Clam dissection
General information:
Clams belong to phylum Mollusca and the class Pelecypoda. The illustration below shows the external structures of a
clam. Clams are also called bivalves, because they have two valves, or shells. These valves are attached at one point by
an elastic tissue called the hinge ligament. Notice the bump called the umbo at the anterior end of the clam. This is the
oldest part of the shell. The circular ridges that extend outward from the umbo across the surfaces of the valves are
growth rings. These rings are formed as the mantle secretes calcium carbonate near the shell’s edges. The oldest rings
are nearest the hinge. The most recent growth has occurred at the shell’s edges. The greater the number of rings, the
older the clam is. Wide bands indicate lots of growth such as what might take place in the summer when the water is
warm and the clam’s metabolism is high. A series of narrow bands indicates little growth due to cold conditions and a
slowed metabolism.
Dorsal
Umbo
Hinge ligament
Growth line
Anterior
Posterior
Ventral
Lab Investigation:
Place a clam in a dissecting tray. Examine the umbo, hinge ligament and growth rings. Be sure you can identify the
anterior and posterior ends of the clam as well as the dorsal, ventral, lateral surfaces and the left and right valve.
Umbo
Left valve
Right valve
Hinge ligament
Hold the clam with its dorsal surface in the palm of your hand. There are two muscles that hold the valves together, the
anterior and posterior adductor muscles. With the scalpel blade pointing toward the dorsal edge, carefully slide your
scalpel between the upper shell and the mantle and cut the anterior adductor muscles. The mantle is the tissue that
lines both valves and covers the soft body of the clam. The mantle cavity is the space inside the mantle. If a grain of
sand or piece of food gets caught in the mantle cavity of an oyster, it will secrete a mineral called nacre to protect itself.
Nacre layers will continue to build until a pearl is formed.
Cut the muscle as close to the shell as possible. Repeat this procedure and cut the posterior adductor muscle. Carefully
lift the left valve. Use your scalpel to separate the mantle from the inner surface of this valve. Bend the left valve back
so it lies flat on the tray.
Anterior adductor muscle
Posterior adductor muscle
Mantle
Valves
Foot
Run your finger along the outside and the inside of the left valve. Compare the textures of the two surfaces. Examine
the inner dorsal edges of both valves near the umbo and notice the tooth like projections. Close the valves and see how
the projections interlock. Locate the muscle “scars” on the inner surface of the left valve. These “scars” are where the
adductor muscles were attached. If you have ever eaten scallops, you are eating the adductor muscles of a relative to
the clam. The innermost, shiny layer of the valve is called the mother of pearl which is made from the before mentioned
nacre.
Locate the two openings at the posterior end of the clam. The more ventral opening, the incurrent siphon, carries water
into the clam. The more dorsal opening, the excurrent siphon, lets water and wastes out of the clam. The clam can also
use the excurrent siphon for movement as quick squirts of water being expelled from the excurrent siphon will propel
the clam forward. With scissors, carefully cut away the half of the mantle that lines the left valve. Once this part of the
mantle is removed, you will see the gills. The gills look like a double fold of tissue with many ridges. Observe the
muscular foot of the clam, which is ventral to the gills. Note its shape. This hatchet-shaped foot can protrude between
the ventral edges of the valves and use it to burrow into soft mud or sand.
Unlike other groups of mollusks, bivalves do not have a radula. The gills are responsible for food-getting and respiration.
The cilia on the surface of the gills draw water into the clam through the incurrent siphon. As the water flows over the
gills, oxygen is taken into the gills and carbon dioxide is released. At the same time, food particles in the incoming water
become trapped in the mucous layers on the gills. The cilia on the gills sweep the food anteriorly to the two palps.
Palps are structures that surround and guide food anteriorly to the mouth.
The soft mass of tissue dorsal to the foot is the visceral mass. In order to examine the embedded organs within the
visceral mass, you must remove part of the foot. With scissors, cut off the ventral portion of the foot and carefully cut
the muscle at the top of the foot into right and left halves. The internal organs can be found by peeling away the
muscular tissue of the foot. The spongy, yellow mass is the reproductive organs. These organs produce both eggs and
sperm. The digestive gland, which produces and secretes digestive juices, is the dark-green structure that surrounds the
stomach. Mechanical breakdown of food takes place in the stomach. From the stomach, digested food passes into a
long, coiled intestine, where the absorption of nutrients takes place. The end of the intestine, the anus, is just behind
the posterior adductor muscle. The intestine passes through a reduced coelom called the pericardial cavity. This is the
cavity that holds the heart. There will not be many blood vessels, as the clam has an open circulatory system. The
pathway of food in a clam goes from the incurrent siphon, to the gills, past the palps, into the mouth, into the stomach,
into the intestine, out the anus, into the mantle cavity and finally expelled out the excurrent siphon.
The nervous system of a clam is composed of three pairs of ganglia which are located on each side of the esophagus,
beneath the posterior adductor muscle, and in the foot. There are two pairs of nerve cords connecting the ganglia to
other individual specialized nerve cells.
The excretory system consists of two nephridia, which are located beneath the pericardial cavity. These structures
collect and rid the clam of metabolic waste.
Analysis questions:
1. Why is a clam a bivalve?
2. What muscles control the opening and the closing of the clam valves?
3. How do water and suspended materials move into and out of the clam?
4. When you open the clam, will all the internal organs be visible? Explain.
5. Describe the two ways a clam can move.
6. What is the name of the elastic tissue which attaches the valves together at the umbo?
7. How do clams and other mollusks produce their shells?
8. What do the rings on the clam shell indicate?
9. What is the function of the tooth like projections at the inner dorsal edge of the valves?
10. Explain where the mantle is located in the clam.
11. What is the mantle cavity?
12. What is nacre?
a. How does nacre used in the formation of pearls?
13. Describe two functions of the gills in the clam.
14. Why is a ridged gill surface more advantageous than a flat gill surface?
15. What type of circulatory system does a clam have?
16. Why is it advantageous to have the anus located close to the excurrent siphon?
17. Clams are described as “filter feeders.” Explain why.
18. Most species of clams live underwater on sandy or muddy bottoms. Describe two adaptations which allow the clam
to be adapted for this type of environment.
19. Use the bold words on your SFS to label the structures on the diagram below.