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Phylum Cnidaria. Jellyfish, hydra, sea anemones, & coral. Cnidarian questions. What do we already know about Cnidarians (from the phlyo. Tree) Two tissue layers (germ layers)(ecto and endoderm) “diploblastic” Radial Symmetry 2. Look at the life cycle of a jellyfish…

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phylum cnidaria

Phylum Cnidaria

Jellyfish, hydra, sea anemones,

& coral

cnidarian questions
Cnidarian questions
  • What do we already know about Cnidarians (from the phlyo. Tree)
    • Two tissue layers (germ layers)(ecto and endoderm)
      • “diploblastic”
    • Radial Symmetry

2. Look at the life cycle of a jellyfish…

A. What are the two body types of Cnidarians?

    • Which is mobile? Sessile?
      • Medusa is mobile
      • Polyp is sessile
  • Polyp: cylindrical, vase shaped animals that are usually attached to a rock or sea floor

Ex. Hydras, sea anemones, corals

  • Medusa: bell-shaped body; umbrella

Ex. Jellyfish

  • Radial symmetry
skeletal muscle system
Skeletal/Muscle System
  • NO MESODERM, so…no real muscles!
    • most have primitive muscle cells
    • coral have external skeleton of calcium carbonate
capturing prey let s eat

Nematocyst (harpoon)

Capturing Prey LET’S EAT!!!
  • cnidarians are carnivores (eat meat)
    • use tentacles arranged in a ring around their mouth to capture prey
  • when a “trigger” is stimulated on a stinging cell called the cnidocyte, a harpoon or nematocyst shoots out & injects poison into the prey

Cnidocyte (stinging cell)

capturing prey cnidocyte and nematocyst at work
Capturing Prey: Cnidocyte and Nematocyst at work

Nematocyst (harpoon)

Cnidocyte (stinging cell)

  • Sac w/ a digestive cavity (gastrovascular cavity) w/ a single opening that acts as a mouth/anus



digestion cont
Digestion (cont.)
  • food is pushed into the gastrovascular cavity where digestion begins (extracellular digestion)
  • undigested remains leave through the mouth/anus
nervous system
Nervous System
  • No brain or even ganglia, no cephalization, etc
  • Nerve Net- have simple sensory receptors that detect & respond to stimuli
  • Diffusion- oxygen from water moves into cells (high to low)
  • Asexual – budding- formation of outgrowths that pinch off from parent to live independently
  • Sexual- forms a zygote and free swimming form that settles on ocean floor (usually a polyp)
  • Diffusion to release waste

Class Hydrozoa

  • Ex- Hydra
  • both asexual polyps and sexual medusa stages
  • Spends most of its life in the polyp stage
  • Reproduction:
    • Sexual: eggs and sperm are shed into the water and form fertilized eggs; planula is by passed with eggs hatching into young hydras
    • Asexual reproduction via budding

Class Hydrozoa cont.

Other Hydrozoans

Portuguese man-of-war:

Single gas-filled float with tentacles

Tentacles house the polyps and modified medusae of the colony


Class Scyphozoa

  • Jellyfish
  • The medusae are large and contain massive amounts of mesoglea
  • Sexual and Asexual reproduction

Class Anthozoa

  • include sea anemones and coral
  • Exclusively marine; there is no medusa stage
  • Most anthozoans are colonial (e.g. corals) and secrete external skeletons composed of calcium carbonate.