Class notes (Cnidaria)

Hydrozoans, jellies, sea anemones, coral polyps

Cnidarian characteristics

  • Radially symmetric
  • Diploblastic (epidermis, gastrodermis)
  • No cephalization, no central nervous system
  • No special respiratory, excretory or circulatory organs
  • Primarily carnivores
  • Polyp (sessile) and medusa (pelagic) body forms
  • Have no separate gut and only one body opening. This has several consequences:
    • Food and undigested waste pass through same opening in different directions
    • Movements of body cause distortion of GV cavity
    • GV cavity serves for both circulation and digestion
    • Gametes and embryos must be released through the same opening
  • The hydrostatic skeleton
    • Uses fluid (water) in body compartment for support and transmission of muscular forces
    • Fluid is compressible, force generated by displacement of fluid in one region, can be used to do work in another

Epithelio-muscular cells (longitudinal)

Nutritive-muscle cells (circular)

Many cnidarians have endosymbiotic dinoflagellates (zooxanthellae) within their cells – this is a very important mutualism

Almost all cnidarians have cells called cnidocytes – which have organelles called nematocysts



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