Practice question

Echinoderm adults possess pentaradial symmetry yet originate from bilaterally symmetrical ancestors, making their radial condition secondary. Planktonic larvae like bipinnaria, auricularia and dipleurula show bilateral organization with distinct anterior-posterior axis, mouth, anus and paired ciliary bands for feeding and locomotion. During metamorphosis, the left side proliferates, larval tissues are resorbed, and the adult rudiment reorganizes around oral-aboral axis into five radii. This dramatic ontogenetic transition from bilateral larva to radial adult demonstrates secondary acquisition, unlike primary radiality observed in cnidarians, supporting deuterostome phylogeny.