Invertebrates Cnidarians

• Cnidarians


  Radial symmetry
• Domain


  • Eukarya
• Kingdom


  • Animalia
• Phylum


  • Cnidaria
• Class


  • Hydrozoa
  • Scyphozoa
  • Anthozoa
• Cnidarians are sometimes called "coelenterates" and include the sea anemones, jellyfishes, corals, and their relatives
• Level of organization


  • Tissues
• Cnidarians display radial symmetry, where similar parts of the body are arranged and repeated around a central axis
• General Morphology


  • Epidermis
  • Gastrodermis
  • Mesoglea
  • Mouth
  • Nematocysts
• Two Basic forms


  • Polyp - a sac-like attached stage with the mouth and tentacles oriented upward
  • Medusa - a bell-like which is like an upside-down polyp adapted for swimming
• The characteristic larva of most cnidarians is the planula, a cylindrical, ciliated stage consisting of two layers of cells. After a time in the plankton, the planula settles on the bottom and metamorphoses into a polyp or develops into a medusa

  
• Types of Cnidarians


  • Hydrozoans (Class: Hydrozoa) - consist of feathery or bushy colonies of tiny polyps
  • Scyphozoans (Class: Scyphozoa) - larger jellyfishes common in all oceans
  • Anthozoans (Class: Anthozoa) - solitary or colonial polyps that lack a medusa stage

  
• Hydrozoans


  • Siphonophores - drifting colonies of polyps, some may be specialized as floats, as in the Portuguese man-of-war (Physalia physalis)

  
• Some scyphozoan medusae are among the most dangerous marine animals known, giving extremely painful and sometimes fatal stings

  
• The anthozoan polyp is more complex than hydrozoan or scyphozoan polyps
• Anthozoans


  • Sea anemones, corals, gorgonians
• Gorgonians, such as sea fans, are colonial anthozoans that secrete a tough, branching skeleton made in part of protein
• Feeding and Digestion


  Cnidarians are carnivorous and use their nematocysts primarily to capture prey. After ingestion, food passes into the gut, where it is digested. The initial phase of digestion is extracellular, and intracellular digestion within cells lining the gut completes the breakdown of food
• Behavior


  • Though cnidarians lack a brain or true nerves, they do have specialized nerve cells
• Comb jellies (Phylum: Ctenophora) are radially symmetrical and gelatinous, and they swim using 8 rows of ciliary combs

  
• Cnidarians


  A phylum of animals that includes jellyfish, sea anemones, corals, and hydroids. They are known for their stinging cells called cnidocytes.
• Radial symmetry


  A type of body plan in which an animal has identical halves when divided along only one axis. Cnidarians typically have radial symmetry along the oral-aboral axis, meaning they are symmetrical from the mouth (oral) to the opposite end (aboral).