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 shell. 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 shell as follows:-- a. Break a thick clamshell and examine the broken edge. Identify the inner or pearly layer and the outer or chalky layers. What gives color to the shell in the living clam? b. Burn a small piece of shell in an evaporating dish over a bunsen burner. What is the appearance of the shell after burning? What has been burned, animal or mineral matter? What then is the residue? c. Place a small piece of shell in acid. What results? Is there a large amount of residue? What constitutes the greater part of the shell, animal or mineral matter? d. (Optional) Devise some method and determine the approximate per cent of mineral and of animal matter in the clamshell. _Summary._ 1. Why did we study the clam? (See title of exercise.) 2. How has the heavy shell 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 shell. f. Diagram of cross sections. 2. THE SNAIL--A TYPE OF MOLLUSCA _To show Another Type of Exoskeleton_ _Materials._ Specimens of pond snails, edible snails, and "slugs," and other land snails, and a collection of shells of various types. _Definitions._ _Gasteropoda_, the name of the class to which the snail belongs. _Spire_, the coiled portion of the snail shell. _Aperture_, the opening of the shell. _Lip_, the edge of the shell forming the margin of the aperture. _Whorl_, a single coil of the spire. _Suture_, the depression between the whorls. _Foot_, the flat disk-like structure on which a snail creeps. _Breathing pore_, an opening in the mantle used in respiration. _Lingual ribbon_, the rasp or file like tongue of the snail. _Observations._ 1. Why is a snail called a univalve?