This gives us some idea of how important a factor these humble creatures are, working mainly at night, in burying the works of man. It is evident that in two or three centuries portions of buildings could be concealed. In England numbers of ancient Roman villas have been discovered, beautiful floors and foundations of ancient buildings which have been lost to sight by being covered by these night workers. To give an adequate idea of the work they accomplish, Darwin says that the amount of vegetable mold brought to the surface in a single year amounts to ten tons to a single acre. They rarely descend below six feet, and Darwin estimated that in favorable localities there are 100,000 in every acre. In New Zealand 348,480 have been found in a very rich acre. The worms eat the earth, and drag leaves and soft twigs into their holes at night. They plant seeds and bury stones. Some of the casts of giant worms of India are a foot in length. They live entirely beneath the ground, lining their burrows with very soft fine earth, which appears to be powdered for the purpose. All their operations are carried on at night, when they come to the surface and eject the casts. They have a habit of lying near the surface at the entrance of their burrows, a fact which the birds have discovered, robins and mocking birds particularly being very clever in hunting them out. One of the most remarkable features of these worms is their phosphorescence, which I have found is more brilliant than that emitted by any other animal. Crossing an orange grove in southern California one dark and rainy night in January, I stumbled over a clod of earth, and if I had kicked a mass of live coals, the result could not have been more marked, as flashes of vivid light darted in every direction with the earth, caused by several earthworms which had exuded so much phosphorescent matter that it had pervaded the entire mass of surrounding soil. The phenomenon on a small scale can often be seen in southern California, especially in winter, when the ground is moist and wet. [Illustration: FIG. 75.--Marine worm (_Cirratulus_).] [Illustration: FIG. 76.--A marine worm (_Nereis_).]