Coordination and Movement
4. Skeletons and Support Systems
Learning outcomes
- I can compare hydrostatic, exoskeleton, and endoskeleton support systems.
- I can explain the functions of skeletons.
- I can describe how skeletons support movement and protection.
- I can identify adaptations of skeletal systems in different animals.
- I can explain the relationship between support and locomotion.
Why Do Animals Need Support Systems?
Animals need some form of structural support to maintain their shape and move effectively.
Without a support system, muscles would have little to pull against and many animals would be unable to maintain a stable body shape.
Support systems can perform several important functions:
- Support the body.
- Maintain body shape.
- Protect delicate organs.
- Provide attachment points for muscles.
- Allow movement.
- Support locomotion.
- In some animals, store minerals.
- Provide protection against predators or physical damage.
Animals have evolved several different solutions to the problem of support.
Three major types are:
- Hydrostatic skeletons
- Exoskeletons
- Endoskeletons
Each system has advantages and limitations and is suited to particular body forms and lifestyles.
What Is a Skeleton?
A skeleton is a structural support system that helps maintain body shape and allows forces produced by muscles to create controlled movement.
The word "skeleton" does not necessarily mean bones.
An earthworm, for example, has no bones but still has a support system that allows it to move effectively.
The three major systems work differently:
Hydrostatic skeleton → fluid provides support
Exoskeleton → rigid structure outside the body provides support
Endoskeleton → internal framework provides support
Hydrostatic Skeletons
A hydrostatic skeleton uses fluid contained within the body to provide support.
The fluid is largely incompressible, meaning that it does not easily decrease in volume when pressure is applied.
Muscles surrounding the fluid-filled space can change the animal's shape and produce movement.
Hydrostatic skeletons occur in animals such as:
- Earthworms.
- Many other worms.
- Sea anemones.
- Jellyfish.
Hydrostatic support also contributes to movement in structures such as the arms of octopuses.
How Does a Hydrostatic Skeleton Work?
Imagine a water-filled balloon.
The water inside cannot easily be compressed.
If one part of the balloon is squeezed, pressure is transferred through the fluid and the shape changes.
A hydrostatic skeleton uses a similar principle.
Muscles contract around a fluid-filled body cavity.
Because the fluid resists compression, muscular contractions can change the shape of the body and generate movement.
This allows the fluid to act as something for the muscles to push or pull against indirectly.
Earthworm Movement
Earthworms provide an excellent example of hydrostatic support.
They have two important sets of muscles:
- Circular muscles.
- Longitudinal muscles.
When circular muscles contract, a body segment becomes:
- Longer.
- Thinner.
When longitudinal muscles contract, the segment becomes:
- Shorter.
- Thicker.
Coordinated waves of contraction travel along the body.
Small bristles called setae help grip the ground.
The sequence can be simplified as:
Muscles contract → body shape changes → parts of body grip surface → animal moves forward
Advantages of Hydrostatic Skeletons
Hydrostatic skeletons have several advantages.
They are:
- Flexible.
- Lightweight.
- Capable of producing a wide range of body shapes.
- Useful for burrowing and squeezing through narrow spaces.
- Able to support soft-bodied animals without heavy rigid structures.
Flexibility can be particularly useful in aquatic environments and underground habitats.
Limitations of Hydrostatic Skeletons
Hydrostatic skeletons also have limitations.
They generally:
- Provide less rigid protection than hard skeletons.
- Depend on maintaining suitable internal fluid pressure.
- Provide fewer rigid attachment points for powerful muscles.
- May provide less support for large terrestrial animals.
For many small or soft-bodied animals, however, these limitations are not major disadvantages.
The effectiveness of a support system depends on the animal's environment and lifestyle.
Exoskeletons
An exoskeleton is a hard supporting structure located on the outside of the body.
Arthropods such as insects, spiders, crabs, and lobsters have exoskeletons.
Arthropod exoskeletons contain a material called chitin, often combined with other substances that change its strength and flexibility.
Functions of an Exoskeleton
An exoskeleton can perform several functions at once.
It can:
- Support the body.
- Maintain body shape.
- Protect internal organs.
- Provide attachment surfaces for muscles.
- Form joints for movement.
- Reduce water loss.
- Provide protection against predators and physical damage.
This combination has been highly successful. Arthropods are among the most diverse groups of animals on Earth.
Exoskeletons and Water Conservation
An important advantage of the arthropod exoskeleton is its ability to reduce water loss.
The outer surface can act as a barrier that limits evaporation.
This is especially important for terrestrial arthropods such as insects.
Without a protective outer covering, a small animal with a high surface-area-to-volume ratio could lose water rapidly.
The exoskeleton therefore contributes to both:
Support + water conservation
Exoskeletons and Movement
A rigid outer covering might seem as though it would prevent movement.
However, arthropod exoskeletons contain joints.
Flexible membranes between rigid sections allow body parts to move relative to one another.
Muscles attach to the inner surface of the exoskeleton.
When muscles contract, they move different sections of the exoskeleton around joints.
This allows movements such as:
- Walking.
- Running.
- Jumping.
- Swimming.
- Flying.
- Grasping.
A Major Problem: Growth
A rigid exoskeleton cannot simply expand continuously as an animal grows.
Arthropods therefore periodically shed their old exoskeleton.
This process is called molting, or ecdysis.
During a molt:
- A new exoskeleton develops beneath the old one.
- The old exoskeleton splits.
- The animal emerges.
- The new exoskeleton expands.
- The new covering eventually hardens.
The Risks of Molting
Molting allows growth, but it creates a period of vulnerability.
