A phylum of animals with hard "valves" (shells) on the upper and lower surfaces, unlike the left- and right-sided arrangement in bivalve molluscs. Brachiopods live only in the sea, and most species avoid locations with strong currents or waves. The larvae of articulate species settle quickly and form dense populations in well-defined areas, whereas the larvae of inarticulate species swim for up to a month and have wide ranges.