Sea pen
Sea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 16 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide. Sea pens are grouped with the octocorals ("soft corals"), together with sea whips and gorgonians.
The earliest accepted fossils are known from the Cambrian-aged Burgess Shale (Thaumaptilon), similar fossils from the Ediacaran (ala Charnia) may represent the dawn of sea pens. Precisely what these early fossils are, however, is not decided.
As octocorals, sea pens are colonial animals with multiple polyps (which look somewhat like miniature sea anemones), each with eight tentacles. Unlike other octocorals, however, a sea pen's polyps are specialized to specific functions: a single polyp develops into a rigid, erect stalk (the rachis) and loses its tentacles, forming a bulbous "root" or peduncle at its base.
The other polyps branch out from this central stalk, forming water intake structures (siphonozooids), feeding structures (autozooids) with nematocysts, and reproductive structures. The entire colony is fortified by calcium carbonate in the form of spicules and a central axial rod.