(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 [Pg 380] 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. Fig. 233.—Fresh-water Mussel. A, from left side; B, from behind. ft, foot; in. sph., inhalant siphon; ex. sph., exhalant siphon; lg, ligament; m, mantle; um, umbo. (× ⅔.) (After Howes.) Molluscs.—Molluscs are soft-bodied animals, in most cases protected by a hard, external shell, but they differ essentially from crustaceans and all other arthropods in not being segmented, and in not possessing jointed limbs. Familiar and instructive examples of two classes of the group are found in the fresh-water mussel and the garden snail.