During the day, the earthworm generally remains in its burrow in the soil, with its head just inside the entrance. Its method of forming the burrow depends upon the texture of the ground. In loose soil the earth is simply pushed aside, but where the material is too compact for this, the animal actually eats its way through. The burrow is lined with soft earth or little stones, and is plugged at the mouth with leaves or other convenient objects. The animal was found by Darwin to display distinct intelligence in its manner of drawing leaves into the mouth of its burrow, seizing them in most cases by their narrow ends, so that they could be pulled in with as little difficulty as possible. Objects are generally grasped between the lobe, which overhangs the mouth, and the lower part of the first segment, the hold being maintained by a sucking action. The inner end of the burrow is enlarged to allow the worm room to turn round. At night, the fore part of the body is protruded in search of food, the tail being generally retained in the [Pg 386] burrow, ready for instantaneous retreat in case of alarm. The earthworm feeds upon leaves—which are first softened by a fluid discharged over them, and then sucked into pieces small enough to be swallowed, for the animal has no jaws—and upon the half-decayed organic matter which is always present in ordinary soil. The soil itself is swallowed in large quantities; the nutritious portion is extracted, and the undigested matter deposited upon the surface of the ground, near the mouth of the burrow, in the form of castings. As a result of numerous experiments, Darwin estimated the weight of castings thus thrown up by earthworms on an acre of land as 15 tons annually. The following passage is worthy of very careful attention: “When we behold a wide, turf-covered expanse, we should remember that its smoothness, on which so much of its beauty depends, is mainly due to all the inequalities having been slowly levelled by worms. It is a marvellous reflection that the whole of the superficial mould over any such expanse has passed, and will again pass, every few years through the bodies of worms. The plough is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organised creatures.”[39]