Phylum: Cnidaria

Phylum: Ctenophora (comb jelly)


  • Hydrozoans, Jelly, Sea Anemone, Coral Polyps

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Jelly” by Wikimedia Commons under CC by 2.0

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“Sea Anemone” by Pixabay under CC by 2.0

Characteristics of Cnidarians

  • Radially Symmetric
  • Diploblastic (epidermis, gastrodermis)
  • No cephalization, no central nervous system, special respiratory, excretory or circulatory organs
  • Primarily Carnivores
  • Polyp and Medusa body forms


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“Polyp” by Wikimedia Commons under CC by 2.0

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“Medusa”  by Wikimedia Commons under CC by 2.0


This Organism does not have a separate gut they just have GV cavity/body wall) and only one body opening


  • Food and undigested waste pass through same openig 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 in a body compartment for support and transmission of muscular forces
  • Fluid is in-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- Mutualism

-Almost all Cnidarians have cells called Cnidocytes- which have organelles called nematocytes

-Mostly non-polar synapses, impulses can travel in either direction

-Signal diminishes with distance

Subphylum of Cnidaria:

  • Class Scyphoxoa (Jelly Medusa) sexual and asexual phase, Dioecious (male or female). Look the same until they produce gametes
  • Class Cubozoa
  • Class Hydrozoa
  • Subphylum Anthozoa
  • Subclass Hexacorallia
  • Subclass Octocorallia




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