: an order of Hydrozoa consisting of various free-swimming or floating pelagic mostly delicate transparent and often showily colored forms that are usually regarded as compound animals composed of zooids modified to perform various functions for the colony (such as feeding, defense, locomotion), that sometimes have two or more zooids in the form of a bell which by their contractions cause the colony to swim, and that often have a hollow float which keeps the colony afloat compare diphyes , porpita , portuguese man-of-war , velella
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