Exoskeleton

An exoskeleton, in contrast to an endoskeleton, is an external anatomical feature that supports and protects an animal's body. All arthropods (such as insects, spiders and crustaceans) and many other invertebrate animals (such as shelled mollusks) have exoskeletons. Lobsters, for example, have tough outer shell systems which provide rigidity and shape to their bodies, in addition to an endoskeleton; the endoskeleton of arthropods is continuous with, and shed at the same time as, the exoskeleton.

An insect's exoskeleton (integument) serves not only as a protective covering over the body, but also as a surface for muscle attachment, a water-tight barrier against desiccation, and a sensory interface with the environment. It is a multi-layered structure with four functional regions: epicuticle, procuticle, epidermis, and basement membrane.NC State University Note that while the arthropod exoskeleton is chitin-based, it is not composed entirely of chitin; in fact, the membranous portions of the exoskeleton and endoskeleton are closer to pure chitin than the hardened external sclerites.

Humans have long used armour as an artificial exoskeleton for protection, especially in combat. Exoskeletal machines are also used for medical and industrial purposes. Powered human exoskeletons are a feature of science fiction writing. Orthoses are a form of exoskeleton.

Natural exoskeletons

Types of exoskeletons

Bone and cartilage
- Armoured fish
- Turtles, Terrapins and Tortoises
Chitin
- Arthropods
- some Bacteria
Calcium compounds
- Corals
- Echinoderms
- Mollusks
- some Polychaetes
Silicate
- Diatoms
- Radiolaria

Moulting

An exoskeleton may interfere with an animal's growth. To overcome this, arthropods go through a process called ecdysis in which they shed their exoskeleton and replace it with a new, larger one.

Excellent as a principle of defence, exoskeletons may nevertheless cause problems where entities carry an excessive weight to surface-area ratio; or whenever organism growth requires an enlarged exoskeleton.

Artificial human exoskeletons

Exoskeletons in history

Medieval armour (in the case of mounted knights) is not load-bearing, but furnishes the appearance of an artificial human exoskeleton.

Exoskeletal devices in medicine

An orthosis (plural orthoses) is a device which attaches to a limb, or the torso, to support the function or correct the shape of that limb or the spine. Orthotics is the field dealing with orthoses, their use, and their manufacture. An orthotist is a person who designs and fits orthoses.

A limb prosthesis (plural prostheses) is a device that substitutes for a missing part of a limb. If the prosthesis is a hollow shell and self-carrying, it is exoskeletal. If internal tubes are used in the device and the cover (cosmesis) to create the outside shape is made of a soft, non-carrying material, it is endoskeletal. Prosthetics is the field that deals with prostheses, use, and their manufacture. A prosthetist is a person who designs and fits prostheses.