CEPHALOPODA, a class of mollusks rep resented by the squid, cuttle-fish, octopus, nau tilus, argonauts, etc. In these mollusks the head-lobe bears arms or lobes, as the animal has no afoot* or creeping-disc like that of other mollusks, though its homologue is found in the siphon and tentacles. They have an unpaired muscular mantle, which forms the walls or out side, so that as in the sguids, where there is no outer shell, the body is naked. The nervous system is much concentrated, for not only are the cerebral ganglia, pedal and visceral ganglia in the head, but also the ears and osphradia, or olfactory organs. The large complicated brain, thus composed of the three primary pairs of !ganglia with some accessory ones, are enclosed in a cephalic cartilage which suggests a com parison with the cartilaginous skull of the lam= prey and sharks. In the body behind are the sympathetic and stellate ganglia. The eyes as a rule are highly developed, with a retina, cho roid, iris, cornea, vitreous body and lens. The gills are well developed, either as one or two pairs situated within the mantle-cavity. The water is forced from the mantle-cavity, which is open behind the head, through the siphon. There are two kinds of hearts. The systematic heart consists of two or four (nautilus) auricles receiving the blood from the gills, and a• median ventricle from which arise the anterior and posterior aorta. There is also, at the base of each gill, a bronchial heart, which receives the blood from the vena cava and pumps it into the gill. These bronchial hearts are not known to exist in other mollusks, and no other mollusks possess an ink-sac. The armature of the mouth, however, as in gaitropods, consists of two horny jaws, enormous in most cephalopods, and an odontophore with its lingual ribbon for cut ting flesh, etc. In many forms one of the arms of the wall is peculiarily modified for sexual purposes — the so-called hectocolylieed arm. The eggs in developing undergo a super
ficial or discoidal development; and the young undergo no metamorphosis. The shell of cephal opods is either chambered, as in orthoceratites, nautiloids and anunonoids, or, as in argonauts, forms a simple deep basin; in all the Dibranchi (Oa it is at least partly internal. In the squids and cuttle-fish it takes the form of an internal pen or The cephalopods are divided into two orders, according to number of their gills: Order 1. Tetrabranchiata.— This group, in which the gills are four in number, is rep resented by the nautilus, the sole living repre sentative of a number of fossil forms, such as Orthoceras, Goniatites and Ammonites. Nau tilus pompilius and Nautilus umbilicatulus are the only survivors of about 1,500 extinct species of the order. See NAUTILUS.
Order 2. Dibranchiata.—The dibranchiates are so called from possessing but two gills, while the tetrabranchiates had, as in Nautilus, numerous unarmed tentacles; these are now represented by 10 (Decapoda) or 8 (Octopoda) arms, provided with numerous suckers. To the 10-armed forms belong Spirula, a diminutive cuttle with an internal coiled shell. The shells of Spina° peronii are rarely thrown ashore on Nantucket; it lives upon the high seas. The ex tinct Belemnites had, like the recent Moroteu this, a straight conical shell, the fossil. Allied to Loligo and Ommastrephes are gigantic cuttle-fishes which live in mid-ocean, but whose remains have been found at sea or cast ashore on Newfoundland and the Danish coast. Their jaws Also occur in the stomachs of sperm whales.
Fossil Cephalopods.— The greater propor tion of cephalopod mollusks are fossil. They began to exist in the Cambrian period, and, as nautiloids and ammonoids, flourished in great profusion in the Palaeozoic and Mesozoic eras, the ammonites (q.v.) of the Jurassic and Creta ceous beds numbering about 5,000 species. For bibliography see MOLLUSCA.