Name___________________________________________
Marine Biology--Mr. Nelson
PHYLUM MOLLUSCA-- CLAM DISSECTION
BACKGROUND
The Phylum Mollusca (Latin molluscus, "soft") has over 100,000 living species and includes
organisms such as snails, clams, chitons, slugs, limpets, squid and octopi. The success of Mollusks is
largely due to their adaptability to different environments. Even though most mollusks live in saltwater
and freshwater habitats, there are a few that live on land. In spite of their diversity, mollusks share some
basic characteristics. As mollusks develop from a fertilized egg to an adult, most pass through a larval
stage known as the trochophore stage. The trochophore is a ciliated, free-swimming larva. Most
mollusks also have a muscular foot adapted for locomotion, a radula or file like organ for feeding, and a
mantle, a thin tissue that covers the body of a mollusk. In most mollusks, this mantle secretes a shell.
Some mollusks have a head, which may have sensory structures like eyes and tentacles.
In this investigation you will observe the external structure of a representative mollusk, the clam.
You also will dissect a clam to observe its internal structure. Clams are pelecypods or bivalves, and
have a two-part hinged shell. Clams are found in fresh water in streams, ponds, and lakes. They also
are very common burrowed into the mud of ocean mud flats. Clams are often used for food.
GOALS:
In this exercise, you will:
*Demonstrate that a clam shell is made of calcium carbonate
*Identify the external parts of a clam
*Locate the bands on clam shells.
*Find out the ages of the clams.
*Dissect a clam and identify the important organs in each system
*Relate the structure of organs to their function.
PROCEDURE:
Part A: External Anatomy of the Clam.
How Old Is a Clam?
Clams are mollusks whose soft bodies are often inside shells. The shells give them protection.
As a clam grows each year, so does its shell. A new ring or thick band surrounded by finer bands
shows on the shell each year. Each thick band is a year's growth. By counting the thick bands, you can
learn how old the clam is. This technique may not be totally accurate, but band variation corresponding
to yearly growth will show on the shells.
1. Obtain a preserved clam and rinse it thoroughly to remove excess preservative. Place the clam in a
dissecting tray. Observe the bivalve shell. Notice the hinge ligament which holds the two shells
together. CAUTION: The preservative used on your clam can irritate your skin. Use your rubber
gloves and avoid rubbing your eyes while working with your preserved clam.
2. Describe the location and function of the ligament in relation to the two halves of
the shell.
3. The small, pointed area near the hinge ligament is called the umbo. It is the oldest part of the clam.
See Figure 40-1. The umbo is situated dorsally toward the anterior end of the clam and is surrounded by
concentric growth lines. The lines represent alternating periods of slow and rapid growth.
4. Examine the clam shell diagram below left. It shows a shell that is 15 years old. Count the bands
(Each band has a line on both sides of it).
5. Examine the shell diagram below right. Be sure to include the crown of the shell as one years
growth. How old is this shell?
6. Measure the straight line distance from the umbo to the edge of the shell. Write the distance, for your
clam, in the chart.
7. If needed use the dissection scope to help you count the bands on your clam's shell. Write this count
in the table on the next page.
8. Add to the chart below, the age of your clam. Check with the other groups and complete the chart.
Shell
Length (cm)
# of Bands
Age of Clam
Your Clam
2nd Group
3rd Group
4th Group
5th Group
9. Do shells with similar lengths have similar ages? Explain.
10. Before you continue with this investigation, it is important to know the orientation of the clam shell.
Recall that the umbo is near anterior end. The posterior of the clam shell is at the opposite end. In
reference to the clam shell, dorsal is the side, or edge, with the umbo. Ventral is the side, or edge
opposite the umbo. Locate the posterior, anterior, dorsal, and ventral surfaces of your clam shell. Hold
the clam shell with the anterior end up and the hinge facing toward you. Locate the posterior, right
valve, and left valve of the clam shell.
11. Using the following terms to label the drawings below:
dorsal, ventral, anterior, posterior, hinge, left valve, and right valve
Each term may be used more than once.
12. The shell of a clam is made up of three layers: the horny outer layer, the thick, middle layer called
the prismatic layer, and the innermost layer called the pearly layer.
