Page:The American Cyclopædia (1879) Volume IV.djvu/215

 CEPHALOPODA 207 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, but have a shell of chambers, numerous according to age, the animal dwelling in the anterior one and form- ing 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 Argonaut without the Shell 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 un- Argonaut within the Shell. known, and some, the so-called hectocotyli, resemble a detached arm of the animal, modi- fied for sexual purposes. All the living cepha- lopods, 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 pres- ent, is internal. These are the typical cepha- lopods, and higher than the tetrabranchiate or 168 VOL. iv. 14 four-gilled, creeping, many-armed shell-bearing nautilus and ammonites. Having ordinarily no external shell for defence, they have power- ful arms provided with suckers, a eland for secreting an inky fluid to discolor the water, and acute senses and rapid movements. The acetdbula 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 ef- fected; 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 con- tents near the base of the funnel ; the ink was formerly used as a pigment, and it is so inde- structible 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 flashes 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 Ibs. The dibranchiate cephalo- pods have been divided into the octopods, with eight arms, like the octopus and the ar- gonaut, and the decapods, with ten arms and more elongated .body, like the squids, cuttle- fishes, and fossil belemnites. The tetrabran- chiate cephalopods are protected by shells, at- tached 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 nauti- lus ; 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 be- long at the bottom, where they creep like a snail ; the discoidal forms were not well calcu- lated for swimming by their respiratory jets, and the straight ones, from the buoyancy of their shells, must have remained head down- ward. The use of the chambers is generally believed to be to render the animal of nearly the specific gravity of water, and thus facili- tate 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,