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Hatschek 1888


Phylum Cnidaria


  • Size, shape and colour are all highly variable.
  • Mostly semi-transparent and bell-shaped.
  • Fragile, often less than 5% solid organic matter.
  • Two forms; polyp (benthic) and medusa (planktonic).
  • Greek word ‘Cnidos’ means stinging nettle.

Distinguishing characteristics

  • Diploblastic (with ectoderm and endoderm).
  • Acoelomate (no body cavity); lack real organs.
  • Usually radially symmetrical, with body parts radiating from central axis.
  • Between the outer body layer (epidermis) and the inner layer (gastrodermis) is a layer of thick, elastic, jelly-like substance called mesoglea.
  • Simple digestive cavity (coelenteron) that acts as a gullet, stomach and intestine. Digestive cavity has only one opening.
  • 4 to 8 oral arms located near mouth used to transport food captured by the tentacles.
  • All armed with stinging cells called nematocysts.
  • Four major groups:
    • Anthozoa: true corals, anenomes, seapens.
    • Cubozoa: box jellies.
    • Hydrozoa: siphonophores, hydroids, fire corals, medusae.
    • Scyphozoa: true jellyfish.


  • Occur worldwide.
  • Mostly shallow coastal waters.
  • Can withstand a wide range of temperature and salinity.
  • They drift in water with limited control over movement. Ocean currents, tides and wind determine horizontal movement.
  • However muscles allow jellyfish to contract bell and give some regulation over vertical movement. Vertical movement is important as some jellyfish descend to deeper waters during the day.


  • Cnidarians possess a nervous system that consists of receptors that are able to detect light, odour and other stimuli.
  • Cnidarian larva is called the planula; pear-shaped and entirely ciliated.
  • Male and female medusae spawn freely into the sea, where fertilization occurs and the planula is formed.
  • Carnivorous; feeds on a variety of zooplankton, comb jellies and other cnidarians.
  • Jellyfish are eaten by some fish, loggerhead turtles and even by humans. Jellyfish such as the mushroom jelly are consumed in large quantities, either fresh or pickled, in China and Japan.