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Dasycladales (class Ulvophyceae) An order of green algae in which the thallus contains a single nucleus, becoming multi-nucleate prior to reproduction. The thallus consists of an erect axis with branches, the whole sharing radial symmetry. The group appeared in the Ordovician, where elaborate and ornate forms occurred. They were more diverse in the Permian and Jurassic, but became reduced in variety during the Cretaceous and survive now only in the tropics.