General patterns of the ontogenesis of Ambulacraria

The life cycle with two larvae (planktotrophic and reserve-trophic) is assumed to be a synapomorphy of Ambulacraria. Planktotrophic larvae have a complex ciliary band and use the “up-stream” filtration for nutrition. Planktotrophic larvae undergo metamorphosis and turn into non-feeding larvae with complete or incomplete ciliary rings. These larvae do not use the yolk, but the reserves stored at the previous planktotrophic stage, therefore they can be called reservetrophic. Among echinoderms, holothurian life cycle is assumed to be plesiomorphic. It includes a planktotrophic larva (auricularia) followed by a reserve-trophic larva (doliolaria). Most echinoderms have lost their reserve-trophic larvae, although rudimentary doliolarians are known in some sea urchins and brittle stars. Most crinoids have doliolaria larvae only, although stalked sea lilies have auricularia followed by doliolaria, but both larvae are lecithotrophic. In enteropneusts, Spengelidae and Ptychoderidae have planktotrophic tornarians followed by reserve-trophic Agassiz larvae. Species of the Harrimaniidae have lost tornarians, but have retained the lecithotrophic Agassiz larvae. The giant planktotrophic larvae were recorded both in echinoderms and hemichordates. The deep-sea holothurian Protankyra brychia has giant larva known by the name Auricularia nudibranchiata. It is possible that the giant tornarian known by the name Planctosphaera pelagica is a larva of some species of deep-sea enteropneust.

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