Fig. 238.—Octopus vulgaris Lam., Naples: A, At rest; B, in motion; f, funnel, the arrow showing the direction of the propelling current of water. (After Merculiano.)
The common Octopus vulgaris Lam., of British and south European coasts, inhabits some rocky hole, the approaches to which, like the den of a fabled giant, are strewn with the bones of his victims. Homer himself knew how hard it is to drag the polypus out of his hole, and how the stones cling fast to his suckers. The colour-changes, which flit across the skin of the Octopus, appear, to some extent, expressive of the different emotions of the animal. They are also undoubtedly protective, enabling it to assimilate itself in colour to its environment. Mr. J. Hornell[396] has noticed an Octopus, while crawling over the rock-work in his tank, suddenly change the colour of the whole right or left side of its body, and of the four arms on the same side, to a snowy whiteness. They have also been seen to change colour, as if involuntarily, according to the material on which they crawl. The nerve-centres which control the chromatophores or pigment cells, causing them to expand or contract, are found to connect with the optic ganglia; hence the changes of colour may be regarded as a reflex result of the creature’s visual perception of its surroundings.
Order Dibranchiata.
Cephalopoda with two symmetrical branchiae, funnel completely tubular, mouth surrounded by 8 or 10 arms furnished with suckers or hooks, ink-sac and fins usually present, eyes with a lens; shell internal or absent.
The Dibranchiata are not known from Palaeozoic strata, and first appear (Belemnites, Belemnoteuthis) in the Trias. Whether they are to be regarded as derived from some form of Tetrabranchiata, e.g. Orthoceras, or as possessing an independent origin from some common stock, cannot at present be decided. They attain their highest development at the present time. The earliest representatives of the Order (the Phragmophora) possessed a shell chambered like that of the Tetrabranchiata. These chambered Dibranchiates rapidly reached their maximum in the upper Lias and as rapidly declined, until at the close of the Cretaceous epoch they were comparatively scarce, only a few genera (Beloptera, Spirulirostra) surviving into Tertiary times.
The ordinary Dibranchiate Cephalopod may be regarded as consisting of two parts—(a) the head, in which are situated the organs of sense, and to which are appended the prehensile organs and the principal organs of locomotion; (b) a trunk or visceral sac, enclosed in a muscular mantle and containing the respiratory, generative, and digestive organs. The visceral sac is often strengthened, and the viscera protected, by an internal non-spiral shell. The ‘arms’ which surround the mouth are modifications of the molluscan foot (p. [200]), and are either eight or ten in number. In the former case (Octopoda) the arms, which are termed ‘sessile,’ are all of similar formation, in the latter (Decapoda), besides the eight sessile arms there are two much longer ‘tentacular’ arms, which widen at their tips into ‘clubs’ covered with suckers.
Remarks have already been made on the generative organs of Cephalopoda (p. [136] f.), the branchiae (p. [170]), the nervous system (p. [206]), the eye (p. [182]), the radula (p. [236]), and the ink-sac (p. [241]).
One of the most characteristic features of the Dibranchiata are the acetabula, or suckers, with which the arms are furnished. They are usually disposed on the sessile arms in rows (of which there are four in most Sepia, two in Octopus, and one in Eledone), and become more numerous and smaller at the tip of the arm. They are massed together in large numbers of unequal size on the ‘clubs’ in the Decapoda, particularly in Loligo. In most Octopoda their base is flush with the surface of the arm, but in Decapoda the acetabula are pedunculate, or raised on short stalks. In Octopoda again, the acetabula are fleshy throughout, but in the Decapoda they are strengthened by a corneous rim with a smooth or denticulate edge (Ommastrephes, Architeuthis). Many of the acetabula on the tentacular and sometimes on the sessile arms of the Onychoteuthidae enclose a powerful hook, which is retractile like the claws of a cat.
In mechanical structure the acetabula consist of a disc with a slightly swollen margin, from which a series of muscular folds converge towards the centre of the disc, where a round aperture leads to a gradually widening cavity. Within this cavity is a sort of button, the caruncle, which can be elevated or depressed like the piston of a syringe; thus when the sucker is applied the piston is withdrawn and a vacuum created (Owen).
