Pennatularia

(sea pens), an order of marine invertebrates of the class Actinozoa. A colony consists of a large main polyp, which forms the trunk of the colony, and usually numerous small secondary polyps, located on the upper end or on special lateral processes of the main polyp. In the latter case, the colony has the form of a feather. The base of the main polyp is embedded in the sea bottom. There are approximately 300 species of sea pens, distributed predominantly in tropical and subtropical waters from the littoral zone to a depth of 6 km. In the USSR, six species are found in the seas of the Far East, and four in the northern seas (including Umbellula encrinus —which is up to 2.6 m tall). Many sea pens are luminescent.