=General Questions.=—The living parts of the mussel are very soft, the name mollusca being derived from the Latin word _mollis_, soft. Why is it that the softest animals, the molluscs, have the hardest coverings? To which class of molluscs is the name acephala (headless) appropriate? Lamellibranchiata (platelike gills)? Why is a smooth shell suited to a clam and a rough shell suited to an oyster? Why are the turns of a snail’s shell so small near the centre? Why does the mussel have no use for head, eyes, or projecting feelers? In what position of the valves of a mussel is the hinge ligament in a stretched condition? How does the shape of the mussel’s gills insure that the water current and the blood current are brought in close contact? The three main classes of molluscs are: the pelecypoda (hatchet-footed); gastropoda (stomach-footed); and cephalopoda (head-footed). Give an example of each class. Comparison of Mollusks ═════════════════╤═════════════════╤═════════════════╤═════════════════ │ MUSSEL │ SNAIL │ SQUID ─────────────────┼─────────────────┼─────────────────┼───────────────── Shell │ │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── Head │ │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── Body │ │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── Foot │ │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── Gills │ │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── Eyes │ │ │ ═════════════════╧═════════════════╧═════════════════╧═════════════════ =Comparative Review.=—(To occupy an entire page in notebook.)