A Guide for the Study of Animals Part 26

_Hinge ligament_, the elastic structure which fastens the valves together at the dorsal margin.

_Umbones_, a pair of elevations near the anterior end of the sh.e.l.l.

_Lines of growth_, concentric lines around the umbones.

_Siphons_, two openings at the posterior end of the clam, the upper opening is the _excurrent_ opening and the lower the _incurrent_. In the salt water clam the siphons form a long tube, usually called the "neck."

_Hinge teeth_, projections near the dorsal margin on the inner surface of the sh.e.l.l. The anterior irregular structure is the _cardinal_ and the more posterior blade-like structure is the _lateral_ tooth.

_Muscle scars_, scar-like markings on the inner surface of the sh.e.l.l indicating the point where muscles were attached. The large scar just in front of the cardinal tooth is the _anterior adductor muscle scar_, and the one just back of the lateral tooth is the _posterior adductor muscle scar_.

_Pallial line_, a line connecting the two muscle scars.

_Mantle_, folds of skin covering the body of the clam and lying close to the inner surface of the valves.

_Foot_, a hatchet-shaped structure extending from the ventral edge of the body.

_Gills_, broad flap-like structures for respiration, situated each side of the body in the mantle cavity. They consist of a double fold of membrane through which run many perforations lined with cilia. The waving of these cilia cause the current of water needed for respiration.

_Palps_, small flap-like structures near the anterior end of the clam. They surround the mouth. On their surface are cilia which cause currents of water toward the mouth.

_Adductor muscles_, large muscles extending from valve to valve.

_Observations._

Identify anterior and posterior ends, dorsal and ventral surfaces, right and left sides.

1. Why may a clam be called a bivalve?

2. What is the position of the clam in the mud? What is the position of the foot if the clam is undisturbed? Are the two valves tightly closed or slightly open at this time?

3. What changes take place in the sh.e.l.l as the clam grows? What markings on the surface of the sh.e.l.l indicate this?

4. Where is the clam sensitive to touch or tactile stimulus? Why has the clam no eyes? Zoologists have found a structure in clams which they have supposed to be an ear. Where do you think the structure is located? Why is the clam successful without eyes? (There are many bivalves which have them.)

5. Examine several clams until you find some with enlargements in the gills. Break off a small part of an enlargement with your forceps and examine under the compound microscope. Describe what you see.

6. Drop some powdered chalk or carmine in the water just above the siphon, watch the siphons for several minutes, and note what happens.

What do you conclude to be the use of the siphons? Recalling what took place in sponges, what would you suggest as the probable cause of these currents? What does the clam thus probably obtain? How do the two siphons differ? Why?

7. Place a clam in water sufficient to cover it and heat slowly to about 40 degrees Centigrade, until the valves open slightly. Remove and proceed as follows: Raise one valve, separate the mantle from it, and then cut through the two large firm structures (adductor muscles) found at each end. What does the valve do when the muscles are cut?

What is the cause of this? State your theory as to how a clam opens and closes its sh.e.l.l.

8. Note the texture of the mantle. How many lobes has it? What is their extent? How are the lobes related to the valves?

9. Remove or lift up one mantle lobe. Identify the soft body, the foot, the gills, the palps, and the mouth. Which of these structures are arranged in pairs?

10. Determine the structure and composition of the sh.e.l.l as follows:--

a. Break a thick clamsh.e.l.l and examine the broken edge. Identify the inner or pearly layer and the outer or chalky layers. What gives color to the sh.e.l.l in the living clam?

b. Burn a small piece of sh.e.l.l in an evaporating dish over a bunsen burner. What is the appearance of the sh.e.l.l after burning? What has been burned, animal or mineral matter? What then is the residue?

c. Place a small piece of sh.e.l.l in acid. What results? Is there a large amount of residue? What const.i.tutes the greater part of the sh.e.l.l, animal or mineral matter?

d. (Optional) Devise some method and determine the approximate per cent of mineral and of animal matter in the clamsh.e.l.l.

_Summary._

1. Why did we study the clam? (See t.i.tle of exercise.)

2. How has the heavy sh.e.l.l of the clam affected:--

a. The character of the clam's body,

b. the locomotion,

c. the development of sense organs.

3. What special problems has the clam as regards getting food and oxygen? How are these problems solved?

4. How does the clam protect the young clam during development?

_Suggested drawings._

a. Dorsal margin of the clam.

b. Side view of the clam.

c. The clam with one valve removed or lifted back.

d. The clam with one valve and one mantle lobe removed.

e. The edge of a broken sh.e.l.l.

f. Diagram of cross sections.

2. THE SNAIL--A TYPE OF MOLLUSCA

_To show Another Type of Exoskeleton_

_Materials._