Download Clam Dissection

1
Clam Dissection
Introduction
The phylum Mollusca includes snails, clams, chitins, slugs, limpets, octopi, and squid. They all have
soft bodies and make hard shells (2 external shells or 1 external shell or 1 internal shell called a beak).
You will be dissecting a clam or a mussel. Both have two valves or shells. Mollusks with two shells are
called bivalves.
Development: 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.
Major Organs
Mantle: Like all mollusks, a clam has a mantle which surrounds its soft body. The Mantle contains
special tissue that emit calcium carbonate CaCO3 . Calcium carbonate is the chemical that makes the
shell.
Foot: Mollusks have a muscular foot which enables the clam/mussel to burrow itself in mud or sand.
Thus, the muscular foot is for locomotion. The foot is large, muscular, and generally hatchet-shaped. It is
used to pull the animal through the substrate (typically sand, gravel, or silt) in which it lies partially buried. It does
this by repeatedly advancing the foot through the substrate, expanding the end so it serves as an anchor, and then
pulling the rest of the animal with its shell forward. It also serves as a fleshy anchor when the animal is stationary. In
marine mussels, the foot is smaller, tongue-like in shape, with a groove on the ventral surface which is continuous
with the byssus pit. In this pit, a viscous secretion is exuded, entering the groove and hardening gradually upon
contact with sea water. This forms extremely tough, strong, elastic, byssus threads that secure the mussel to its
substrate. The byssus thread is also sometimes used by mussels as a defensive measure, to tie up a predators such
as dog whelks, that invade mussel beds, immobilising them and thus starving them to death. In cooking, the byssus of
the mussel is known as the "beard" and is removed before the mussels are prepared.
2
Taxonomy
Kingdom - Animalia
Phylum - Mollusca
Class - Bivalvia or Pelecypoda
Purpose
To study the internal and external anatomy of a bivalve mollusk.
Experiment
Safety Warning…see safety poster
Materials…
Plate, dissecting tools, plastic gloves, safety glasses, preserved clam
Procedure…
1. Put on safety glasses, and plastic gloves.
2. Place the mollusk on a plate and identify the anterior and posterior ends of the mollusk as well as the dorsal,
ventral, & lateral surfaces.
Figure 1
3
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.
Figure 2
4. Pry the mollusk open.
5. Locate the muscle "scars" on the inner surface of the left valve. The adductor muscles were attached here
to hold the mollusk closed.
6. Identify the mantle, the tissue that lines both valves & covers the soft inner body of the mollusk. Find the
mantle cavity, the space inside the mantle.
7. Carefully cut away the mantle that lined the valve. After removing this part of the mantle, you can see the
gills, respiratory structures.
8. Observe the muscular foot of the clam, which is ventral to the gills. Note the ax like shape of the foot.
9. Locate two openings on the posterior end of the clam. The more ventral opening is the incurrent siphon
that carries water into the mollusks’ two shells and the more dorsal opening is the excurrent siphon where
wastes & water leave.
10. 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.
Figure 3
4
11. Cut the muscle at the top of the foot into right and left halves.
12. Carefully peel away the muscle layer to view the internal organs.
13. Find the digestive gland, a greenish structure that surrounds the stomach.
14. Locate the long, coiled intestine extending from the stomach.
15. Locate the spongy, yellowish reproductive organs (gonads).
16. Follow the intestine through the mollusk. Find the area near the dorsal surface that the intestine passes
through called the pericardial area. Find the clam's heart in this area.
17. Continue following the intestine toward the posterior end of the mollusk. Find the anus just behind the
posterior adductor muscle.
Conclusion:
1. Why are clams called bivalves?
2. What type of symmetry do Mollusks have?
3. What controls the opening and closing of the bivalves’ shell?
4. Name the structure and describe the process to how the shell is made.
5. Describe with pictures and words the mollusk’s foot?
6. How does your mollusk breathe?
7. Why are these mollusks referred to as "filter feeders"?