This is a group of alycyonaria in which the substance keratin (or Gorgonin ) closely allied to horn enters into the composition of the skeletal structures. In a few gorgonians, there is simply a long, unbranched stem attached by a basal expansion to some substratum. Usually the main stem is soon divided into secondary branches and these ramify again before they terminate in numerous delicate twigs, but the branching throughout takes place in one plane only. This results in a fan-shaped structure from which these forms derive their popular name "Sea fans".