contraction of the web-like membrane, and then but for a very short distance. Still less frequently does the octopus reverse its usual course; but I have twice seen one swim with its arms extended in advance of it, by bending the syphon tube beneath its body so as to present the orifice in a direction exactly contrary to its normal position. [Illustration: Fig. 5. The Octopus swimming.] M. Hugo forcibly refers to the remarkable property of rapidly changing its colour possessed by this animal. He writes:— “Its under surface is yellowish; its upper, earthy. Its dusty hue can neither be imitated nor explained: it might be called a ‘a beast made of ashes, which inhabits the water.’ Irritated, it becomes violet. It is a spider in form, a chameleon in coloration.” When quiescent, the general tone of colour of the octopus is a mottled brown, but it assimilates itself as much as possible to the rock to which for the time it may be holding. The moment it commences to swim it assumes a deeper hue, which usually becomes a dark, dingy red, but sometimes tends to purple. Mr. Darwin, in his delightful “Journal of Researches made during the Voyage of H.M.S. Beagle,” mentions his having noticed its endeavour to escape detection by the use of this chameleon-like power of changing colour so quickly as to cause it to vary with the nature of the ground over which it passes. This is effected in the same manner as the similar mutation of colour in the chameleon. Through the thin and transparent outer skin are visible cells in the inner layer beneath it, which contain pigment-matter of yellow, blue, red, and brown. By the contraction and expansion of the cells, prominence is given to one or another of these colours, at the will of the owner; and not only do the spots appear, and fade, and alternate in position, but, like human beings, the octopus turns pale when exhausted, and flushes red under the influence of anger or excitement. A curious play of colour, which I have elsewhere compared with the flashing and dying out of sparks in tinder, often takes place on the skin of the cephalopods by the continued action of the pigment cells, long after the death of the animal. The ancients were well acquainted with this colour-changing habit of the octopus. Aristotle mentions it, and Oppian describes it as follows:— “All fishers know the changing prekes’ deceit, How, clung to rocks, when coming dangers threat New forms they take, and wear a borrowed dress, Mock the true stone, and colours well express. As the rock looks they take a different stain, Dapple with grey, or branch the livid vein. Thus they, concealed, the dreaded danger shun By borrowed shapes obscured, and lost in seeming stone.” It was also frequently referred to by other writers. Athenæus quotes Theognis of Megara as saying in his Elegies:—