Final answer:
a. Calcium carbonate. Land snails need calcium carbonate in the soil to build their shells. Calcium carbonate is a key element for shell growth and strength. A calcium-rich environment is essential for the survival of these gastropods.
Step-by-step explanation:
Land snails require a certain element in the soil to construct their shells, and this element is calcium carbonate. The shells of most mollusks, including land snails, are primarily made up of calcium carbonate (CaCO3). This compound is essential for them to produce their protective shells. In a soil dig rich in calcium carbonate, you can often find numerous shells of bivalves, chips of corals, and other indicators of a calcium-rich environment. For gastropods, which include snails, their shells serve a vital role similar to how bones provide structure to vertebrates.
When snails are unable to extract enough calcium carbonate from their environment, their shell growth can be adversely affected. This can lead to weaker shells or even failure to develop a shell at all, which jeopardizes their survival. So, for land snails, a presence of calcium carbonate in soil is not just beneficial but necessary for their well-being and continued existence.