Clam DissectionThe phylum
Mollusca includes snails, clams, chitons, slugs, limpets, octopi, and squid. As mollusks develop from a fertilized
egg to an adult, most pass through a larval stage called the trocophore. The trocophore is a ciliated, free-swimming
stage. Mollusks also have a radula or file-like organ for feeding, a mantle that may secrete a shell, and a
muscular foot for locomotion. Clams are marine mollusks with two valves or shells. Like all mollusks, a clam has a
mantle which surrounds its soft body. It also has a muscular foot which enables the clam to burrow itself in mud or sand.
The soft tissue above the foot is called the visceral mass and contains the clam's body organs.
Taxonomy
Kingdom
- Animalia
Phylum - Mollusca
Class - Bivalvia or Pelecypoda
Objective
To
study the internal and external anatomy of a bivalve mollusk.
Materials
Dissecting
pan, dissecting kit, screwdriver, lab apron, plastic gloves, safety glasses, preserved clam
Procedure
1. Put on your lab apron, safety
glasses, and plastic gloves.
2. Place a clam in a dissecting tray and identify the anterior
and posterior ends of the clam as well as the dorsal, ventral, & lateral surfaces. Figure 1
Figure 1
3. Locate the umbo, the bump at the anterior end of the valve.
This is the oldest part of the clam shell. Find the hinge ligament which hinges the valves together and observe the
growth rings.
4. Turn the calm with its dorsal side down and insert a screwdriver between the ventral edges of the
valves. Carefully work the tip of the screwdriver between the valves so you do not jab your
hand.
5. Turn the screwdriver so that the valves are about a centimeter apart. Leave the tip of the screwdriver between
the valves and place the clam in the pan with the left valve up.
6. Locate the adductor muscles. With your blade
pointing toward the dorsal edge, slide your scalpel between the upper valve & the top tissue layer. Cut down through the
anterior adductor muscle, cutting as close to the shell as possible.
7. Repeat step 6 in cutting the posterior
adductor muscle. Figure 2
Figure 2
8. Bend the left valve back so it lies flat in the tray.
9. Run your
fingers along the outside and the inside of the left valve and compare the texture of the two surfaces.
10. Examine the
inner dorsal edges of both valves near the umbo and locate the toothlike projections. Close the valves & notice how the
toothlike projections interlock.
11. Locate the muscle "scars" on the inner surface of the left valve. The adductor muscles
were attached here to hold the clam closed.
12. Identify the mantle, the tissue that lines both valves & covers
the soft body of the clam. Find the mantle cavity, the space inside the mantle.
13. Locate two openings on the posterior
end of the clam. The more ventral opening is the incurrent siphon that carries water into the clam and the more dorsal
opening is the excurrent siphon where wastes & water leave.
14. With scissors, carefully cut away the half of
the mantle that lined the left valve. After removing this part of the mantle, you can see the gills, respiratory
structures.
15. 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.
16. 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.
18. Carefully peel away the
muscle layer to view the internal organs.
19. Locate the spongy, yellowish reproductive organs.
20. Ventral
to the umbo, find the digestive gland, a greenish structure that surrounds the stomach.
21. Locate the long, coiled
intestine extending from the stomach.
22. 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.
23.
Continue following the intestine toward the posterior end of the clam. Find the anus just behind the posterior adductor
muscle.
24. Use your probe to trace the path of food & wastes from the incurrent siphon through the clam to the excurrent
siphon.
25. 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.
What distinguishes this animal from others
you have observed?
Can you observe any body symmetry? Describe
it.
Compare the anatomy and organs of the
clam with that of the annelid and yourself.
What organs are present in both human
and mollusc bodies?
How is the heart connected to the respiratory
organs?
How is the digestive system similar to
yours? How is it different?
Be thorough and complete in your answers.
What distinguishes this animal from others
you have observed?
Can you observe any body symmetry? Describe
it.
Compare the anatomy and organs of the
clam with that of the annelid and yourself.
What organs are present in both human
and mollusc bodies?
How is the heart connected to the respiratory
organs?
How is the digestive system similar to
yours? How is it different?
Be thorough and complete in your answers.