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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

Advantages of the exoskeleton

1. Muscle attachment:The exoskeleton forms the point of attachment of internal muscles needed for locomotion thereby providing better leverage for muscle action.
2. Protection:The exoskeleton protects the soft internal tissues and organs.
3. Support:The exoskeleton provides structural support and shape.
4. Prevents Dessication:The exoskeleton prevents desiccation (drying out) on land.
5. Light-weight:The exoskeleton of insects has a low density and is therefore lightweight, to allow for flight.
6. 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

1. 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.
2. 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.
3. 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.
4. Sites of structural weakness:Exoskeletons are weaker at the joints.

Endoskeleton

This skeleton is found inside the body and can consist of bone (all vertebrates except sharks) or cartilage (sharks) and some endoskeletons consist of both.

Advantages of the endoskeleton

1. Living:Endoskeletons consist of living tissue, so it is able to grow steadily within the animal enabling some to reach a large size.
2. Structure and support:The endoskeleton provides shape and structural support.
3. Structural diversity and adaptation:The bones can vary in size and shape to support the animal’s mass.
4. Flexible:The endoskeleton is jointed which allows for flexible movement and support.
5. Muscle attachment:The muscles attach directly to the skeletal bones to allow for movement and support.
6. Protection:The endoskeleton protects the vital organs such as the heart and lungs which are protected by the ribcage.
7. Diversified locomotion:The development of an endoskeleton has allowed for animals to become successfully adapted to locomotion in the environment in which they live. Vertebrates (organisms with a vertebral column and an endoskeleton) have become adapted to move in a number of different modes of locomotion, e.g. running, jumping, swimming, and flying

Disadvantages of the endoskeleton

1. Vulnerable to external environment:The endoskeleton does not offer the animal any protection from the exterior, be it a physical attack or changes in environmental conditions. The animal is therefore very vulnerable.
2. Susceptible to disease:The skeleton consists of living tissue so is susceptible to infections and disease.