A shell is considered as biotic (living) because it is a hard, rigid covering of many animals such as snail, sepia, pila, turtles, sea urchins. A shell is protective outer layer of these soft bodied animals. Shells are made of calcium carbonate in the form of calcite or aragonite crystallized in an organic matrix. Shells of other animals are composed of chitin, bone or cartilage. That's why a shell is considered as biotic (living).
