Cnidaria

=**Phylum Cnidaria (meaning "stinging cells")**= The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea anemones, and corals. Cnidarians are radially or biradially symmetric, a general type of symmetry believed primitive for eumetazoans. They have achieved the tissue level of organization, in which some similar cells are associated into groups or aggregations called tissues, but true organs do not occur. Cnidarian bodies have two or sometimes three layers. A gastrovascular cavity (coelenteron) has a single exterior opening that serves as both mouth and anus. Often tentacles surround the opening. Some cells are organized into two simple nerve nets, one epidermal and other gastrodermal, that help coordinate muscular and sensory functions.

Cnidarians have two basic body forms, medusa and polyp.
 * Medusae**, such as adult jellyfish, are free-swimming or floating. They usually have umbrella-shaped bodies and tetramerous (four-part) symmetry. The mouth is usually on the concave side, and the tentacles originate on the rim of the umbrella.
 * Polyps**, in contrast, are usually sessile. They have tubular bodies; one end is attached to the substrate, and a mouth (usually surrounded by tentacles) is found at the other end. Polyps may occur alone or in groups of individuals; in the latter case, different individuals sometimes specialize for different functions, such as reproduction, feeding or defense.

Reproduction in polyps is by asexual budding (polyps) or sexual formation of gametes (medusae, some polyps). Cnidarian individuals may be monoecious or dioecious. The result of sexual reproduction is a planula larva, which is ciliated and free-swimming.


 * Nematocysts** define cnidarians. These tiny organelles, likened by Hickman to cocked guns, are both highly efficient devices for capturing prey and extremely effective deterrents to predators. Each contains a coiled, tubular thread, which may bear barbs and which is often poisoned. A nematocyst discharges when a prey species or predator comes into contact with it, driving its threads with barb and poison into the flesh of the victim by means of a rapid increase in hydrostatic pressure. Hundreds or thousands of nematocysts may line the tentacles or surface of the cnidarian. They are capable even of penetrating human skin, sometimes producing a painful wound or in extreme cases, death.


 * Classes of Cnidaria:**
 * //Cubozoa//** encompasses about 20 species, among them are the species //Chironex fleckerii// and //Chiropsalmus quadrigatus//, known as sea wasps, which possess a highly potent toxin.
 * //Scyphozoa//** contains about 200 species, commonly known as jellyfish. The name Scyphozoa means "cup animals," reflective of the dominant medusa form (Towle 1989).
 * //Anthozoa//** includes over 6,000 species, including sea anemones and corals. Anthozoa means "flower animals." The medusa stage is not known among this class.
 * //Hydrozoa//** contains about 3,700 mostly marine species, and is a broad spectrum stretching from the tropical fire corals to the hydroids.

**Student ZooKeeper Pages of Species: (Enter Your Species Pages Below)**
Blue Button Jellyfish (Zookeeper: Sidney Baker) Cannonball Jellyfish (Zookeeper: Taylor Cebuly) Portuguese Man O' War (Zookeeper: Kenzie Davidson) White/Australian Spotted Jellyfish (Zookeeper: Shaiann Etwara) Lion's Mane Jellyfish (Zookeepeer: Erin Holbert) Sea Slugs(Zookeeper:Syed Islam) Moon Jellyfish (Zookeeper: Marissa Jenkins) Silky Medusa (Zookeeper: Hannah Jones) //Polypodium hydriforme (Zookeeper: James K. Kanthachack)// Man-of-War Jellyfish (Zookeeper: Savannah Kelley) Renilla reniformis (Zookeeper: Tori Lopez) Erenna Jellyfish (Zookeeper: Whopper) Irukandji Jellyfish (Zookeeper: Heath McGregor) Hydra (Zookeeper: Ramon Prieto) Freshwater Jellyfish (Zookeeper: Lindsey Rafus) Tube Coral(Zookeeper: Sarah Reimer) Sea Anenome (Zookeeper: Paco Rodriguez) Crystal Jelly (Zookeeper: Paige Ryder) Pink Hearted Hydroid (Zookeeper: Haley Shelton) __Yellow Sea Anemone__ (Zookeeper: Jeffrey Slape) Purplestripped Jelly (//Pelagia noctiluca//) (Zookeeper: Riley Stewart)) Plate Coral- Fungia scutaria (Zookeeper: Jessica Tolliver) Octocoral Polyps (Zookeeper: Rachel Tolliver) Sea Wasp- //Chironex fleckerii// (Zookeeper: Ashley Veilleux) Box Jelly(Zookeeper: Jayme Willingham) __Fire Coral__ ( Zookeeper: Jesse Wilson) Atlantic Mushroom Coral (Zookeeper: Katie Woolen) Giant Green Anemone (Zookeeper: Elisabeth Young)

Myers, P. 2001. "Cnidaria" (On-line), Animal Diversity Web. Accessed March 06, 2011 http://animaldiversity.ummz.umich.edu/site/accounts/information/Cnidaria.html. http://www.newworldencyclopedia.org/entry/Cnidaria seanet.stanford.edu
 * Information and Photo References:**