The head protrudes from the top of the mantle; it has a pair of large eyes on sockets, and some species of these animals have eyelids. The ears are cavities under the cartilage of the skull, containing a small sac and an otolite. The mouth, which is terminal, and surrounded by the tentacles, has powerful jaws like a parrot’s beak reversed, acting vertically. The tongue is large, the posterior part is covered with recurved spines, and the organ of smell is a cavity near the eyes. The Naked Cephalopods are remarkable for having three hearts, or propelling vessels—one for the circulation of arterial blood through the body, the others for projecting venous blood through the gills, at whose base they are situated. The arms or tentacles of all the Naked Cephalopoda are formidable organs of defence and prehension, but are most powerful in the Loligo vulgaris, the Poulpe, and the Cuttle, on account of one pair of the tentacles being long slender arms, dilated at their extremities into flat clubs. On the inner surface of each of the tentacles, and upon the lower surface of the dilated extremities of the long ones, there are multitudes of sucking disks, which, once fixed to an object, adhere so firmly that it is easier to tear off a portion of the animal’s tentacle than to make it release its hold. These sucking disks, which are placed in parallel rows, are represented magnified in fig. 180. Each sucker consists of a firm cartilaginous or fleshy ring (_e_), across which a disk of muscular membrane (_f_) is stretched, having a circular opening (_g_) in its centre. A cone-shaped mass of flesh fills the opening like a piston, capable of being drawn backwards; the membranous disk can also be drawn in. When one of these sucking-disks touches a fish, the fleshy piston is instantaneously retracted, a vacuum is formed, and the edges of the disk are pressed against the victim with a force equal to the pressure of the superincumbent water and that of the atmosphere. The fish is powerless when embraced by the eight tentacles and their hundreds of suckers; but, if large enough still to struggle, the force is increased by drawing in the membranous disk. The Poulpe, the most powerful of the group which swims far from land, and has to contend with large slippery fishes, has a hooked claw in the centre of each sucking-disk, which is clasped into the fish the instant the vacuum is formed. The expansions at the extremities of their two long arms, which are thickly and irregularly beset with hooked sucking-disks, not only drag the fish into the embrace of the eight short tentacles, but they clasp round it and interlock, so that the fish can be torn to pieces by the parrot-like jaws, and eaten at leisure. The tentacles, long and short, have strong nerves, and a little nerve-mass occupies the centre of each sucking-disk, which gives the tentacles great power. [Illustration: Fig. 180. Arm of Octopus.]