Phylum Cnidaria

Phylum Cnidaria

1. Characteristics

   • eumatozoa = animals w/ true tissues
   • acoelomate
   • hydrostatic skeleton
   • radially symmetrical
   • mostly marine
   • polymorphism - generally 2 body forms
   • no organ systems
   • nerve receptors, sensitive to touch
   • Carnivores
   • stinging cells
   • diploblastic
   • mesoglea - not tissue, jelly flotation support

2. Classes

   • Hydrozoa
     • 3000 sp
     • predators
     • mostly marine, some freshwater
     • hydra = polyp only
     • obelia = colonial polyp and medusae
     • physalia = colonial polyp only
   • Scyphozoa
     • true jellyfish
     • marine
     • both polyp and medusa in lifecycle
     • medusa - tentacles around periphery of bell
     • 4 gastric pouches, 4 oral arms to capture prey, nematocysts
     • rhopalium (sense organ containing statocysts and sometimes ocelli)
   • Cuboza
     • box jellyfish
     • cube shaped medusa with tentacles arising from each corner
     • medusa stage dominate
     • bell margin with a velarium
     • rhopalia with ocelli and statocysts
     • predators - have nematocysts
   • Anthozoa
     • anemone, hard coral, soft coral, sea fan, sea pen
     • all marine
     • solitary or colonial
     • polyp form only
     • predators & have symbiotic relationship with zooxanthellae
   • Staurozoa
     • star jellyfish
     • marine
     • only medusa
     • medusa trumpet shaped with thin stalk (attaches to substrate)
     • sexual reproduction - release gametes into water column
     • planula larvae not free swimming!

3. Nerve net

   • simple
   • no brain or concentration of nerves in any one region
   • respond to stimuli
   • contraction controlled by neurons arranged in a diffused nerve net

4. Solitary vs. Colonial corals

   • solitary
     • individual polyps
     • free living or attached
     • single mouth (in some)
   • Colonial
     • polyps joined together
     • generally attached
     • many mouths
   • Framework of coral reefs
     • coral skeletons made from calcium carbonate
     • corals extract calcium carbonate from seawater to build skeleton
     • skeleton lies underneath thin layer of tissue

5. Polyp and medusa

   • Polyp form
     • sessile
     • cylindrical body, mouth directed up
     • tentacles surrounding mouth
     • some sexual, others asexual
   • Medusa form
     • free swimming
     • bell shaped body, mouth directed down
     • tentacles on periphery of bell
     • sexual

6. Nematocysts

   • tentacles of cnidarians armed with stinging cells that contain stinging capsules called nematocysts
   • structure containing a spirally folded venom filled thread with a sensor
   • sensor stimulated --> capsule ejects thread with force and barb penetrates prey
   • some entangle prey, some inject toxins

7. Zooxanthellae

   • single celled algae that live in tissues of animals
   • corals provide protected environment and compounds needed for photosynthesis to zooxans
   • zoos provide food as products of photosynthesis to corals

8. How do coral reefs form?

   • egg & sperm --> fertilization -- > planula larvae --> attach to hard surfaces --> polyp
     • polyps secrete layers of calcium carbonate
       • polyps divide asexually and form colonies
         • most corals are made up of hundreds of thousands of individual polyps

9. Soft corals

   • 8 feathery tentacles surrounding mouth/anus
   • marine
   • colonial only
   • cylindrical body
   • fleshy internal skeleton (sclerites for support) and a hydrostatic skeleton
   • carnivores

10. Hard corals

    • reef builders!
    • tentacles
      • catch prey - nematocysts
      • multiples of 6
      • surround mouth/anus
    • mouth - food is ingested and waste excreted
    • gastrovascular cavity - food digestion
    • zooxanthellae algae in corals tissue photosynthesize
    • limestone skeleton excreted by coral polyps builds reefs
    • cylindrical body
    • hydrostatic skeleton and rigid skeleton of calcium carbonate
    • attach to substrate
    • marine
    • solitary or colonial - no medusa stage
    • carnivores & symbiotic with zooxanthellae