Exoskeleton

Exoskeleton

An exoskeleton is an external skeleton that supports and protects an animal's body. The skeleton is non-living and consists of a cuticle strengthened by chitin, a substance secreted by the epidermis (skin). Crustaceans such as crabs have their exoskeleton further strengthened by calcium carbonate. There are muscles attached to the inside of the exoskeleton which provides the resistance needed for muscle action.

The exoskeleton is confined to animals such as insects, spiders, scorpions, crabs etc., all of which belong to the Phylum Arthropoda (jointed-legged and jointed-bodied animals). The exoskeleton acts as a hard outer covering, and is made up of a series of plates or tubes. We often call large exoskeletons `shells'. Exoskeletons first appeared in the fossil record during the time of the Cambrian explosion and comprises a substantial portion of our fossil record (as you will learn in tutorial 10).

Picture of a spider, a type of arthropod.

Advantages of the exoskeleton

• Muscle attachment: The exoskeleton forms the point of attachment of internal muscles needed for locomotion thereby providing better leverage for muscle action.
• Protection: The exoskeleton protects the soft internal tissues and organs.
• Support: The exoskeleton provides structural support and shape.
• Prevents Dessication: The exoskeleton prevents desiccation (drying out) on land.
• Light-weight: The exoskeleton of insects has a low density and is therefore lightweight, to allow for flight.
• Diversity: The mouth-parts can be modified for biting, sucking, piercing grasping thus providing for a diversified diet for organisms possessing an exoskeleton compared to those that do not.

Disadvantages of the exoskeleton

• Size restriction: The final body size is limited because as the body size increases, the surface area to volume ratio decreases. The larger the animal, the heavier the exoskeleton, making movement more difficult.
• Non-living skeleton does not grow with animal: The overall growth of the animal is restricted due to periodic moulting. Since the exoskeleton restricts growth, moulting is required to accommodate for increases in the size of the animal.
• Vulnerability during moulting: The animal is vulnerable when it is in the moulting process, because the new skeleton is very soft until the new exoskeleton has dried and hardened.
• Sites of structural weakness: Exoskeletons are weaker at the joints.