An external skeleton, commonly referred to as an exoskeleton, is an external covering that supports and protects the body of an animal. Many animals, including crabs, cockroaches, snails, clams and tortoises, have exoskeletons.
The functional roles of exoskeletons include protection, support, excretion, sensing and feeding. It also acts as a barrier against predators and pests. Exoskeletons contain chitin and calcium carbonate, which makes them stronger and harder.
Different species produce exoskeletons made of various materials. For example, chitin forms the exoskeleton in anthropods, arachnids and crustaceans. Calcium carbonates form the shells of molluscs and brachiopods. Silica forms the exoskeleton in diatoms and radiolaria. Bacteria and fungi also have mineral exoskeletons. Bone, cartilage and dentine form the exoskeleton of some animals, including turtles.
Exoskeletons are inflexible and, therefore, restrict growth to some extent. For example, molluscs, such as snails and bivalves with open shells, grow by adding new material to the aperture of their shells. Anthropods, on the other hand, shed their exoskeleton once it is outgrown in a process known as molting or ecdysis. A new exoskeleton starts to form beneath the old one. If the outgrown exoskeleton is not shed, the organism is likely to suffocate inside the shell.