13. Hold the clam in the dissecting tray as shown in Figure 40-2a. With a scalpel carefully scrape away
some of the horny outer layer of the shell. CAUTION: Scrape in a direction away from your hand
to avoid cutting yourself. Politely ask Mr. Nelson to come over to your lab bench. Place one drop
of acid on the exposed prismatic layer as shown in Figure 40-2b. CAUTION: Do not let any acid
contact your skin to avoid acid burns. The bubbling of the acid indicates that calcium carbonate is
present. Rinse with water and wipe the shell dry with paper towels.
14. a. What happened when acid was placed on the exposed prismatic layer?
b. What is the shell made of?
The chart below summarizes some of the easily recognizable characteristics of the three main classes of
mollusks. Study this chart and use it to answer the questions below:
Class
Gastropoda
Head
Foot
well-developed
head; may have
eyes and short
tentacles
Shell
large, flat foot
spread beneath
body
usually one shell,
often spiraled
Pelecypoda
no head
hatchet-shaped
foot
two shells hinged
together
Cephalopoda
large head, may
have eyes
foot evolved into
rasping tentacles
and arms
usually no external
shell; shell is often
internal rod
15. Name the Class to which each of the above organisms belongs.
A.
B.
C.
16. To which class does your quahog belong? Why?
17. Look at how each of the two shells fit together. What do you think is the
function of the tooth like projections along the inside edges of the valves?
Part B: Internal Anatomy of the Clam
18. Read Table 40-1. Refer to it as you proceed through the following investigation.
TABLE 40-1:
Structure
Description/Location
Anterior adductor muscle
Anterior edge inside shell
Holds two halves of shell together
Posterior adductor muscle
Posterior edge inside shell
Holds two halves of shell together
Mantle
Membranous tissue that covers
entire body; yellow or cream colored
Secretes shell
Foot
Hatchet-shaped; hard; anterior and
ventral to gills
Locomotion, movement
Gills
Folded, ridged tissue with microscopic cilia
Respiration, gas exchange
Incurrent siphon
Fold in mantle; posterior end ventral to excurrent siphon
Regulates flow of water into clam
Excurrent siphon
Fold in mantle; posterior end; dorsal to incurrent siphon
Regulates flow of water out of clam
Palps
Leaf like structures anterior to gills
and posterior to anterior adductor
muscle
Directs water carrying food into mouth
Mouth
Slit between palps
Passage of food into digestive system
Stomach
Saclike structure near mouth
Digestion of food
Intestine
Coiled tubule from stomach
through body to anus
Absorption of digested food
Digestive gland (liver)
Light green mass surrounding
stomach
Secretes enzymes into digestive system
to digest food
Anus
End of intestine near excurrent
siphon
Removal of undigested food
Reproductive organ
Spongy reddish mass ventral to
palps
Production of eggs and sperm
Pericardial cavity
Area between visceral mass and
hinge; dorsal to foot
Protects and houses heart
Heart
Inside pericardial cavity
Pumps blood throughout body
Kidneys
Spongy brownish organs found
below pericardial cavity
Waste removal
Function
19. Look at the partially opened shell. Observe the anterior adductor muscle, posterior adductor muscle,
mantle, and foot. The opening between the two shells is called the gape. Have your best group member,
carefully insert the scalpel between the mantle and the left valve of the shell. Cut the anterior adductor
muscle as close to the shell as possible. See Figure 40-3. Repeat this procedure to cut the posterior
adductor muscle. Open the shell. If necessary, carefully run your fingers or scalpel between the shell
and the mantle to separate the mantle from the shell. The space between the two halves of the mantle is
the mantle cavity and contains all the "guts" of the clam. Open the left valve as far as possible.
20. Observe the hinge. Notice the interlocking teeth that hold the two valves of the shell together.
Locate the "scars" from the anterior and posterior adductor muscles on the inner surface of the left valve.
These scars indicate where the posterior and anterior adductor muscles were attached.
21. You observed the scars on the left valve. Where else in a clam would scars from the
anterior and posterior adductor muscles be found?
