Snail evolution. Snails are hatched from eggs. When they hatch, they eat most of the shell, but a small part of it stays with them and that is the beginning of their shell. As the snail grows, so does its shell — in a spiral shape. New shell is added at the opening of the shell. In the center of the spiral is the part of the shell that the snail began life with. The shell is made from several layers. The thin outer layer is made from an organic substance known as conchiolin. The thicker layer underneath is formed from three layers of crystalline calcium carbonate, similar to the chicken egg of which you may be more familiar. The color of the shell depends on the types of food the snail is eating and also the type of snail.