Does the mussel have bilateral symmetry? Can you find a horny covering, or epidermis, over the limy shell of a fresh specimen? Why is it necessary? Does water dissolve lime? Horn? Find a bare spot. Does any of the shell appear to be missing there?
Fig. 189.—Diagram of Shell open and closed, showing muscle, m, and ligament, b.
The bare projection on each shell is called the umbo. Is the umbo near the ventral or the dorsal line? The posterior or anterior end? Is the surface of the umbones worn? Do the umbones rub against the sand as the mussel ploughs its way along? How are the shells held together? Where is the ligament attached? (Fig. [189].) Is it opposite the umbones or more to the front or to the rear? (Fig. [189].) Is the ligament of the same material as the shell? Is the ligament in a compressed condition when the shell is open or when it is closed? (Fig. [189].) When is the muscle relaxed?
Fig. 190.—Mussel crawling in sand.
Notice the lines on the outside of the shell (Figs. [188] and [190]). What point do they surround? They are lines of growth. Was each line once the margin of the shell? If the shell should increase in size, what would the present margin become? (Fig. [191].) Does growth take place on the margin only? Did the shell grow thicker as it grew larger? Where is it thinnest?
Draw the outside of the shell from the side. Draw a dorsal view. Near the drawings write the names of the margins of the shell (p. [98]) and of other parts learned, using lines to indicate the location of the parts.
Study the surface of the shell inside and out. The inside is called mother-of-pearl. Is it of lime? Is the deeper layer of the shell of lime? (When weak hydrochloric acid or strong vinegar is dropped on limy substances, a gas, carbon dioxide, bubbles up.) Compare the thickness of the epidermal layer, the middle chalky layer, and the inner, pearly layer.
