OCTOPUS (Gr. ὀκτώ, eight and πούς, foot), the name in scientific zoology belonging to a single genus of eight armed Cephalopoda (q.v.), one of whose distinguishing characters is that it has two rows of suckers on each arm. This true octopus occurs occasionally on the British coasts, at least the south coast, but is usually rare. It is more common on the southern coasts of Europe, including those of the Mediterranean. The usual species of Octopoda on the British south coast is Eledone cirrosa, which has only one row of suckers on each arm, and is a smaller animal. The celebrated account of the octopus given by Victor Hugo in his Travailleurs de la mer is not so fictitious as some critics with a knowledge of natural history have maintained. It is true that the great French author has made the mistake of using the name Cephaloptera, which belongs to a large tropical fish similar to a skate, instead of Cephalopoda, and that he applies the term devil-fish, which belongs to Cephaloptera, to the octopus. His description is exaggerated, imaginative and sensational; but it is correct in its most important particulars, and bears evidence that the author was to some extent personally acquainted with the animal and its habits, although he was not a scientific observer. The octopus feeds on crabs, and crabs feed on carrion, and, therefore, there is nothing impossible in Hugo's account of the skeleton of a drowned man surrounded by the shells of numbers of crabs which the octopus had devoured. Whether an octopus would attack and kill a man is another question, but it certainly might seize him with its arms and suckers while holding to the rocks by other arms, and a man seized in this way when in the water might be in danger of being drowned.
The octopus and many of the Octopoda move about by means of their arms on the sea bottom, and are not free-swimming, though like other Cephalopods they can propel themselves on occasion backwards through the water by means of the funnel. Other Octopoda, however, are pelagic and free-swimming, and such habits are not confined to those forms which are provided with lateral fins. The Argonaut (see Nautilus) is one of the Octopoda. The separation of one of the arms of the male for purposes of reproduction is one of the most remarkable peculiarities of the Octopoda. It does not occur, however, in octopus nor in many other members of the group. One arm is always considerably modified in structure and employed in copulation, but it is only in three genera, one of which is Argonauta, that the arm spontaneously separates. The detached arm is found still alive and moving in the mantle cavity of the female, and when first discovered in these circumstances was naturally regarded by the older naturalists as a parasite. Cuvier, on account of the numerous suckers of the detached arm, gave it the name Hectocotylus (hundred suckers). When the arm is not detached but only altered in structure it is said to be hectocotylized. In Octopus and Eledone it is the third right arm which is hectocotylized. The extremity of this arm is expanded and assumes the shape of a spoon. Whether detached or not the modified arm possesses a cavity into which the spermatophores are passed and the arm serves to convey them to the mantle cavity of the female.
It has been mentioned above that the true octopus (Octopus vulgaris) is usually rare on the English coast. In 1899 and 1900, however, they became so abundant on the south coast as to attract general notice, and to constitute a veritable plague which threatened complete ruin to the shell-fish fisheries. This visitation and its effects were described by W. Garstang in the Journal of the Marine Biological Association. The abnormal abundance occurred all along the west coast of France, whence it extended to the Channel, and was probably due to a succession of unusually warm summers and mild winters, beginning with the warm spring and hot summer of 1893. The octopus in the years mentioned entered the lobster pots of the fishermen and devoured or killed the crabs and lobsters captured. The pots when hauled contained usually only living octopus and the mutilated remains of their victims. One fisherman took in a single week 64 specimens of octopus and only 15 living uninjured lobsters. The octopus also almost exterminated the swimming crabs (Portunus) in Plymouth Sound, and in the tanks of the Plymouth aquarium attacked and devoured all the specimens of its smaller relative Eledone cirrosa.
With regard to the size which the octopus may attain, the dimensions of the body are not usually given in records, but it is stated that the arms in the largest specimens measured 3½ ft., and in numerous cases were 3 ft. in length. This would enable the eight arms to extend over a circle 6 ft. in diameter, but the globular body is not more than about a third of the length of an arm in diameter. When not in pursuit of prey the octopus hides itself in a hole between rocks and covers itself with stones and shells. Like its victims it seems to be active chiefly at night and to remain in its nest during the day.
For a technical account of the Octopoda see Cephalopoda; also W. Garstang,“The Plague of Octopus on the South Coast, and its Effect on the Crab and Lobster Fisheries,” Journ. Mar. Biol. Assoc. vol. vi. (1900) p. 260.