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one of the larval stages in the development of coel-enterates. The body is oval, elongate, or wormlike; it consists of two layers. The outer layer, or ectoderm, consists of flagellate cells, among which are distributed musculoepithelial cells, nerve cells, and nemocysts. The inner layer, or endoderm, bounds the closed cavity of the gut. The planula swims in the water, subsequently attaching itself to the bottom and proceeding to the next stage of development—the polyp.