A superficial bilateral organization has evolved twice, in irregular echinoids and holothuroids, but is based on an underlying five-fold organization of skeleton and most organ systems, and is clearly secondary. The evolutionary origins of five-fold symmetry remain obscure. All extant echinoderms live in the ocean, and there is no fossil evidence of any exception to this. Within the marine realm, echinoderms occupy nearly all habitats, where they often constitute a major proportion of the biomass. Pelago-benthic life cycle.
Fertilization is almost always external.
Larvae are plesiomorphically bilaterally symmetrical, have a recurved gut and transparent ectoderm, and feed by upstream particle capture using the ciliated band. Metamorphosis is typically radical and occurs during settlement onto the benthos. In most species, the coeloms are trimerous, and initially bilaterally symmetrical. In some extant forms, the larval mouth is preserved as the adult mouth, while in others the entire digestive system is re-plumbed during metamorphosis and a new mouth and anus form. This organ system shows similarities, and may be homologous, to those of other deuterostome phyla.