CLAM DISSECTION HANDOUT
Fig 6-6a Fig 6-6b
PROCEDURE 1. If the shells of the clam are closed tight, steam the clam in hot water until it opens. Lift it from the hot water with tongs. Drain the excess water out of the clam. Throughout the following procedures, refer to Table 6—1 as needed for definitions of terms.
2. Cut the clam open. a. Lay the clam on newspapers or a dissecting pan with the umbo to the left. Insert a scalpel between the valves and move the blade dorsally along the valve margin to cut the adductor muscles at positions (A) and (B) in Fig. 6—6. b. Carefully slit the hinge ligament at the umbo.
3. Hold the clam so that its anterior end faces you and its umbo faces up. See Fig.6 —7.
a. Find the mantle and the gills on both sides of the soft body cavity. b. Observe, sketch, and identify the structures shown in Fig. 6—7. c. Turn the clam around and look at it from the posterior end. Observe, draw, and identify the structures. d. Observe, draw, and identify the structures from the ventral side. e. Expose the clam’s interior anatomy by gently separating the mantle from the upper valve with a blunt probe and then lifting the upper valve.
4. Lift and fold the top mantle to expose the gills and body cavity. Find the body structures listed below. Draw and label the internal parts of your clam. See Table 6—1 for definitions of the parts. a. gills g. adductor muscles h. palp h. incurrent siphon c. foot i. excurrent siphon d. body j. hinge ligament e. mantle k. hinge teeth f. mouth 1. umbo
5. Using different-colored pencils or crayons, sketch arrows to show the pathway of food and water into and out of the clam’s body. See Fig. 6—9.
6. Draw the interior features of the valve. a. Cut away and remove all soft body materials. Rinse the valve. b. Position the clam with its umbo to the left as shown in Fig. 6—6. Sketch its outline. c. Find the hinge teeth, the hinge ligament, the muscle scars, the pallial line, and the nacre. Draw and label them in the outline. 7. Turn the valve over and trace another outline. Draw and label the exterior features of the valve. Find and examine its umbo, lines of growth, and anterior and posterior ends.
QUESTIONS 1. What is an invertebrate? 2. What are the features that differentiate mollusks from other phyla’? 3. How does a bivalve filter-feed’? What does it eat? What should humans keep in mind when eating bivalves’? 4. The bivalve exoskeleton consists of two hard valves (shells). a. What are the advantages to these organisms of having such an exoskeleton’? What are some possible disadvantages’? b. As a bivalve grows larger, what happens to its exoskeleton? 5. What is the function of hinge teeth on the bivalve shell’? 6. What holds the two valves of a bivalve together’? How do bivalves open and close their valves’? 7. In terms of evolutionary development, earlier organisms evolve before more complex ones. Give specific examples of how might this apply to the clam?
Table 6—1. Glossary of parts of bivalves Adductor muscles. Muscles that close the valves and hold them shut. Bivalve anterior end. The end of a bivalve with the mouth and main sense organs. Bivalve posterior end. The end of a bivalve where the siphons and anus are. Bivalve dorsal side. The hinged side of a bivalve. Bivalve ventral side. The open end of the clam from which the foot protrudes. Excurrent siphon. A tube that pumps out filtered water and waste materials. It is dorsal to the incurrent siphon and opens to the posterior end of the bivalve. Foot. The large muscle on the ventral or anterior part of the body, used for locomotion.
Gills. Respiratory organs, the site of O2 and CO2 exchange and food particle entrapment. Hinge ligament. A tough, elastic ligament that connects the valves like a door hinge. Hinge teeth. Interlocking projections on the dorsal surface or top of both shells that keep the shells aligned and prevent them from slipping out of position. lncurrent siphon. A tube that pumps in nutrients and oxygen-rich water. It is ventral to the excurrent siphon at the posterior end of the bivalve. Lines of growth. Major concentric ridges that show the shape of the animal at previous stages of development. Mantle. Tissue that lines and produces the shell and encloses the soft body parts. Mouth. The external opening to the digestive system. It is at the anterior end of the bivalve inside a groove between the palps. Muscle scars. Circular indentations on the inner valve lining where the adductor muscles attach to the valve. Nacre. The shell’s interior wall produced by the mantle; commonly known as mother-of-pearl. PaIIiaI line. The line on the interior of the valve showing where the mantle stopped secreting the nacre (pearly substance). Palps. Liplike structures that sort out material collected by the gills and transfer edible food to the mouth. They are at the anterior end of the bivalve. Pearl. A secretion produced by the mantle in response to an irritating foreign particle. Found in only a few bivalves. Umbo. The smallest and oldest part of the bivalve shell, located near the hinge. It lies offcenter, protrudes slightly, and is outlined by the smallest line of growth (see above). Valve. The exoskeleton, which protects the soft body and provides for muscle attachments. Commonly called a shell. Because clams have two shells, they are called bivalves.
6. (Optional) If you have a live, unsteamed clam, take these steps: a. Remove one valve and roll back the mantle. Put the clam in a bowl of seawater. Using an eyedropper, slowly add a carmine solution to the posterior region of the gill. Observe the flow of particles. Record your observations. b. Remove a 3-cm x 3-cm piece of gill from your live clam. Spread it out and pin it in a dissecting pan with a waxed bottom. Cover the gill with seawater. Add carmine solution and watch the flow pattern under a dissecting microscope. Modify by adding water of varying temperatures and salinities. Record your observations.