A soft coral in the order Pennatulacea and subclass Alcyonaria. Sea pens are colonial octocorals which exhibits polyp dimorphism. One polyp grows very large, loses its tentacles and forms a central axis. The base of this primary polyp forms a bulb which may be expanded or contracted. The bulb is used to anchor the colony. Branching off this primary polyp are various secondary polyps. Some, called autozooids, are typical feeding polyps. Others, called siphonozooids, serve as intakes for water, which circulates within the colony and helps keep it upright. Calcareous spicules and frequently a central axial rod of calcium carbonate also provide support. One group of sea pens have secondary polyps grouped into "polyp leaves," giving these species a feather-like appearance, from which we get the common name as they look something like old-fashioned quill pens. Most species, however, do not have polyp leaves, and look more like clubs, umbrellas, or pinwheels.