Cnidarians: Hydrozoans, jelly, sea anemone, coral polyps

General Cnidarian Characteristics

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

Have no separate gut, (just GV cavity/body wall) 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 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

-Longitudinal and circular muscles are enough to cause many different kinds of movements and shape changes

-Epithelio-muscular cells (longitudinal)

-Nutritive-muscle cells (circular)

-Many Cnidarians have endosymbiotic dinoflagellates (zooxanthellae) within their cells – Very important mutualism!

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

  • Used for prey capture
  • Why jellyfish “sting”

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

-Signal diminishes with distance

Phylum Cnidaria

  • Subphylum Medusozoa
    • Class Scyphozoa
    • Class Cubozoa
    • Class Hydrozoa
  • Subphylum Anthozoa (used to be Class)
    • Sunclass Hexacoralia 
    • Subclass Octocoralia

-Most Scyphozoans are Dioecious