such small fish as may chance to swim towards them. It also changes its colour[2530] according to the aspect of the place where it is, and more especially when it is alarmed. The notion is entirely unfounded that it gnaws[2531] its own arms; for it is from the congers that this mischance befalls it; but it is no other than true that its arms shoot forth again, like the tail in the colotus[2532] and the lizard.[2533] CHAP. 47.—THE NAUTILUS, OR SAILING POLYPUS. Among the most remarkable curiosities is the animal which has the name[2534] of nautilus, or, as some people call it, the pompilos. Lying with the head upwards, it rises to the surface of the water, raising itself little by little, while, by means of a certain conduit in its body, it discharges all the water, and this being got rid of like so much bilge-water as it were, it finds no difficulty in sailing along. Then, extending backwards its two front arms, it stretches out between them a membrane[2535] of marvellous thinness, which acts as a sail spread out to the wind, while with the rest of its arms it paddles along below, steering itself with its tail in the middle, which acts as a rudder. Thus does it make its way along the deep, mimicking the appearance of a light Liburnian[2536] bark; while, if anything chances to cause it alarm, in an instant it draws in the water, and sinks to the bottom.[2537] CHAP. 48. (30.)—THE VARIOUS KINDS OF POLYPI; THEIR SHREWDNESS. Belonging to the genus of polypi is the animal known as the ozæna,[2538] being so called from the peculiarly strong smell exhaled by the head;[2539] in consequence of which, the murænæ[2540] pursue it with the greatest eagerness. The polypi keep themselves concealed for two months in the year; they do not live beyond two[2541] years, and always die of consumption, the females even sooner,[2542] and mostly after bringing forth. I must not omit here the observations which L. Lucullus, the proconsul of Bætica, made with reference to the polypus, and which Trebius Niger, one of his suite, has published. He says that it is remarkably fond of shell-fish, and that these, the moment that they feel themselves touched by it, close their valves, and cut off the feelers of the polypus, thus making a meal at the expense of the plunderer. Shell-fish are destitute of sight, and, indeed, all other sensations but those which warn them of hunger and the approach of danger. Hence it is, that the polypus lies in ambush[2543] till the fish opens its shell, immediately upon which, it places within it a small pebble, taking care, at the same time, to keep it from touching the body of the animal, lest, by making some movement, it should chance to eject it. Having made itself thus secure, it attacks its prey, and draws out the flesh, while the other tries to contract itself, but all in vain, in consequence of the separation of the shell, thus effected by the insertion of the wedge. So great is the instinctive shrewdness