Learning Goals

Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The muscles in a hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces movement. For example, earthworms move by waves of muscular contractions of the skeletal muscle of the body wall hydrostatic skeleton, called peristalsis, which alternately shorten and lengthen the body.  Lengthening the body extends the anterior end of the organism. Most organisms have a mechanism to fix themselves in the substrate.  Shortening the muscles then draws the posterior portion of the body forward. Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals.

• Fluid shape: This allows organisms with hydrostatic skeletons to fit through oddly shaped passages, which is useful for burrowing or swimming.
• Strength: Creatures with hydrostatic skeletons can squeeze between spaces and expand, making a ‘prying open’ movement which allows them to force their way into various regions of rock and soil surfaces.
• Healing: Healing takes place faster in organisms with hydrostatic skeletons than in organisms with bone structures. This is because the haemocoel contained within the hydrostatic skeleton is made up mostly of water, and thus, can be refilled quickly. This allows many organisms with hydrostatic skeletons such as earthworms to grow back their body mass after damage.
• Lightweight: The hydrostatic skeleton allows the animal to move in a more flexible manner as it requires very little muscle mass for movement.
• Circulation: The fluid cavity allows the circulation of nutrients and waste.
• Protection: The hydrostatic skeletons cushions the internal organs of the animal from shock.
• Suited to the environment: Hydrostatic skeletons are suited for life in moist or aquatic environments, depending on the animal’s adaptations.

Disadvantages of a hydrostatic skeleton

• Structure and surface for attachment: The hydrostatic skeleton lacks a structure and does not have surfaces for the attachment of muscles or limbs.
• Lack of protection: There is very little protection for the internal organs.
• Desiccation: A moist or water habitat is essential for the survival of these animals in order to prevent desiccation (drying out).
• Limited strength: Terrestrial animals with hydrostatic skeletons cannot increase their body size as they would collapse under their own body weight.

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’.

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 Desiccation: 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.
• The non-living skeleton does not grow with animals: 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.

An endoskeleton is a skeleton that consists of hard, mineralized structures located within the soft tissue of organisms. The bones of vertebrates are composed of tissues and can consist of bone (all vertebrates except sharks) or cartilage (sharks) and some endoskeletons consist of both. Endoskeletons provide support for the body, protect internal organs,  and allow for movement through the contraction of muscles attached to the skeleton.

Advantages of the endoskeleton

• Living: Endoskeletons consist of living tissue, so it is able to grow steadily within the animal enabling some to reach a large size.
• Structure and support: The endoskeleton provides shape and structural support.
• Structural diversity and adaptation: The bones can vary in size and shape to support the animal’s mass.
• Flexible: The endoskeleton is jointed which allows for flexible movement and support.
• Muscle attachment: The muscles attach directly to the skeletal bones to allow for movement and support.
• Protection: The endoskeleton protects the vital organs such as the heart and lungs which are protected by the ribcage.
• Diversified locomotion: The development of an endoskeleton has allowed 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

• Vulnerable to the 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.
• Susceptible to disease: The skeleton consists of living tissue so is susceptible to infections and disease.

The three types of skeleton designs are hydrostatic skeletons,  exoskeletons, and endoskeletons. A hydrostatic skeleton is formed by a  fluid-filled compartment held under hydrostatic pressure; movement is created by the muscles producing pressure on the fluid. An exoskeleton is a hard external skeleton that protects the outer surface of an organism and enables movement through muscles attached to the inside. An endoskeleton is an internal skeleton composed of hard, mineralized tissue that also enables movement by attachment to muscles.