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DescriptionA large, colonial hydroid that grows up to 20 cm high, with broad unbranched stems. The side branches are long and straight in alternating series on opposite sides giving a feather-like colony. The colour of living specimens is brown with a green tinge due to zooxanthellae. The feeding polyps are borne in a single series along the side branches, each surrounded by three defensive polyps. The hydrothecae, a supporting, nonliving, protein-chitinous envelope surrounding the stalk, is slightly broader than long and the rim has nine projecting teeth. The corbula or basket is up to 3 mm long, at the base of the branch, close to the main stem with up to ten per stem. Some of the ribs forming the basket are unfused leaving open gaps.