Crustaceans have a number of larval forms. The earliest and most characteristic is the nauplius. In most groups, there are further larval stages, including the "zoea" (pl. zoeæ or zoeas). This name was given to it when naturalists believed it to be a separate species. It follows the nauplius stage, and often has spikes on its carapace. These may assist these small organisms in swimming. In many decapods, due to their accelerated development, the zoea is the first larval stage. In some cases, the zoea stage is followed by the "mysis" stage, and in others, by the "megalopa" stage, depending on the crustacean group involved.