Shells are outer calcareous skeletons of shell dwellers that consist of three layers: the outer one (conchiolin), middle (porcelain) and inner (pearl, built of tiny calcareous blades). Shells are of various sizes, shapes and colors, it depends among others on the kind of a shell dweller, its age and the place of occurrence. Mollusks are the best known shell dwellers and the variety of their shells is the largest. Shells – especially of rare species – are considered to be collectibles. Rumor has it that when one attaches a shell to an ear it is possible to hear sea humming. However, in reality it is a hum of blood in blood vessels located in a human hearing organ that is echoed from the inside of a shell.
