sea pen, long, slender colonial organism of the same phylum as the jellyfish. Sea pen colonies are formed by several genera of the order Pennatulacea. The colony consists of a stalk formed by an organism called a primary polyp (see polyp and medusa) and short branches formed by secondary polyps. The stalk, embedded in sand or mud, holds the colony upright. Sea pens differ from the closely related sea pansies and sea feathers by the form of the colony. Sea pens are marine organisms; they are found on Atlantic and Pacific coasts in shallow to moderately deep water. Some reach a length of 2 ft (61 cm) or more. They belong in the phylum Cnidaria, class Anthozoa, order Pennatulacea.
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