The highest group of mollusks belongs to the class Cephalopoda, which signifies head-footed, the name being given to them because the head is surrounded by a circle of eight or ten arms, which act both as arms and feet. Let us take as an example of this class the common squid of the Atlantic coast (Ommastrephes illecebrosa), and see how it is formed. The body is long and cylindrical and ends at the tail in a point; the dorsal side of the tail end has a pair of triangular fins. The body is practically a hollow cylinder or sac which contains the vital organs of the animal. The neck is in many genera fastened to this cylinder or mantle by an apparatus which may be likened to a button and button-hole. The head is rounded, has on either side the large, round eyes, and at the end it is split up into ten arms, two of which are longer than the others and are called the tentacular arms. On the inner side, the arms are provided with two rows of suckers, which are little, rounded cups placed on pedicels or stems and which form a vacuum when they touch an object and so cling to it. The two long arms are expanded and club shaped at the end, each club being armed with four rows of suckers. Directly in the center of the circle of arms the mouth is placed and is provided with two sharp beaks like those of a parrot, only inverted. In addition to these organs there is a large siphon or tube on the ventral side, which is really an organ of locomotion, for it expels water from the mantle cavity with great force, thus rapidly sending the animal backward, its usual direction of propulsion. The body has no shell for protection, but in its place there is a long rod called a pen, which acts as a backbone to support the body of the animal, although of course not in the same sense as the backbone of vertebrated animals.

In some cephalopods this pen is hard and stiff but in Ommastrephes it is thin and soft. Such is, the general form of a cephalopod, familiar names of which are the Octopus, Squid, Nautilus, Paper-nautilus and Devil-fish. In this class, also, the majority of the shelled species are extinct, only a few living at the present time. The Ammonite is an example of the extinct cephalopods.

The most familiar member of this class to the layman is the Pearly Nautilus, the shell of which may be found on the mantel shelf or what-not of very many dwellings. The shell of the Nautilus is formed in a spiral and is made up of many chambers, all connected by a tube called a siphuncie, the outer chamber containing the animal and hence called the living chamber. The shell is called the "Pearly Nautilus," but the pearly tints cannot be seen until the outer layer — which is yellowish-white with brown markings — is taken off, when the exquisite, rainbow-like colors may be observed.

While the shell of Nautilus is well known the animal is very rare in our museums, although the natives of the Fiji Islands, New Hebrides and New Caledonia are able to obtain it in large quantities for food and it is highly esteemed by them. During the voyage of H. M. S. Challenger around the world, a living Nautilus was captured by dredging in some three hundred and twenty fathoms near Mateeka Island, one of the Fiji group. This was placed in a tub and it swam about in a lively manner by ejecting water from its funnel. The tentacles, of which there are a larger number than in the other cephalopods, were spread out radially, like those of the sea anemone. The Nautilus lives among the coral reefs, at depths varying from three to three hundred fathoms or more.