Clam Dissection Questions

Pre-lab:
1. Give the kingdom, phylum, and class for the clam.

2. Give several examples

2.  What is the mantle and give its function?

3. What controls  the opening and closing of the clam's shell?

4. How do clams move?

5. How do clams feed?

6. Why are clams called bivalves?

Introduction

The following outline can be used as a guide to doing the dissection or as a substitute for the dissection in the event that clams are not available.

Purpose: To observe the structure of the clam and relate it to the animal's niche.

Procedure: Follow the steps below. The diagrams should help guide you.

1. Place the clam on a dissecting tray so that the left side is up. The shells are hinged on the dorsal side. If you can see the siphons it will be easy to determine which is the left side. The siphons are at the posterior end. If the siphons are not visible, determine which is the front end by using the umbo. The umbo is the bump on the top of the shell. It leans toward the front end.

Notice that the siphon is actually made of two siphons. If you cannot see the siphon now, remember to look closely at it after you open the shell. The ventral siphon carries water into the shell and the dorsal siphon removes the water.



2. If the shells are relaxed enough, notice the edge of the mantle. The mantle has a ruffled appearance along its edge, otherwise it is very thin. The mantle is against the shell and secretes the shell. Some bivalves produce pearls when particles like grains of sand get in between the mantle and the shell. We try to open the clam without disturbing the mantle. It is easy to tear because it is quite thin.

3. The shells are held together by two muscles called the anterior and posterior adductor muscles. The posterior adductor muscle is above the siphons, quite close to the edge of the shell. Remove the left shell by inserting the scalpel blade between the shell and the mantle and cutting the adductor muscles away from the left shell. It is important to keep the blade against the shell so that you are scraping the adductor muscle off the shell more than cutting it. If the shells are still tightly shut, you will have to work the scalpel blade in between the shells carefully. Get help from your instructor if you are having difficulty.

4. Once both adductor muscles have been cut away from the left shell, the shell can be opened easily. As the shell is opened the mantle is pushed away from the shell so that it is not torn. You can remove the left shell to get it out of the way if you want. Once you have the shell opened, lay the clam out on the tray, find the following parts and draw Diagram 1 Left Side With Shell Removed:

The large muscle attached to the siphons is called the siphon retractor muscle. There would be another one on the right side. These muscles pull the siphon in. Most clams can retract the siphons completely into the shell. Some large clams like the Geoduck have such a large siphon that it doesn't fit into the shell. The siphon brings water into the clam so the siphons have to reach from the clam's body to the water. If the clam is buried 15 cm below the surface, the siphons would have to be able to stretch that far. At low tide when the siphon hole is visible we can sometimes see water spurt up as the clam retracts its siphons. As it retracts the siphons, the water they contain is pushed out.

5. The next step is to remove the left mantle along with the siphon retractor muscle. Cut the mantle away with a razor blade or a pair of sharp scissors. Be careful not to cut the gills. Lift the mantle up as you cut and notice how thin it is. Removing the mantle exposes the gills and the foot. You can see the visceral mass better now as well. In clams like the one pictured, the labial palps are close to the anterior adductors muscle. The labial palps are two flap-like structures that are close to the mouth. Notice the structure of the gills. Bivalves are filter feeders. The gills are used to strain plankton out of the water, as well as to remove oxygen from the water. If the clam is alive you can get an idea of how many cilia are on the gills by making a wet mount of a small piece of gill and observing under low power. The cilia are what set up the water current that brings the water in through the ventral siphon and pushes it out the dorsal siphon. There are another pair of gills on the right side of the clam. The accompanying diagram shows these parts. You can see the edge of the right mantle below the foot and visceral mass.

Draw Diagram 2 Left Side With Mantle Removed, label the following parts:

·                     labial palps (if you see them)

6. To see where the heart is located look above the visceral mass above the gills. There is a clear looking region near the top of the clam. The heart is contained in a thin-walled sac called the pericardium. To get a better view of this region, take the clam out of the shell so you can look at the dorsal side. The clam should come out of its shell fairly easily if you cut the adductor muscles away from the right shell. Looking down on the dorsal side will enable you to see the pericardium. The tube-like structure that runs through the pericardium is the intestine.

7. Observe the muscular foot of the clam, which is ventral to the gills. Note the hatchet shape of the foot used to burrow into mud or sand.
8. Locate the palps, flaplike structures that surround & guide food into the clam's mouth. The palps are anterior to the gills & ventral to the anterior adductor muscle. Beneath the palps, find the mouth. 17. With scissors, cut off the ventral portion of the foot. Use the scalpel to carefully cut the muscle at the top of the foot into right and left halves.
9. Carefully peel away the muscle layer to view the internal organs.
10. Locate the spongy, yellowish reproductive organs.
11. Ventral to the umbo, find the digestive gland, a greenish structure that surrounds the stomach.
12. Locate the long, coiled intestine extending from the stomach. 
13. Follow the intestine through the calm. Find the area near the dorsal surface  that the intestine passes through called the pericardial area. Find the clam's heart in this area.
14. Continue following the intestine toward the posterior end of the clam. Find the anus just behind the posterior adductor muscle.
15. Use your probe to trace the path of food & wastes from the incurrent siphon through the clam to the excurrent siphon.
16. Answer the questions on your lab report & label the diagrams of the internal structures of the clam. Also, use arrows on the clam diagram to trace the pathway of food as it travels to the clam's stomach. Continue the arrows showing wastes leaving through the anus.

Be sure to dispose of the clam as instructed and clean the equipment!

Lab Questions:
1. What is the oldest part of a clam's shell called and how can it be located?

2. What do the rings on the clam's shell indicate?

3. What is the function of the toothlike projections at the dorsal edge of the clam's valves?

4. Where is the mantle located in the clam?

5. What is the mantle cavity?

6. Where are the incurrent & excurrent siphons located and what is their function.

7. How do clams breathe?

8. Describe the shape of the clam's foot.

9. Where are the palps found and what is their function?

10. Describe the movement of food from the current siphon through the digestive system of the clam.

11. Where is the clam's heart located?

12. Why are clam's referred to as "filter feeders"?

13. Label the internal structures of the clam and draw arrows showing the pathway of food as it travels to the clam's stomach: