ment for the sudden concealment from enemies which the more active two-gilled forms demand. The many-chambered shell of the pearly nautilus exhibits a flat, symmetrical, spiral shape. Its many-chambered state is explained by the fact that as the animal grows it successively leaves the already formed chambers, and secretes, a new chamber to accommodate the increasing size of body. Each new chamber is partitioned off from that last occupied by a shelly wall called a septum (g). Through the middle of the series of septa runs a tube named the siphuncle, (s, s), whose function has been credited with being that of maintaining a low vitality in the disused chambers of the shell.
All other living cuttle-fishes possess, on the contrary, two gills, never more than ten arms provided with suckers, an ink-sac, unstalked eyes, a completely tubular funnel, and an internal shell. If, however, the nautilus represents in its solitary self the four-gilled cuttle-fishes of to-day, it likewise, like "the last of the Mohicans," appears as the descendant of a long line of famous ancestors. In its distribution, the nautilus is limited to the southern seas. It is still the rarest of animals in our museums, although its shells are common enough.
This, according to Mr. Moseley, is no doubt due to the fact that the animal is mostly an inhabitant of deep water. The shells of Spirula (Fig. 6) similarly occur in countless numbers on tropical beaches, yet the animal has only been procured two or three times.