(b) The regions of the shell.—The rounded end is anterior, the more pointed end posterior, the straight hinge-line dorsal, and the gape ventral. Notice the knob-like umbo on each side near the anterior end of the hinge; it is the oldest part of the valve. Make out the concentric lines of growth, showing the successive positions of the margin. Draw the shell from the right side and from above. (c) General structure.—Kill the mussel by putting it for a few minutes into hot, but not boiling, water. Notice that the valves of the shell now gape apart somewhat. Hold up the animal and see, lining the shell, a thin soft membrane, the mantle. Carefully separate the mantle from one valve and notice, near each end, a thick white pillar which passes from one valve to the other. These pillars are the closing muscles. Pass the blade of a knife between the mantle and valve, and cut through each closing muscle quite near to the valve. Turn the valve back and remove it from the other valve by cutting through the elastic ligament (at the hinge) with strong scissors. Clean the inside of the separated valve and examine it; notice (i) the line of attachment of the mantle; (ii) the impressions of the closing [Pg 379] muscles; (iii) the lines of shifting of the closing muscles—triangular depressions stretching from the umbo to the muscles. Examine the animal as it lies in the other valve. Make out: (i) The right and left lobes of the mantle which line the valves and (in the natural position of the mussel) hang down from the sides of the body; (ii) the two plate-like gills on each side, lying between the mantle and the foot; (iii) the median foot; (iv) the reddish, triangular palps surrounding the mouth—an aperture at the anterior end of the foot. Push a stout pin into the mouth and upwards into the gullet. Make a drawing showing the relative positions of the parts. 2. The garden snail.—(a) Habits.—Where have you seen snails? At what time of the year are they most active? Upon what do they feed? Place a snail upon a lettuce leaf and carefully watch its method of eating. How does it move about? Put a live snail upon a sheet of glass, and look through the glass to see the wave-like action of the flat sole—the foot—by means of which it moves. Have you ever found snails in winter? How much of the animal was visible? How is the mouth of the shell closed in winter?