CEPHALOP'ODA (Gr. head-footed), a class of mollusks, the highest in organization of that division of the animal kingdom. To this class belong the nautili, spiruhe, argo nauts, poulpes, squids or calamaries, cuttlefish, etc., of the present time, and the ammonites, belemnites, etc., of former geological periods. The C. are all marine, and only a few of them are capable of leaving the water, and moving about in search of food on shore.
The C. receive their name from having organs of prehension and locomotion attached to the head, an arrangement towards which a gradual approach may be traced iu the highest gasteropod (q.v.) mollusks. These organs have been variously designated arms, feet, and tentacula. They "have no true homology" with the limbs of vertebrate animals, but are only analogous to them in respect of the purposes which they serve.—The body of the C. is a bag, formed of the mantle (see MoLLuscA), open only at the end to which the head is attached. In some, this bag is almost spherical, and locomotion is accomplished only by the appendages of the head; in others, the body is elongated, and furnished with two fin-like expansions, which are the principal instruments of locomotion. In locomo tion by the fins, a cephalopod swims like a fish, with the head first, and often very rapidly; in locomotion by the arms, it draws itself along, laying hold of any object within reach by means of suckers, with which the arms are furnished. Some C. are capable also of moving backwards through the water by alternate contractions and expansions of a muscular web which unites the bases of the arms; some appear to depend for a similar power of swimming backwards upon the forcible ejection of water from the ' funnel" below the eye.
The head of a cephalopod is roundish, generally furnished with two large and prominent eyes, very similar in structure to those of vertebrate animals. There are also ears, but they consist merely of little cavities, one on each side of the brain, in each of which is suspended a membranous sac containing a small s:one. The organs of smell
are not very certainly known, but it appears that the C. possess this sense, as well as that of taste, of which the character of the tongue is much more indicative than in many vertebrate animals.—The brain forms a ring around the gullet. The whole nervous system is more complex than in the lower mollusks.—The mouth opens in the midst of the circle of arms. It is furnished with a strong horny beak of two mandibles. moving vertically, not unlike the 'bill of a parrot, but the upper mandible the shorter of the two. —The digestive apparatus is very complicated. The gullet swells out into a crop, and there is a gizzard as muscular as that of a bird. The intestine, after a few convolutions, terminates in the cavity which contains the gills, at the base of the funnel by which the water is ejected after having supplied air for respiration. This cavity is situated within the mantle or bag, and separated from the other viscera by a membranous partition. Into it the water is freely admitted by means of a slit or valvular opening, being drawn in by muscular action, and again expelled .with considerable force through the funnel, which opens at the neck, and with its current all secretions, eggs. and eicrements are carried forth. There are only two gills in the greater number of existing C., the only exceptions being the two or three known species of nautilus, which have four gills; and two-gilled C.—the order dibranchiata—are in many respects of higher organization than the four-gilled—the order tetrabranchiata—which, although containing so few recent, con tains a vast number of fossil species. Each gill consists of many membranous plates, fixed to two sides of a stalk.—The heart in the tetrabranchiata consists of a single ventricle only; but besides this systemic heart, the dibranchiata have two branchial or respiratory hearts, contractile reservoirs, one for each gill, by which the blood is forced into these organs.