While many sea animals produce exoskeletons, also known as sea shells, usually only the shells of mollusks are considered to be "sea shells." Over time, as the animal inside grows, it will add its building material to the leading edge of the shell near the opening. This causes the shell to become longer and wider. Each species of mollusk animals will build the external shell in specific shape, pattern, ornamentation, and color. Mollusk shells (especially those formed by marine species) are very durable and greatly outlast the otherwise soft-bodied animals that produced them. Shells of marine mollusks (some of which wash up on beaches and are therefore easily found without specialized equipment) are called "seashells," and are collected by a large number of enthusiasts.