We now pass to the last and highest class of the mollusca, called the Cephalopoda because they have a number of arms attached to the head, round the mouth. Unlike the majority of molluscs they are bilaterally symmetrical: and are much more highly organised, in some respects even making an approach to the vertebrates. Thus they generally have an internal hard structure, either horny or calcareous in structure, representing the vertebral column, and the circulatory system consists of arteries and veins, connected by minute capillaries. The corpuscles of the blood are also similar in form to those of the vertebrates. Externally they are all naked, with the exception of the nautilus and argonaut of the warmer seas.
The arms, so characteristic of the class, are eight or ten in number, long and muscular, and provided with numerous suckers by which the animal can cling with remarkable tenacity. These suckers are situated on the inner surface of the arms, and the disc of each one displays a series of muscular fibres, all converging from the circumference towards the centre, which is occupied by a softer structure that works inwards and outwards like the piston of a pump. Thus the suckers form a system of exhausting air-pumps by which a vacuum can be produced, and the tenacity of the grip, maintained by atmospheric pressure, is so great that the arms, strong as they are, may be torn asunder by attempting to pull them from their hold; and yet the animal can release its grip with the greatest of ease by simply releasing the pistons of its pumps.
The cephalopods are further distinguished by their very large, glaring eyes, situated on the sides of the well-formed head, and by powerful jaws that work in a vertical plane, like those of the vertebrates, but somewhat resembling the beaks of certain birds. The tongue is also very large and fleshy, and in part armed with numerous hooked spines or teeth.
The class is usually divided into two orders, one characterised by the possession of two gills, and the other of four; but the British species belong to the former, known technically as the Dibranchiata. This order is subdivided into two sections according to the number of arms; and the divisions are called the Octopoda and Decapoda respectively.
Fig. 183.—Octopus
The former section includes the Octopods, of which some species inhabit our seas. They all have eight arms, of unequal size, with the suckers arranged in two rows, and their round or oval bodies seldom have any fins, locomotion being effected by means of the arms, and by the sudden expulsion of water from the siphon. The shell is rudimentary, being represented merely by two short ‘styles’ within the mantle. The species vary considerably in size, some being only about an inch long when fully grown, while others measure two feet or more, and are looked upon as formidable creatures by man. Sometimes they are washed up on our beaches, but the best way to make their acquaintance is to examine the contents of the fishermen’s drag nets as they are hauled on the beach.
In the same manner we may secure various species of the Decapods or Ten-footed Cephalopods, which comprise the Calamaries, Squids, and Cuttlefishes. These, too, properly speaking, have but eight arms, the other two appendages being really tentacles, which are usually longer than the arms, and more or less retractile; they are also expanded at the ends. The decapods are also to be distinguished from the octopods by their elongated bodies, and a flattened, fin-like appendage on either side. Their eyes, also, are capable of being rotated within the orbits, while those of the octopods are fixed; and the shell consists of one or more horny ‘pens,’ or of a calcareous ‘bone,’ contained in a cavity so loosely that it drops out of its place when the cavity is opened.