The members of this class are defined by the possession of eight or more "arms"placed in a a circle round the mouth ; the body is enclosed in a muscular mantle-sac, and there are two or four plume-like gills within the mantle. There is an anterior tubular orifice (the "infundi-bulum" or "funnel") through which the effete water of respiration is expelled. The flexure of the intestine is neural.
The Cephalopoda, comprising the Cuttle-fishes, Squids, Pearly Nautilus, etc, constitute the most highly organised of the classes of the Mollusca. They are all marine and carnivorous, and are possessed of considerable locomotive powers. At the bottom of the sea they can walk about, head downwards, by means of the arms which surround the mouth, and which are usually provided with numerous suckers or "acetabular They are also enabled to swim, partly by means of lateral expansions of the integument or fins (not always present), and partly by means of the forcible expulsion of water through the tubular "funnel," the reaction of which causes the animal to move in the opposite direction.
The majority of the living Cephalopods are naked, possessing only an internal skeleton, and this often a rudimentary one; but the Argonaut (Paper Nautilus), and the Pearly Nautilus, are protected with an external shell, though the nature of this is extremely different in the two forms.
The integument in the Cuttle-fishes is provided with numerous mobile cells, containing pigment-granules of different colours, and termed "chroniatophores." By means of these many species can change their colours rapidly, under irritation or excitement.
The body in the Cephalopoda is symmetrical, and is enclosed in an integument which may be regarded as a modification of the mantle of the other Mollusca. Ordinarily there is a tolerably distinct separation of the body into an anterior cephalic portion (pro-soma), and a posterior portion, enveloped in the mantle, and containing the viscera (metasoma). The head is very distinct, bearing a pair of large globular eyes, and having the mouth in its centre. The mouth is surrounded by a circle of eight, ten, or more, long muscular processes, or "arms" (fig. 224), which are generally provided with rows of stalked or sessile suckers. Each sucker, or "acetabulum," consists of a cup-shaped cavity, the muscular fibres of which converge to the centre, where there is a little muscular eminence or papilla. When the sucker is applied to any surface, the contraction of the radiating muscular fibres depresses the papilla so as to produce a vacuum below it, and in this way each sucker acts most efficiently as an adhesive organ. In some forms (Decapoda) the base of the papilla, or piston, is surrounded by a horny dentated ring, and in some others (as in Onychoteuthis) the papillae are produced into long claws. In the Octopod Cuttle-fishes there are only eight arms, and these are all nearly alike. In the Decapod Cuttle-fishes there are ten arms, but two of these - called "tentacles" - are much longer than the others, and bear suckers only at their extremities, which are enlarged and club-shaped. In the
Fig. 224. - Cephalopoda. Sepiola Atlanlica, one of the Cuttle-fishes. (After Woodward.)
Pearly Nautilus the arms are numerous and are devoid of suckers.
In all the Cuttle-fishes, the mouth is placed in the centre of the "foot," and it is by a splitting up of the margins of the foot into long muscular processes, that the "arms* are produced. The arms are always symmetrically arranged in a dorsal, a ventral, and two lateral pairs; and the "tentacles" (when present) are placed on the ventral surface, between the 3d and 4th pairs of arms. The tentacles may or may not be retractile into pouches placed below the eyes, and their length may be many times greater than that of the body. They are organs of prehension; and the arms are in addition employed by the animal in locomotion, enabling it to creep along the sea-bottom head downward. In all the Decapod, and in some of the Octopod forms, the sides of the body are produced into muscular expansions or fins (figs. 224 and 231), with which the animal swims head foremost. In all the Cephalopods, also, the lateral margins of the foot ("epipodia") are either placed in apposition (Nautilus) or are actually united (Cuttle - fishes), in such a manner as to form a muscular tube, known as the "funnel." The funnel is placed on the lower surface of the body, with its anterior extremity projecting beyond the mantle, while it opens behind into the pallial chamber. It serves for the elimination of the water which has been used in respiration, and the out-going currents also carry away with them the excretions of the kidneys and of the ink-sac, together with the faeces. By the contractions of the mantle, the water contained in the pallial sac can also be driven through the funnel in a succession of jets, driving the animal backwards through the water.
Fig. 225. - Diagram of a Cuttle-fish (altered from Huxley), m Mandibles; n Cerebral ganglia ; / Liver ; p Intestine ; i Ink-bag ; g Gill; f Funnel; o Ovary; s Cuttle-bone.
The mouth leads into a buccal cavity (fig. 225) containing two powerful mandibles, working vertically, resembling the beak of the parrot in shape, and either horny (as in the Cuttlefishes) or partially calcareous in composition (as in the Nautilus). There is also a muscular tongue which appears to be in part an organ of taste, whilst in part it is developed into a lingual ribbon or "odontophore." The buccal cavity (fig. 225) conducts by an oesophagus - into which salivary glands usually pour their secretion - to a stomach, from which an intestine is continued, with a neural flexure, to open on the ventral surface of the animal at the base of the funnel. A large and well-developed liver is present. In many cases there is also a special gland, called the "ink-bag," for the secretion of an inky fluid, which the animal discharges into the water, so as to enable it to escape when menaced or pursued. The duct of the ink-bag opens at the base of the funnel; but this apparatus is entirely wanting in the Tetra-branchiate Cephalopods, where, in consequence of the presence of an external shell, this means of defence is not needed.