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    Cnidarians Cnidarians Presentation Transcript

    • Cnidarians
    • Core Concepts
      • The cnidarians (or coelenterates), phylum Cnidaria (or Coelenterata), are soft bodied animals that exhibit radial symmetry and posses true tissues.
      • The basic structure of cnidarians includes a sac with a central digestive area (gastrovascular cavity or coelenteron) and a single opening surrounded by tentacles bearing stinging cells.
      • Cnidarians have two body forms: the sessile/sedentary polyp and the free-swimming medusa. In many cnidarians, the life cycle alternates between these two body forms.
      • Phylum Cnidaria is divided into three classes: Class Hydrozoa (hydras), Class Scyphozoa (jellyfish) and Class Anthozoa (corals and sea anemones
    • Keywords
      • Cnidaria/Coelenterata
      • Eumetazoa
      • polyp
      • medusa
      • gastrovascular cavity/coelenteron
      • epidermis
      • mesoglea
      • gastrodermis
      • cnidoblast
      • nematocyst
      • nerve net
      • Hydrozoa
      • Scyphozoa
      • Anthozoa
    • Characteristics
      • the name: Cnidaria  from KNIDE (nettle)
      • Coelenterata  from COEL (cavity) + ENTERON (gut)
      • includes hydras, jellyfish, corals and sea anemones
      • aquatic (mostly marine, few fresh water species)
      • basic structure: hollow sac w/ single opening (mouth) surrounded by tentacles bearing stinging cells (cnidoblasts)
      • exhibits radial symmetry
      • diploblastic, with two cell layers of the body wall separated by mesoglea  EPIDERMIS & GASTRODERMIS (+ MESOGLEA – jelly-like, non-cellular)
      • exist as both free-swimming medusae and as sessile/sedentary polyps
    • Digestive and nervous system
      • Incomplete digestive system
      • Nerve net
    • Structure and function
      • diffuse nervous system  NERVE NET (nerve cells distributed all over body)
      • tentacles possess CNIDOBLASTS (stinging cells) containing NEMATOCYSTS (w/ poison) for defense and catching food
      • gastrovascular cavity (coelenteron) is sac shaped with one opening acting as both mouth and anus  INCOMPLETE gut/digestive tract
      • digestion  EXTRACELLULAR (within the gut) and intracellular (in cells lining the gut)
      • no respiratory, circulatory, excretory systems; only through diffusion
    • Body form
      • POLYP
        • sessile, cylindrical body w/ mouth & tentacles at upper free end
        • may be solitary or colonial
        • e.g. corals, hydras, sea anemones
      • MEDUSA
        • free swimming, like an upside down bowl w/ mouth & tentacles facing downward
        • e.g. jellyfish
    • Reproduction
      • polyp  asexual by budding (polyp or medusa)
        • regeneration of Hydra
      • medusa  sexual
        • larval stage (PLANULA) elongate & ciliated
        • settles, attaches & develops into a polyp
    • Life cycle
    • Class Hydrozoa
      • alternation between polyp and medusa forms, both stages equally well-developed where the polyp stage is more conspicuous than the medusa
      • e.g. Obelia
      • Physalia (Portuguese man-of-war)
      • Hydra (only exists in polyp form)
    • Class Scyphozoa
      • medusa is the conspicuous stage; polyp stage reduced to small larva
      • although in some, there is no polyp stage
      • e.g. true jellyfish  Aurelia
    • Class Anthozoa
      • “ flower animals”
      • sedentary or sessile polyp; no trace of medusa stage
      • e.g. sea anemones
      • corals (secrete hard external skeletons of CaCO­ 3 )
        • Cnidarians, which are among the most primitive eumetazoans, were the first animals capable of movement. The presence of true tissues, albeit a simple nerve net and contractile cells, has enabled these organisms to sense and react to stimuli.