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Cnidarians. Cnidarians - General Characteristics. Diverse: ~10,000 living species described. cnidoblasts unique to this phylum.  Contains Nematocyst. Threads may wrap, stick, or poison. Cnidarians - General Characteristics. Body plan:

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cnidarians general characteristics
Cnidarians -General Characteristics
  • Diverse: ~10,000 living species described.
  • cnidoblasts unique to this phylum.  Contains Nematocyst. Threads may wrap, stick, or poison
cnidarians general characteristics3
Cnidarians -General Characteristics
  • Body plan:
    • gelatinous layer (mesoglea) between inner and outer tissues usually with scattered cells and collagen fibers.
    • Distinct internal cavity with a single opening (enteron)
    • The mouth is often, but not always, surrounded by a ring of tentacles.
cnidarians general characteristics5
Cnidarians -General Characteristics
  • rudimentary ‘nerve-nets’
  • Sensory organs:
    • Statocysts and ocelli.
  • lack circulatory

and respiratory structures

cnidarians life history
Cnidarians –Life History
  • solitary or colonial
  • All aquatic, most marine
  • primarily carnivorous, but some filter-feed on phytoplankton, and some with endosymbiotic algae
  • May have symbiotic relationships with other species as well:
  • "A" Portuguese man-of-war is
  • composed of various types of polyps
  • for feeding, reproduction
  • and buoyancy.
cnidarians reproduction
Cnidarians -Reproduction
  • Asexual and sexual.
  • Cnidarians have acquired a bi-morphic life-style:
    • sessile polyp, which is attached

to the substrate,

    • a free-swimming/floating medusa
cnidarians major classes
Cnidarians –Major Classes
  • Class Hydrozoa
    • Typically display alternation of generations, polyp (asexual) and medusal (sexual),though some only exist as polyps (e.g. hydra) 
    • No cells in mesoglea
    • No nematocysts in gastrodermal tissue
  • fire "corals“
  • Freshwater "jellyfish"
  • Craspedacusta sowerbii
  • Obelia (a hydroid)
cnidarians major classes9
Cnidarians –Major Classes
  • Class ScyphozoaThe true jellyfish
    • Medusa stage predominates and most  inhabit the water column in medusa stage.
    • extensive gastrovascular canal systems
cnidarians major classes10
Cnidarians –Major Classes
  • Class CubozoaSmall class in size and diversity and recently considered in a different order than Scyphozoa
    • approximately cuboid in shape
    • margin of the bell is frequently

turned inward

    • feed predominantly upon fish and

can even kill humans!

  • Sea Wasp (Chironex fleckeri)
cnidarians major classes11
Cnidarians –Major Classes
  • Class Anthozoa

‘flower animals’ contain majority of cnidarian species

    • enteron (coelenteron) is divided  into a succession of paired vertical partitions.
  • simple net. Why so simple?
  • no medusal stage; sessile or  colonial except for planktonic planula larval stage
  • asexual budding is extensive in colonial forms and some  secrete a  supportive skeleton of calcium carbonate or chitinous material.  Not many other types of organisms have so altered the landscape.