Unattached Octocorallia; colonies live upright with their stalks thrust into mud or sand. Each colony consists of an enlarged axial polyp which is divided into two regions: a proximal peduncle or stalk, which never bears secondary polyps and functions as a burrowing organ, and a distal rachis, from which numerous secondary polyps bud. The axial polyp contains a slender, unbranched, calcareous axis. There are two main types of secondary polyps: siphonozooids, which usually lack tentacles, and "normal", tentacled autozooids. Autozooids are often fused together in bunches or leaves, which are arranged in two opposing lateral rows. The ventral surface of a colony is that towards which the expanded autozooids face. Sclerites may be present or absent.
Sea-pens are well known for their brilliant luminescence, which may occur as isolated flashes or rhythmic pulses of light passing along the colony. Nicol (1958) described, and analysed by photo-electric recording, the luminescence of Pennatula and Virgularia.
Many early natural historians believed that sea-pens were free-swimming animals. Grant (in Johnston, 1847) conjured the fascinating but erroneous picture of a colony: "with all its delicate transparent polypi expanded and emitting their usual brilliant phosphorescent light, sailing through the still and dark abyss by the regular and synchronous pulsations of the minute fringed arms of the whole polypi". The more prosaic belief of the fishermen of the time "that they are fixed at the bottom with their ends immersed in the mud" was, however, the correct conclusion.