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. [Illustration: FIG. 206.--PEARLY NAUTILUS. (Shell sawed through to show chambers used when it was smaller, and siphuncle, _S_, connecting them. Tentacles, _T_.)] [Illustration: FIG. 207.--PAPER ARGONAUT (female). × ¹⁄₃ (_i.e._ the animal is three times as long and broad as figure).] [Illustration: FIG. 208.--PAPER ARGONAUT (male). × ¹⁄₂.] =General Questions.=--The living parts of the mussel are very soft, the name mollusca having been derived from the Latin word _mollis_, soft. Why is it that the softest animals, the mollusks, have the hardest coverings? To which class of mollusks 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 center? Why does the mussel have no use for head, eyes, or projecting feelers? In what position of the valves of a mussel is the hinge ligament in a stretched condition? How does the shape of the mussel’s gills insure that the water current and blood current are brought in close contact? The three classes of mollusks are: the pelecypoda (hatchet-footed); gastropoda (stomach-footed); and cephalopoda (head-footed). Give an example of each class. Comparison of Mollusks. =====+==========+==========+========== | MUSSEL | SNAIL | SQUID -----+----------+----------+---------- | | | Shell| | | | | | -----+----------+----------+---------- | | | Head | | | | | | -----+----------+----------+---------- | | | Body | | | | | | -----+----------+----------+---------- | | | Foot | | | | | | -----+----------+----------+---------- | | | Gills| | | | | | -----+----------+----------+---------- | | | Eyes | | | | | | =====+==========+==========+========== =Comparative Review.=--(To occupy an entire page in notebook.)