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Pennatulidae is a family of around 50 described species of octocorals (or "soft corals") in the cnidarian order Pennatulacea and suborder Subselliflorae, the sea pens. Various sea pens are found throughout the world's oceans from relatively shallow waters to several thousand meters in depth. Like other octocorals, sea pens are actually colonies of individuals known as polyps. Most sea pens exhibit striking polyp polymorphism: One polyp grows very large and loses its tentacles, forming the central axis of the colony and anchoring itself in soft sediment; other polyps taking various forms and serving various functions (feeding, water circulation) branch off from this central polyp. The global diversity of Pennatulacea was recently reviewed by Williams (2011).
The common name “sea pen” and the order and family names “Pennatulacea” and "Pennatulidae" are derived from the perceived resemblance of the species in some genera (such as Pennatula, Pteroeides, and Virgularia) to quill pens.
(Williams 2011 and references therein)