Exoskeleton
Exoskeletons are external skeletons providing support and protection.
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An exoskeleton is a skeleton located on the exterior of an animal, formed as a hardened integument that supports the body's shape and protects internal organs. It contrasts with an internal endoskeleton, such as that of humans, which is enclosed by soft tissues.
Examples include the cuticle skeletons of arthropods and tardigrades, the skeletal cups of stony corals, and the mollusc shells of snails, clams, and nautilus. Note that sea squirts have a tunic made of cellulose that is living tissue, not a hardened external skeleton, so they are not considered to have an exoskeleton. Turtles have a shell derived from their endoskeleton (modified ribs and vertebrae), not an exoskeleton, and sea urchins possess an internal endoskeleton (test), not an exoskeleton.
Quick Facts
- Examples
- Arthropods
- tardigrades
- stony corals
- molluscs
- Materials
- Chitin
- calcium carbonate
- silica
- iron sulfides
- Functions
- Protection
- structural support
- muscle attachment
- osmotic barrier
- defense against predators and parasites
- Growth mechanism
- Moulting (ecdysis) in panarthropods; aperture addition in molluscs
Facts from the source article.
Lore & Background
Exoskeletons contain rigid components that serve multiple roles beyond structural support, including protection, as an osmotic barrier against desiccation in terrestrial organisms, defense against parasites and predators, and attachment points for musculature. Arthropod exoskeletons contain chitin; adding calcium carbonate increases hardness and strength at the cost of weight. Ingrowths called apodemes serve as muscle attachment sites and are stronger and stiffer than vertebrate tendons, capable of storing elastic energy for jumping, as in locusts.
Reader's Guide
Exoskeletons have evolved independently many times, with 18 lineages developing calcified exoskeletons alone. Their rigid nature limits growth, requiring moulting in panarthropods or aperture addition in molluscs.
Mineralized exoskeletons are crucial for fossil preservation, as hard parts resist decay and can form molds or be mineralized. However, reliance on fossilized skeletons limits evolutionary understanding, as two-thirds of living animal phyla have never been found as fossils due to soft bodies. Ocean chemistry influences which calcium carbonate form (calcite or aragonite) is used, with most lineages using the form stable at the time of their first mineralization.
Did You Know?
- Some molluscs, like the scaly-foot gastropod, use iron sulfides greigite and pyrite in their exoskeleton.
- Arthropod growth is limited by the space within their current exoskeleton; failure to moult can prevent reproduction.
- Some Precambrian organisms like Cloudina had calcified exoskeletons, with evidence of predation in the form of borings.
Frequently Asked Questions
What is an exoskeleton?
An exoskeleton is a rigid outer framework that sits on the exterior of an animal's body, giving it structural shape and shielding its internal organs. It is the external counterpart to the internal endoskeleton found in vertebrates like humans.
What materials are exoskeletons made of?
Depending on the organism, exoskeletons can be built from chitin, calcium carbonate, silica, or even iron sulfides. For instance, arthropod cuticles are chitin-based while mollusc shells are typically calcium carbonate.
How does an exoskeleton grow with the animal?
Panarthropods must periodically shed their old exoskeleton through a process called moulting (ecdysis) to allow for further growth. Molluscs take a different approach, adding new material at the aperture of their shell over time.
More in Animal Physiology & Morphology
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Exoskeleton (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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