Sea cucumbers symmetry (Echinodermata: Holothuroidea)

The types of symmetry that are characteristic for the class Holothuroidea and reflect different stages in the evolution of this group of echinoderms and the role of paedomorphosis in its origin are analyzed. A hypothesis is proposed to explain the distinctive structure of the water vessel (ambulacral) system of sea cucumbers and the origin of the class Holothuroidea. The development of holothurians was interrupted at the stage of five primary tentacles. It resulted in changing of the mode of metamorphosis. The catastrophic metamorphosis became evol.: the body of an adult organism became to develop based on the body of larva, and the larval tissues were partly included in the body of an adult holothurian. The body of holothurian larva became partially included in the body of adult organism. This process resulted in the modification of the place and way of radial complex organs origination and development and strengthening of the heterochrony processes. All this led to the formation of the holothurian body plan. Data presented support the homology between the radial ambulacral canals of sea cucumbers and other Eleutherozoa.