General Patterns of Ambulacraria Development

A comparative analysis of the larval development of Echinodermata and Hemichordata was performed. A similarity between the life cycles of Echinodermata and Hemichordata is the formation of planktotrophic larvae with a similar organization of the adoral ciliary field and ciliary bands as a result of embryonic development. In both taxa, the planktotrophic larva transforms into a reserve-trophic larva, which is characterized by a reduction of ciliary bands and swimming based on ciliary rings. There are two evolutionary trends in the phylogeny of Ambulacraria: (1) the reduction of planktotrophic larvae and (2) reduction of reserve-trophic larvae while preserving the planktotrophic ones. The phenomenon of giant planktotrophic larvae was recorded in both groups. An example of giant larvae in echinoderms is Auricularia nudibranchiata, which is the larva of the deep-sea holothuroid Protankyra brychia. It is assumed that Planctosphaera pelagica is a giant larva of a species of deep-sea acorn worms from the family Torquaratoridae. A life cycle with two forms of larvae (planktotrophic and reserve-trophic ones) is assumed to be a synapomorphy of Ambulacraria.