On the Origin of Tentacles and Limbs in Deuterostomia

Abstract

The paper considers the origin of segmentation, tentacles, limbs, and ciliary bands of larvae of Deuterostomia. The analysis of the expression of regulatory genes (along with the data of classical comparative anatomy) allows us to prove the homology of the pharyngeal plane of Anthozoa and the sagittal plane of triploblatic Bilateria and to confirm the origin of the coelomic segments from the peripheral gastric pockets of coelenterone. At the same time, an end-to-end homology of the segments is assumed within the Bilateria. The homology of the preoral segment and the tentacular segment in all Bilateria is emphasized. It is assumed that the ancestors of Bilateria had two circles of tentacles, labial and marginal. The marginal tentacles of the coelenterates are homologous to the metameric appendages of Bilateria, and the labial tentacles are homologous to the perioral ciliated tentacles and the ventral ciliated sole. The ciliary bands of larvae are homologous to the labial ciliated tentacles transferred to the larval stage. Possible homologues of metameric appendages and ciliated tentacles in Deuterostomia are discussed. In chordates (taking into account the “upside-down theory” of their origin), the radials of the median fin are considered as homologues of the labial tentacles, while the radials of paired fins are considered as homologues of the marginal tentacles. Two-phase expression of Hox genes allows us to prove the homology of tetrapod digits and fin radials of primitive fish. The similarity of the genetic mechanisms regulating the development of limbs makes it possible to stretch the threads of homology from the coelenterate tentacles to the limbs of vertebrates.