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".