Phylum Cnidaria

The beautiful corals, nasty jellyfish and mysterious sea life…Solar powered and build entire island chains over millions of years. Do you feel modest yet?



  1. Unifying Characteristics
    1. Old name - Coelenterata
    2. Majority marine, but a few freshwater species.
    3. Metazoan, with tissues, triploblastic, acoelomate.
    4. Phylum exhibits polymorphism. The two main structural types are the polyp and the medusa.
    5. May be solitary or colonial.
    6. Typical larval form is planula.
    7. Exhibit some form of radial symmetry.
    8. Generally possess a ring of tentacles around the oral end.
    9. The single body cavity is the coelenteron.
    10. The body wall is three-layered.
    11. Possess nematocysts, special cell organelles used for offense and defense, located in cnidoblast cells of epidermis and gastrodermis
    12. Possess undifferentiated interstitial cells (mesoglea), which gave rise to sex cells and cnidoblasts, and are also involved in regenerative and reproductive processes
    13. Nervous system a network and not centralized (nerve net)
    14. Hemaphrodotic, dioecious.
  1. Coelenterata Classification (3 Classes)
    1. Class Hydrozoa
      1. hydrozoan forms two polyp types:
      2. feeding polyps = gastrozooid,

        reproductive polyps = gonozooid (medusae inside)

      3. statocysts - equilibrium organs on medusa


      Hydra (freshwater);

      Obelia (colonial);

      Physalia = Portuguese Man -of- War, not a true jellyfish , but a colonial, pelagic hydrozoan; velum - distinguishing character of a medusa

    2. Class Scyphozoa

      1. medusa dominant, true jellyfish
      2. no velum


Aurelia – True jellyfish

Life Cycle: planula larvae ® strobila ® ephyra ® mature jellyfish


            C.    Class Anthozoa
      1. solitary or colonial, but only polyps

Examples: Sea pansy; sea fan; true corals, sea anemones

(Picture courtesy of Dr. Robert Richmond of the UOG Marine Lab, click below  to see more of the Marines Lab's world class research in Tropical Marine Biology)

University of Guam Marine Laboratory