Cnidarians

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Cnidarians

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

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