1. The fresh-water mussel.—(a) Habits.—Study the habits of the living animal ([Fig. 233]) in the aquarium. Notice the muscular foot which is protruded between the halves (valves) of the shell, and by means of which the mussel slowly makes its way along the sandy bottom. With a pipette, discharge some water, coloured with carmine or indigo, close to the more pointed end of the shell. Where is the coloured water taken in, and where is it expelled? Notice that the shell closes when the animal is handled.
(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 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?
(b) General appearance.—Make a drawing of the animal from the left side and another from the right side, and notice the differences between the two. Is the shell placed over the middle of the animal, or does it lie to one side? How many turns has the spiral of the shell? In a fully expanded snail observe the fleshy “collar” round the margin of the shell; it is the edge of the mantle. Notice the rounded head, the two pairs of tentacles, the eye-spot at the tip of each of the larger and upper tentacles, the mouth, and, near the base of the shell on the right side, the respiratory pore, which opens into the lung. Touch various parts of the body in turn to see if they are irritable; how are the tentacles retracted? Can the animal close and open its respiratory pore at will?
3. An air-breathing pond-snail.—Obtain several fresh-water snails and study their habits in glass aquaria. The commonest fresh-water snails are species of Limnaea; identify some of these by comparison with [Fig. 234], A, and try to make out the parts already seen in the garden snail. Watch the action of the mouth as the animal feeds on the green scum which often collects on the sides of aquaria. Notice how it creeps over the glass or along the surface-film of the water. When the snail comes to the surface it replaces the air of its lung by fresh air, and a bubble may often be seen escaping from the respiratory opening under the lip of the shell. Look for the spawn (egg-masses) of this snail on leaves or on the sides of the aquarium, and examine the eggs frequently with a hand lens.
