forms in early Paleozoic. On the third evolutionary stage, the ancestors of modern echinoderms became the sedentary organisms, who attached to the solid substrate by the posterior body end. The attached mode of life caused the development of secondary radial symmetry. This caused the contravention of colinearity and order of the Hox genes expression. The left mesocoel formed the circumoral ring. The metameric coeloms of metacoel formed the metacoelomic rings, which are the remnants of metamerism of common ancestor of Deuterostomia. Radial water-vascular canals appeared between the primary perioral tentacles. As in many sedentary animals, the preoral lobe reduced, but the preoral coeloms (protocoels) together with the axial organ immersed deep into the body. Only Holothuroidea retained the primary perioral tentacles (homologues of the tentacles of Deuterostomia and, perhaps, of all Bilateria) along with the radial water-vascular canals. Possibly, the labial podia of Crinoidea are also the homologues of the primary perioral tentacles. Holothuroids began to crawl on the primary ventral side, trivium. The ancestors of Asteroidea, Ophiuroidea, and Echinoidea began to crawl on the oral face and this led to the shifting of the anus to the aboral side. The loop of intestine was retained only in Echinoidea.