Figure 30.21 Diversity among Cnidarians (A) Sea anemones are sessile, living attached to marine substrates. Water currents carry prey into the nematocyst-studded tentacles. (B) The sea pen is a colonial cnidarian that lives in soft bottom sediments and projects polyps above the substrate. (C) This jellyfish illustrates the complexity of a scyphozoan medusa. (D) Polyp colonies of this hydrozoan grow on rocks of intertidal pools.