Oceanography: Starfish Dissection
Background : Echinoderms are radially symmetrical animals that only live in the sea.
Echinoderm means "spiny skin" in Greek. There are over 6,000 species and they usually
have five appendages. Sea stars (class Asteroidea ) are sometimes called starfish, though
they are not real fish. Sea stars move through a complex system of water movement. First,
water enters madreporite and flows through the stone canal and then enters the circular ring canal. Water then is
separated into five radial canals that branch into double rows of bulblike structures called ampullae, which are on
each side of the ambulacral ridge. The ampullae are connected to sucker-like tube feet. Contraction of the ampullae
causes the tube feet to stretch as water is brought into them. This whole process allows for movement, and is quite
powerful but extremely slow.
Part I: External Anatomy
First, obtain a preserved starfish and place it in a dissection pan with its aboral surface facing upward. Use web
resources to access an external diagram of a starfish to guide you through the checklist below.
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_____ Locate the central disc in the center of the starfish.
_____ Locate the small, round hard plate called the madreporite on top of the central disc.
_____ Feel the upper surface of the starfish for spines.
_____ Look at the tip of each arm and find the eyespot.
_____ At the center of the central disc, locate the anus of the starfish.
_____ On the oral surface, locate the mouth in the center of the central disc.
_____ Find the ring of oral spines surrounding the mouth.
_____ Find the groove that extends down the underside of each arm. This is called the ambulacral groove.
_____ Feel the numerous, soft tube feet inside each groove.
External Anatomy Sketch: Examine one of the tube feet and sketch it here:
Check in with the teacher to identify the above structures before
continuing with the dissection.
Part II: Internal Anatomy
With the starfish's aboral surface up, cut off the tip of an arm (about ½ inch up from the end of the arm). On one
edge of the arm, carefully cut a flap of skin back from the tip of the arm towards
the central disc. Repeat the same cut on the other side of the arm. Then, cut
across the arm to connect the two previous incisions and remove the skin flap
entirely. Your purpose in this cut is to reveal the internal anatomy of the starfish
arm (see figure at right).
10. _____ Observe the parts of the tube feet. Find the zipper-like ridge that
extends the length of the ray. The tube feet are attached to this.
11. _____ Locate the bulb-like top of a tube foot called the ampulla. This sac works like the top of an eyedropper
to create suction. The bottom of the tube foot is a sucker.
12. _____ Tube feet are attached to the radial canal.
Next, cut a circular flap of skin from the central disc. (You will have to also cut around the madreporite in order to
remove this flap.)
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_____ Observe the stomach under the central disc.
_____ Locate the skeletal plates called ossicles embedded in the soft body wall.
_____ Find the ring canal in the central disc where the five radial canals, or lateral canals, connect.
_____ Located the stone canal, a short canal that leads from the ring canal to the madreporite
Post-Lab Questions:
1. Name the kingdom, phylum, and class for the starfish you dissected.
2. How is the radial symmetry exhibited by the starfish and other echinoderms different than the radial
symmetry seen in cnidarians?
3. Would you be able to locate the dorsal side of this starfish? Why or why not?
4. On what types of organisms do starfish feed?
5. Describe how the tube feet of starfish aid in their feeding on bivalves.
6. What does “echinoderm” mean and why is this an appropriate name for organisms like the starfish?
7. Where does water enter the starfish? Where does it leave?
8. What accompanying structure allows the tube feet to fill with water and project for movement and feeding?
9. How do starfish reproduce?
10. Describe what happens to the stomach of a starfish as it feeds on a bivalve.
11. What structures make up the nervous system of the starfish?
12. Name two other classes of echinoderms and give an example of each.
13. How does the water vascular system aid in the circulation and respiration of the starfish?
14. How do the feeding mechanisms of the starfish and sand dollar differ?
15. What is evisceration and what echinoderm uses it as its defense system?
16. What is meant by the term “regeneration” and how is this ability used by starfish?
17. The water vascular system of the starfish opens to the outside through a sieve-like structure called a
____________________________.
18. Would you classify a starfish as benthic or pelagic?
19. In what way(s) would you classify the starfish as a more complex organism than the squid?
20. In what way(s) would you classify the starfish as a less complex organism than the squid?
21. Complete the following table:
Structure
Madreporite
Tube Feet
Ampullae
Water Vascular
System
Function
22. Label your starfish diagram with the following terms and staple it to this lab: mouth, anus, madreporite,
oral surface, aboral surface, arm, eyespot, spines, ambulacral groove, central disc.