Cnidarians

Purpose:

  1. Identify the structures of a jellyfish through dissection techniques.
  2. Identify and compare structures of various cnidarians.

Materials:

The following organisms are available for observation and/or dissection:

Aurelia (jellyfish) – also available for dissection

Dissecting microscopeScalpelPins

Dissecting trayRulerScissors

Dissecting probe

Procedure:

  1. Observe, draw, label, and give the function and adaptive value for the following structures for each of the specimens listed in the materials (make sure to indicate color, size, etc.):
  2. Whole organism
  3. Gonads
  4. Marginal tentacles
  5. Oral arms
  6. Mouth
  7. Gastrovascular cavity
  8. Tentacles under microscope showing cnidocytes.
  1. Lay the body upside down so that you can visualize the underside of the jellyfish.
  2. Count the oral arms.
  3. Use the scalpel to carefully slice open the body cavity along the midline. Observe the internal organs and record.
  4. Slice the body cavity along the transverse plane and observe the body structures.
  5. Open one of the oral arms and diagram the structure.
  6. Open the other tentacles and diagram the structure. How do each of the tentacles differ?
  7. Open up the gastrovascular cavity; observe and record.
  8. Identify the gonads. Carefully remove them from the body. Open them up and observe the internal structure.
  9. Place each portion under the dissecting microscope to take a look at the cellular level of organization.
  10. Remove a THIN slice of the gonad and create a wet mount slide. View it under a microscope. Diagram your observations.
  11. Repeat step i for the tentacles. PUT YOUR MICROSCOPE BACK WHEN YOU ARE DONE!!!
  1. Questions to answer for each specimen listed in the materials:

How many oral arms are there? How many gonads? What is the length of one of the oral arms (cm) – Aurelia jellyfish only? What is the length of a marginal tentacle (mm) – Aurelia jellyfish only?

  1. Questions:
  2. Why might coral be included in the Cnidarians? What are some things/structures that might influence you to remove it from this phylum? Where would you put it instead? Support your answer.
  3. Why does a jellyfish look the way it does? Why is it colored the way it is? Why is it shaped the way it is? Why are the structures located where they are? Refer to specific structures and use the language of the theories of evolution and natural selection to answer the questions.
  4. Why does a coral look the way it does? Why is it colored the way it is? Why is it shaped the way it is? Why are the structures located where they are? Refer to specific structures and use the language of the theories of evolution and natural selection to answer the questions.
  5. Explain why Cnidarians can move while Porifera cannot. You may need additional resources to answer this question. Frame your answer using the language of natural selection.
  6. Choose two features from the structures listed in #1 of the procedure and explain why they are in a jellyfish. Frame your answer using the language of natural selection.
  7. What are the defense mechanisms found in each of the specimens studied? Why did they develop they way they did? Frame your answer using the language of natural selection.