Answer: their mantle
Step-by-step explanation:
The external shell is formed in the part of a mollusk known as the mantle, which is a muscular fold of skin that helps it move along. Calcium Carbonate is an element which is naturally produced inside of the mantle. Calcium Carbonate is what makes the shell hard. An example of a mollusk would be a clam, although mollusks can also live on land, such as common snails.