_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 shell. 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 shell 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 shell as the clam grows? What markings on the surface of the shell indicate this? 4. Where is the clam sensitive to touch or tactile stimulus? Why has the clam no eyes? Zoölogists 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?