CLAM (QUAHOG) ANATOMY
External
A clam lives between two shells, or valves. The shell is
made up of calcium carbonate that the clam secretes.
The thickness and toughness of this shell give the
animal its other common name, the hard-shell clam. A
hinge, made up of intermeshing teeth, forms the joint
between the shells. The tough, but pliable hinge
ligament holds the two valves together. At the top of
the hinge, the umbo, is the oldest section of the shell.
Internal
The clam uses adductor muscles to
close the shell. It does this to avoid
predators, and also if water
conditions are not good. The large,
muscular foot can reach outside the
shell so that the clam can burrow.
Water comes into the quahog
through the incurrent siphon and
leaves through the excurrent
siphon. Together, these siphons
make up what we call the "neck" of
the clam. The clam sits buried in
sediment, and sticks the siphons up
into the water above so that it can
suck in and spit out water. The water
that the clam sucks in through the incurrent siphon contains oxygen and food (plankton).
The water that the clam spits out through the excurrent siphon contains the animal's
wastes.
The mantle is the part of the animal that forms the shell. The mantle secretes calcium
carbonate, the compound that we see as the hard substance that makes up seashells.
The quahog's gills serve several important functions: obtaining oxygen and getting food.
The gills have tiny, hair-like structures on them called cilia. By waving the cilia, the clam
can create a current that moves water through its body. The gills also move food through
the body. When water comes in through the incurrent siphon, particles of silt and food are
trapped on the layer of mucous on the outer surface of the gills.
From there, the cilia move the particles
along food grooves toward the labial palps,
where they are sorted. Food particles move
on toward the mouth. Other particles—such
as silt or excess phytoplankton—are
dropped onto the surface of the mantle,
where the clam eventually gets rid of them
in mucous-coated balls. Food particles
move from the mouth and esophagus to a
multi-chambered stomach with numerous
passageways and dead-end sacs.
Located inside the muscular foot are the
intestines, digestive glands, and gonads.
The quahog has an "open" circulatory system, so that once the hemolymph gets to the
outer tissues, it leaves the blood vessels and flows into open sinuses, or cavities, where it
directly bathes the tissues. In contrast, in our circulatory systems, blood always stays in
some kind of blood vessel, such as capillaries.
CLAM DISSECTION
1. Identify the following external structures: umbo, shell, hinge
2. To open the clam, you have to cut through the 2 adductor muscles with the scalpel.
Insert your scalpel before the hinge on one side and move it towards the hinge until
you feel resistance. Then using a slicing motion, keep pushing the scalpel forward
until you are through the muscle. Do the same on the other side
3. Once the adductor muscles are cut, slowly and carefully pry open the shells so you
can see the internal structures
4. Once the shells are pulled open so they are horizontal, identify the following internal
structures: gills, foot, mantle, labial palps
5. To open the foot, run the scalpel the long way through the foot so you can see inside.
Identify the following structures: digestive glands (greenish mass), intestine, gonads
(yellowish mass)
Reference
http://seagrant.gso.uri.edu/G_Bay/HabitatEco/Shellfishing/quahog_dissect.html