This is a slide of many stained specimens of Obelia, a colonial marine cnidarian that shows a distinct polymorphism in the organization of its members.  Feeding polyps called hydranths bear tentacles armed with nematocysts, a mouth and a thin outer covering or hydrotheca.  Gonangia or reproductive polyps consist of a stalk containing medusa buds surrounded by a thin membrane called the gonotheca.  When mature, these medusa buds are released from the gonangium through a central opening called the gonopore.  All individuals are attached to a main stem known as the hydrocaulus, which consists of a cylindrical tube of living tissue called the coenosarc covered by a thin outer membrane or perisarc.