. The " arms" or " feet" are very numerous in the tetrabranchiata, not provided with suckers, but hollow, and with long retractile tentacula; in the dibranchiata they are only eight or ten in number, furnished with suckers (acetabula); two of them, when they are ten in number, being much longer than the rest, and differing from them in form. The suckers are very admirably constructed—an adhesive disk of muscular membrane, often having a cartilaginous circlet, capable of most exact application to any object, with an aperture in the center leading into a cavity, the bottom of which can be retracted like a piston so as to form a vacuum, and render the adhesion of the sucker close and firm, whilst on the muscular action being interrupted or reversed, it immediately lets go its hold. The poulpe has each of its eight flexible arms crowded with 120 pairs of such suckers, and as an animal of this kind exists on some tropical shores, with arms about 2 ft. long, it is not wonderful that it is reckoned dangerous. Still more formidable, however, are the hook-squids of the South seas, the two long arms of which have suck ers furnished in the center with a hook to enter into the flesh of any creature of which they may lay hold, and so more effectually to secure their prey.
The sexes are distinct in all the cephalopoda. The eggs have a horny covering, and after their extrusion from the parent, become. agglutinated into masses of various forms. The young, from the first, very much resemble the mature animals, except in size.
All the dibranchiata are provided with a peculiar organ of defense, called an "ink bag," which is wanting in the tetrabranchiata. This ink-bag is filled with a peculiar secretion, capable of being expelled at will to darken the water, and facilitate the escape of the cephalopod.
The tetrabranchiate C. have a chambered shell. See NAUTILUS. The dibrauchiate C. have no external shell—the shell of the female argonaut (q.v.) being scarcely an exception—but they have an internal shell (cuttle-fish bone, etc.), sometimes merely rudimentary, included between two folds of the mantle, and apparently intended to give support to the soft body of the animal.
The C. are all very voracious, feeding on fish, mollusks, crustaceans, etc. Even a powerful crab is not safe from the attacks of a dibrauchiate C. little bigger than itself; the arms, so abundantly provided with suckers, seize it, and trammel every movement, the parrot-like beak is strong enough to break the hard shell. Cuttlefish and squids are often very troublesome to fishermen, following shoals of fish, and devouring great numbers of them after they are entangled in the net.
Fossil C. exist in all the strata which form the earth's crust. The order tetrabran ehiata is almost exclusively a fossil order, being represented by not more than four recent species. With the exception of two genera, nautilus and aturia, this order is confined to primary and secondary rocks. The two groups into which it is divided are also characteristic of geological epochs. The vanalas, with simple or gently undu lating septa, and siphuncle central or in the inner margin, belong, with the exception +of the two genera just referred to. to the paleozoic rocks. Including a small group while it has the siphon on the external margin, has yet simple septa, the nautili da are represented by 145 Silurian, 158 Devonian, and 91 carboniferous species. The have the sipliuncle always on the outer margin of the shell, and the septa with corrugated or lobed margins. This group, with the exception of goniatites, a pale ozoic genus, is peculiar to, and co-extensive with, the secondary strata. Of the 930 species that have been described, more than the half belong to the genus ammonites (q.v.).
The order dig ranchiata is found first in the lias, and extends through the more recent strata, receiving its full development in our present seas. Scarcely 90 fossil species have been described, while more than double that number are known as recent animals.
See AMMONITES, ARGONAUT, BELEMNITES, CALAMARY, CERATITES, CUTTLE-FISII, GONIATITES, HOOK-SQUID, NAUTILUS, OCTOPODA, ORTROCERAS, POULPE, etc.