Cephalopoda (Gr. head, and foot), the highest of the branch of mollusks. Some have no external shell, like the squid (loligo); others an internal one, like the cuttle fish (sejnii); others a simple shell, like the argonaut; and others chambered shells, like the fossil ammonites and living nautilus, many being in this respect unsurpassed for symmetry and beauty among mollusks. Their most striking feature is that the head is distinct from the body, with large eyes on the sides; the posterior part of the body ends in a sac. The principal locomotive organs, whence their name, are attached to the head in the form of muscular arms or tentacles, usually eight or ten; besides these, many have fins, and all are more or less able to propel themselves backward by the forcible expulsion of water from the branchial sac through the siphon which opens on the lower surface. They are perfectly symmetrical animals, having the right and left sides equally developed, and the shell, when present, usually straight or coiled in a vertical plane, very different from that of the spirally coiled gasteropods.
The cephalopods, except the argonaut, do not occupy the whole of their shells, as do the gasteropods, hut have a shell of chambers, numerous according to age, the animal dwelling in the anterior one and forming another one, with a dividing partition, as it grows larger. They are carnivorous, with powerful parrot-like jaws, moving vertically; the tongue is in part armed with recurved spines for retaining the food; all their senses are acute; they are all marine. The nervous system is more concentrated than in other mollusks, and the anterior ganglia, or brain, are protected by a cranial cartilage, which has been regarded as a commencing cranial cavity. They breathe by two or four plume-like gills, symmetrical on each side, in a cavity beneath the body, opening externally; at the base of the gills are dilatations of the blood vessels, which send the blood to the heart after purification. The sexes are distinct, but the females are the most numerous, the males of many being unknown, and some, the so-called herfocotyli, resemble a detached arm of the animal, modi-tied for sexual purposes.
All the living cephalopods, except the nautilus, belong to the di-branchiate or two-gilled order; these are naked, free swimming animals, with distinct head, prominent eyes, eight or ten arms with numerous suckers, the body round or elongated, usually with two fins; all have an ink gland, and the walls of the funnel entire; with the exception of the argonaut, the shell, when present, is internal. These are the typical cephalopods, and higher than the tetrabranchiate or four-gilled, creeping, many-armed shell-bearing nautilus and ammonites. Having ordinarily no external shell for defence, they have powerful arms provided with suckers, a gland for secreting an inky fluid to discolor the water, and acute senses and rapid movements. The acetahula or suckers are in single or double rows on the inner surface of the arms; the muscles of each cup converge to a central cavity, with a soft piston-like caruncle in the centre, by which a very strong adhesion is effected; these are perfectly under the control of the animal, which can instantly release its thousand suckers, and dart away under cover of its inky secretion, at the approach of danger; in some the base of the piston forms a toothed horny loop, or even a sharp hook.
The ink bag is tough and fibrous, discharging its contents near the base of the funnel; the ink was formerly used as a pigment, and it is so indestructible as to be found fossil, and to permit the naturalist to draw with its own secretion an animal which perished millions of years ago. The skin is of various colors, and remarkable for the sudden changes which dart over it in brilliant Hashes as the creature dies; they have hence been called "chameleons of the sea;" they have water pores on the head and arms. They are nocturnal or crepuscular animals, hiding by day, found in every zone and at all depths of water, as well as in the open sea; they may attain, in tropical seas, a length of six or eight feet, including the arms, and a weight of 300 lbs. The dibranchiate cephalopods have been divided into the octopods, with eight arms, like the octopus and the argonaut, and the decapods, with ten arms and more elongated body, like the squids, cuttlefishes, and fossil belemnites. The tetrabranchiate cephalopods are protected by shells, attached to the body, in the anterior chamber of which the creature dwells. They are now nearly extinct, the nautilus being the only living representative.
More than 1,400 fossil species are known by their shells, which vary from perfectly straight in orthoceras and bacu-lites to the tightly coiled ammonite and nautilus; they performed in the palaeozoic and me-sozoic seas what the carnivorous gasteropods did in the cenozoic and do in the present epoch. The chambers do not act to enable them to rise or sink at will; they are rarely found at the surface of the water, except in storms, but belong at the bottom, where they creep like a snail; the discoidal forms were not well calculated for swimming by their respiratory jets, and the straight oiks, from the buoyancy of their shells, must have remained head downward. The use of the chambers is generally believed to be to render the animal of nearly the specific gravity of water, and thus facilitate locomotion, and that of the partitions to strengthen them against collisions, being most numerous in the delicate ammonites; the animal is probably always growing forward, except during the formation of the partitions, which indicate periods of rest.
All the ce-phalopods below the middle of the mesozoic strata belong to the group with chambered shells; those with internal shells do not date back much beyond the oolite age, like the be-lemnites; the group with chambered shells may, therefore, be regarded as the lower type. Those which have eight arms have never any shell or hard part; those which have ten have always hard parts, varying from cartilage to the horny pen of the squids, and the calcareous plates of the cuttle fishes.
Argonaut without the Shell.
Argonaut within the Shell.