Starfish Lab

Introduction: The starfish has a body that consists of a central disk from which extend a number of rays. Most starfish have 5 rays, but some have multiples of 5. A starfish has no head, and any one of its rays may lead in its slow creeping movements along the sea bottom. The central disk contains a stomach on the ventral side. The starfish has an internal skeleton that has spines which extend through the epidermis to the exterior of the animal. Each ray contains a pair of digestive organs as well as a pair of gonads, skin gills used for respiration, and a nerve cord. Two or more rows of tube feet are also found on each ray.

Purpose: To identify the internal and external structures of a starfish.

Materials:Preserved starfishDissecting trayScalpel

ScissorsTweezersProbe

Procedure:

External Structures

1. Place the starfish in the dissecting tray with the dorsal side up.

2. Locate the arms. Look to see if any are regenerating. On your data sheet, note how many arms there are, and note whether any were regenerating.

3. Stroke the skin of each arm. On your data sheet, describe its texture and what it feels like.

4. If possible, locate the eyespot at the end of each arm. The eyespot may not be seen because their red color becomes dull by the preservative. They are now white in color. On your data sheet, record how many you can see. If you cannot see any, note NONE.

5. Look for the very small circular disk located near the starfish's center. On your data sheet, identify this structure.

6. Now turn the specimen with the ventral side up.

7. Find the hole through which the stomach exits the starfish. On your data sheet, name this structure.

8. Notice the groove that runs from the tip of each arm toward the center of the starfish. On your data sheet, write the name of this structure.

9. The tube feet are arranged on each side of the groove. On your data sheet, identify what helps these tube feet to contract and expand, allowing the starfish to move.

Internal Structures

1. Place the starfish dorsal side up, and cut a cross section through one arm. On your data sheet, draw and label what structures you see.

2. Cut along the top side of 3 of the other 4 arms. Carefully remove the skin, allowing the organs to remain in the arms.

3. In one arm, observe the digestive organs. On your data sheet, describe their appearance, and note how many are in each arm.

  1. In another arm, remove the digestive glands and view the gonads. On your data sheet, describe their appearance and location in relation to the digestive glands.
  1. In the last arm, remove both the digestive and reproductive organs to see the radial canal. On your data sheet, describe its appearance.

Data Sheet

1. How many arms were there, and were any regenerating? When would this regeneration be considered a form of asexual reproduction?

2. Describe the texture of the starfish's skin.

  1. How many eyespots can you see? What can these eyespots detect?
  1. What is the name of the small circular disk located on the dorsal side (near the center) of the starfish? What is its function?

5. Name the hole through which the stomach exits the starfish? What are they likely to feed upon?

6. Name the structure that runs from the tip of each ray toward the center of the starfish, seen from the ventral side.

  1. What helps these tube feet to contract and expand, allowing the starfish to move? What other structures are connected to the tube feet?

8. Draw and label what you see with the cross section of your starfish.

9. Describe appearance of the digestive glands. How many are seen in each arm?

10. Describe the appearance of the gonads, and explain where they are located in relation to the digestive glands.

11. Describe the appearance of the radial canal.

12. List the path that water takes through the water vascular system.

Analysis Questions

  1. What life processes does the water vascular system help the starfish to perform?
  1. Does the starfish exhibit cephalization? If so, explain how. If not, explain what features they do have in their nervous system.
  1. Explain the feeding process of a starfish. How do they get to their food, and what happens in order to digest their food?
  1. Do starfish appear as separate males and females? Can you tell by looking at them?
  1. How do starfish reproduce? What other animal is this similar to?
  1. What kind of symmetry does the starfish have? What other animal is this similar to?
  1. Name 2 other animals that the starfish is related to. How are they similar? How are they different?

8. Why can the starfish be described as having a tube within a tube body plan? What digestive structure does it have?