Pennatulida, or "Sea Pens," an order of Alcyonaria (q.v.), characterised by the free habit of the colonies and the fact that the individuals or zooids occur usually on a long, central, rod-like skeleton. The zooids are developed on two types, so that the colony is said to be dimorphic; the zooids are situated onlyon the upper exposed part of the colony, while the lower part is embedded in sand or mud. In some genera, suqh as the '; Cock'scomb "or Pennatula, the zooids are borne on lateral branches from the main stem; while in. others, such as Veretillum, they are placed directly round the central stem. The best-known English member of the order is Pennatula phosphoria, a small species about three inches in length; it owes its specific name to its phosphorescence when irritated. The English members of the group have been monographed by the late Professor Milnes Marshall. The Pennatulida are all marine; a few fossil forms are known, such as the Graphularia of the London Clay, or Pavonaria of the Chalk. The Silurian fossil Protovirgnlaria, which has been referred to this order, is probably a Graptolite..