22. Describe the appearance and feel of the inner side of the clam shell.
23. Locate the mantle, a thin layer of tissue that covers the visceral mass and foot. The visceral mass is
a soft mass of tissue located dorsal to the foot. The mantle is usually cream or yellow in color with a
brownish edge in a preserved clam. Locate the incurrent siphon and excurrent siphon. The siphons are
folds in the mantle at the posterior of the clam. The incurrent siphon is ventral to the excurrent siphon.
The incurrent siphon takes in water that contains oxygen and microscopic food particles. Water and
waste materials are removed from the mantle cavity through the excurrent siphon. Examine these
structures with a dissection microscope.
24. With a pair of scissors, carefully cut away a portion of the mantle as shown in Figure 40-4. With
the mantle removed you can now observe the gills, folds of tissue covered with microscopic cilia. Gills
are found in pairs, one on each side of the visceral mass. Use a probe and a hand lens to examine the
gills. Observe the muscular, hatchet-shaped foot located anterior and ventral to the gills. Locate the
palps, a pair of leaf like structures ventral to the anterior adductor muscle and anterior to the gills. The
mouth is a slit located between the palps. Water from the incurrent siphon passes over the gills toward
the palps. Mucus and cilia on the palps trap food and direct it toward the mouth. Water then circulates
out of the mantle cavity through the excurrent siphon.
25. The gills are feathery in appearance. This helps to increase the surface area of
the gills. Of what advantage to a clam are gills with many feathery projections on
their surface?
26. Locate the visceral mass. In order to study the visceral mass in detail, remove the gills and set them
aside in your dissecting tray. Then use a pair of scissors to cut off the ventral portion of the foot as
shown in Figure 40-5A. With a scalpel, carefully cut the top portion of the foot into right and left
halves, as shown in Figure 40-5B, to expose internal organs.
27. With the foot removed, see if you can locate any of the following structures: reproductive organs, a
spongy reddish mass; the sac-like stomach near the mouth; the digestive gland, a light green mass
surrounding the stomach; the coiled intestine leading from the stomach to the anus near the excurrent
siphon; the pericardial cavity, an area between the visceral mass and the hinge; the heart contained
within the pericardial cavity; and the kidneys, spongy brownish organs below the pericardial cavity.
28. Label each drawing as described below:
Diagram A: anterior adductor muscle, posterior adductor muscle, incurrent siphon, excurrent siphon,
and right valve.
Diagram B: mouth, palps, gills, and foot. Also draw red arrows to indicate the flow of water from the
incurrent to the excurrent siphon.
Diagram C: gonads, stomach, digestive gland, anus, intestine. Also draw blue arrows to indicate the
path of food through the body.
29. In the manila folder, on your lab bench, are 4 structures that I would like your group member to be
able to identify. Invite Mr. Nelson over and he will ask different members of your group show him
these structures in your clam. He will make a mark below to indicate that you and your group have
completed this part. While waiting, work on the Analysis Questions with your partners.
Mr. Nelson Rules!!!!
30. Mr. Nelson will tell you when it's time to start the clean up process. Thoroughly wash and dry your
dissecting tray, scalpel, probe, scissors, and any other equipment you may have used. Wash your hands
with soap and water.
Analysis Questions:
1. Using the theme of Structure is related to Function, describe how a quahog buries itself in the sand.
2. Research bivalve clams in your text. What type of feeder is a clam and what do clams eat? Explain
your answer.
3. What other organism, that we studied, obtains food in the same manner as the clam?
4. The anus is located very close to the excurrent siphon; what a great design! What is the function of
the excurrent siphen and what is the advantage of having the anus located very close to the excurrent
siphon?
5. There are two types of digestion that can occur in organisms. Mechanical digestion and chemical
digestion. Which do you see in a clam, and what role do the digestive glands play in this form of
digestion?
6. If other students were to look at your shell, why might their age differ using this method of
determining age and how can errors occur using this dating method?
7. Much like the growth rings in trees, the width of the bands varies year to year. Name two abiotic
environmental conditions that might cause the bands to be larger during a given year?
8. Take one of your two sdconditions above and describe how this condition could affect the clam’s
growth.