An order of free-swimming, shrimplike crustaceans, commonly known as opossum shrimps, belonging to the subclass Eumalacostraca. They occur in vast numbers in coastal and open oceanic regions of the world. Mysida were once classified along with the Lophogastrida into an order Mysidacea, but the two, along with a closely allied extinct Paleozoic group, Pygocephalomorpha, are now generally considered to be separate orders. The mysidan adult body length generally averages about 0.6 in. (15 mm), and most species are distributed in shallow coastal and shelf waters of the oceans. A few mysidans live in the surface layers of the ocean bottoms. In addition, some species have invaded freshwaters, including specialized forms strictly confined to caves.