Worms are found in many places, from the forest floor to mountains, and in many locations around the world. Though they are considered terrestrial animals, they are really semi-aquatic, like other annelids; they die quickly in air but survive for months in water. Though inactive during the day, they sometimes come out of their burrows at night. They are eaten by thrushes and other birds in large numbers because they lie close to the surface. They have well developed muscular, nervous, circulatory and digestive systems, the latter being quite unique. Though eyeless, they respond to the intensity and duration of light. They also slowly respond to temperature. They have no hearing, but are sensitive to vibrations. Their sense of smell is feeble, but they are able to find their preferred foods. Omnivorous animals, they swallow much earth and extract food from it. Worms live chiefly on half decayed leaves, partially digested by a pancreatic solution before ingestion. This extra-stomachal digestion is not unlike that which Darwin had previously described as occurring in Insectivorous Plants. The structure and physiology of the calciferous glands of earthworms are described. Many hypotheses had been advanced for their function; Darwin believed them to be primarily for excretion and secondarily a digestion aid.
Thin leaves are seized with the mouth, while thick ones are dragged by creating a vacuum. Leaves and stones are used to plug up the burrow. This may deter predators, keep out water and/or keep out chilled air (the latter is Darwin's preferred function). Leaves are dragged in mostly by the tips, which is the easiest way of doing it, but when the base is narrower the worms change behaviour. They drag pine needle clusters in by the base. Petioles are used to plug up burrows, and for food. Worms drag experimental triangles of paper by the apex most of the time, and do not rely on trial and error. Worms excavate burrows by consuming material or, preferably, pushing it away. They mainly consume soil for nutrients. They are found down to six or more feet, especially in extreme conditions. Burrows are lined, which serve several functions, and terminate in a chamber lined with stones or seeds. Worms are found all over the planet, some on isolated islands; how they got there is a mystery. Darwin draws on correspondence with people from around the world such as Fritz M端ller in Brazil.
The amount of earth brought to the surface by worms can be estimated by the rate at which objects on the surface are buried and by weighing the earth brought up in a given time. Information from farmers on marl, cinders etc. sinking into the ground allowed Darwin to make calculations. He conducted a 29-year experiment on chalk at a field near his house. Objects of all sorts "work themselves downards" as farmers say. Large stones sink because worms fill up any hollows with castings, then eject them beyond the perimeter and the ground around them starts to rise. He visited Stonehenge and found some outer stones partly buried, the turf sloping up to meet them (see figure 7). Darwin weighed castings and had friends do so in other countries. He also weighed castings per unit area per year, then worked out how thick a layer castings would make, compared with rates of sinking. Additionally, he worked out casting weight per worm per year.
Worms have preserved many ancient objects under the ground. Darwin describes an ancient Roman villa in Abinger, Surrey. Worms have penetrated the concrete walls and even mortar. Similar subsidence occurred at Beaulieu Abbey, Hampshire, with worms penetrating gaps between the tiles. His sons Francis and Horace visited Chedworth Roman Villa in Gloucestershire, while William reported on Brading Roman Villa, Isle of Wight. Darwin goes into some detail on the well preserved ruins of Silchester Roman Town, Hampshire, with the help of the Rev. J. G. Joyce. Finally he discusses the case of the Viroconium Roman town ruins at Wroxeter, Shropshire, with the help of Dr. H. Johnson, who made observations including depth of vegetable mould. He concludes that both worms and other causes, such as dust deposition and washing down of soil, have buried such ruins.
Denudation (removal of matter to a lower level) is caused mainly by air and water movement. Humic acids generated by worms disintegrate rock; their burrowing behaviour speeds this up. But as the soil layer thickens, this process is slowed down. Worms swallow hard objects (e.g. stones) to aid digestion, which causes attrition to such objects. This has geological significance, especially for the smaller particles which otherwise are eroded very slowly.
Rain causes castings to move down an incline; Darwin worked out the weight moving a certain distance in a given time. Some also roll down, and collect in drains etc., or get blown. There is a greater effect on casting movement in the tropics, because of increased rain. The finest earth is washed away. Ledges on hillsides, formerly believed to be caused by grazing mammals, are partly due to casting accumulations. High winds, especially gales, are almost as effective as the slope/rain in moving castings. Crowns and furrows of formerly ploughed lands slowly vanish when under pasture, due to worms, but more slowly when there is no incline. Fine earth is washed down from slopes, making a shallow layer. Dissolving of chalk supplies new earth.
Darwin writes in the conclusion that worms "have played a more important part in the history of the world than most persons would at first suppose." They are important for many reasons, including their role in decomposition of rocks, gradual denudation of the land, preservation of archaeological remains, and improving soil conditions for plant growth. Despite their rudimentary sense organs, they show complex, flexible behaviour.

Question: What kind of objects do worms swallow in order to aid digestion?
Answer: hard objects
Alternative answer: Hard objects such as stones.