The largest and most highly evolved superorder of the crustacean class Malacostraca. It contains the orders Euphausiacea, Amphionidacea, and Decapoda. Despite the great morphological and ecological diversity of eucaridans, all share (1) a well-developed carapace that is fused to all the thoracic somites; (2) stalked and movable eyes, although in a few these have been secondarily reduced; (3) mandibles that lack a mobile element between the incisor and molar processes; (4) a telson without a caudal furca; (5) heart and gills that are thoracic in position; and (6) typically metamorphic larval development.