ZOOLOGY A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site cnidarian A dahlia sea anemone (Tealia sp.) – an Atlantic coast cnidarian. Photo: Andy Bruckner A member of the phylum Cnidaria, a group of aquatic (mostly marine) invertebrates which includes corals, Hydra, jellyfish, sea anemones, sea pens, sea whips, and sea fans. Cnidarians (also known as coelenterates) are diploblastic, with two cell layers (ectoderm and endoderm) separated by mesoglea, and have radial symmetry. The body cavity has a single opening that serves as both mouth and anus. Unique to cnidarians are stinging cells known as cnidocytes (nematocytes), which are used for defense or capturing prey. Cnidarians exist both as free-swimming medusae (such as jellyfish) and as sedentary polyps. The latter may be solitary (e.g. sea anemones and Hydra) or colonial (e.g. corals). In many cnidarians the life cycle alternates between these two forms. Related category    • ZOOLOGY Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP