Fig. 206.—Pearly Nautilus. (Shell sawed through to show chambers used when it was smaller, and siphuncle, S, connecting them. Tentacles, T.)
The octopus is more sluggish than the squid. Large species called devilfish sometimes have a spread of arms of twenty-five feet. The pearly nautilus (Fig. [206]) and the female of the paper argonaut (Fig. [207]) are examples of cephalopods that have shells. The cuttlefish is closely related to the squid.
Fig. 207.—Paper Argonaut (female). × ⅓ (i.e. the animal is three times as long and broad as figure).
Fig. 208.—Paper Argonaut (male). × ½.
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?