Cnidarians
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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

  • Diverse: ~10,000 living species described.

  • cnidoblasts unique to this phylum.  Contains Nematocyst. Threads may wrap, stick, or poison


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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.


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Cnidarians -General Characteristics


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Cnidarians -General Characteristics

  • rudimentary ‘nerve-nets’

  • Sensory organs:

    • Statocysts and ocelli.

  • lack circulatory

    and respiratory structures


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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.


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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


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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)


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Cnidarians –Major Classes

  • Class ScyphozoaThe true jellyfish

    • Medusa stage predominates and most  inhabit the water column in medusa stage.

    • extensive gastrovascular canal systems


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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)


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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.


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