Immediately after leaving the old exoskeleton, the new exoskeleton may be relatively soft.
During this period, the animal may be:
- More vulnerable to predators.
- Less protected from physical damage.
- Less able to move normally.
- More vulnerable to water loss.
This is an important trade-off associated with having a rigid external skeleton.
Endoskeletons
An endoskeleton is an internal supporting framework.
Vertebrates have endoskeletons made primarily from:
- Bone.
- Cartilage.
Examples include:
- Fish.
- Amphibians.
- Reptiles.
- Birds.
- Mammals.
Some invertebrates, such as echinoderms, also have internal skeletal structures.
Functions of the Vertebrate Endoskeleton
The vertebrate skeleton performs several functions.
It:
- Supports the body.
- Maintains body shape.
- Protects organs.
- Provides attachment points for muscles.
- Allows movement around joints.
- Stores minerals.
- Contains bone marrow involved in blood-cell production.
The skeleton is therefore an active biological system, not simply a rigid framework.
Protection
Different parts of the vertebrate skeleton protect important organs.
For example:
Skull → protects brain
Vertebral column → surrounds and protects spinal cord
Rib cage → protects heart and lungs
Pelvis → helps protect organs in the lower abdomen and pelvic region
The shape of individual bones often reflects both their mechanical and protective functions.
Endoskeletons and Movement
Skeletal muscles are attached to bones by tendons.
When muscles contract, they pull on bones.
Bones then move around joints.
A simplified system is:
Muscle → tendon → bone → joint → movement
Bones can function as levers, while joints act as pivots.
This allows relatively small changes in muscle length to produce useful movements of limbs.
Antagonistic Muscles and the Skeleton
Because muscles pull rather than push, skeletal movement often requires antagonistic muscle pairs.
At the elbow:
Biceps contracts → forearm flexes
Triceps contracts → forearm extends
The endoskeleton therefore provides the rigid structures against which muscles can generate controlled forces.
Endoskeletons Grow with the Animal
Unlike an arthropod exoskeleton, a vertebrate endoskeleton can grow as the animal grows.
Bones are living tissues.
During growth, bone can:
- Increase in length.
- Increase in thickness.
- Change shape.
- Repair damage.
- Remodel in response to mechanical forces.
This means vertebrates do not need to shed their entire skeleton in order to grow.
Comparing the Three Support Systems
| Feature | Hydrostatic Skeleton | Exoskeleton | Endoskeleton |
|---|---|---|---|
| Main support | Pressurised fluid | Rigid external covering | Internal framework |
| Example | Earthworm | Insect | Human |
| Flexibility | High | Limited by rigid sections and joints | Moderate to high at joints |
| Protection | Usually limited | Strong external protection | Strong protection of selected organs |
| Muscle interaction | Muscles act against fluid | Muscles attach internally to exoskeleton | Muscles attach to bones |
| Growth | Can expand with body | Requires molting in arthropods | Grows with body |
| Water conservation | Usually limited | Can strongly reduce water loss | Skin provides main external barrier |
| Large body support | Usually limited on land | Size can be mechanically constrained | Can support large terrestrial animals |
No system is universally superior. Each represents a different solution to the problems of support, protection, and movement.
Support and Locomotion
Locomotion means movement of an animal from one place to another.
A support system makes locomotion more effective because it provides something against which muscles can generate forces.
The general principle is:
Muscle contraction + support system + interaction with environment → locomotion
For example:
- Earthworm muscles act against fluid.
- Insect muscles act against an exoskeleton.
- Human muscles pull on an endoskeleton.
Different structures, but the same fundamental requirement: muscular force must be translated into useful movement.
Support Systems and the Environment
The effectiveness of a support system depends partly on the environment.
Water provides buoyancy, which helps support an animal's weight.
This means aquatic animals may require less structural support against gravity than similarly sized terrestrial animals.
On land, gravity places greater demands on structures such as:
- Limbs.
- Vertebral columns.
- Joints.
- Muscles.
This helps explain why support structures differ between aquatic and terrestrial animals.
Adaptations for Running
Animals specialised for running often have skeletal adaptations that improve speed or efficiency.
These may include:
- Long limbs.
- Reduced mass toward the ends of limbs.
- Flexible spinal columns.
- Specialised joints.
- Elongated lower limb bones.
In a cheetah, for example, a flexible spine contributes to increased stride length during high-speed running.
Adaptations for Flying
Flight places very different demands on the skeleton.
Bird skeletons have several adaptations related to flight.
These can include:
- Lightweight bones.
- Fusion of some bones for strength.
- Modified forelimbs forming wings.
- Large surfaces for attachment of flight muscles.
- A streamlined body form.
Many flying birds possess a large keel on the sternum.
Powerful flight muscles attach to this structure.
The skeleton therefore provides both lightweight support and strong muscle attachment.
Adaptations for Swimming
Aquatic vertebrates also show specialised skeletal adaptations.
Fish typically have:
- Flexible vertebral columns.
- Fins supported by skeletal structures.
- Streamlined bodies.
- Muscles arranged to produce side-to-side movement.
The skeleton and muscles work together to push water backward.
The resulting forces move the fish forward.
Adaptations for Burrowing
Burrowing animals face yet another challenge.
They must move through soil or sediment.
Useful adaptations may include:
- Flexible bodies.
- Powerful digging limbs.
- Strong claws.
- Compact body shapes.
- Hydrostatic support in some worms.
An earthworm's hydrostatic skeleton is especially effective because the body can lengthen, shorten, and change shape while moving through narrow spaces.
Adaptations for Jumping
Animals specialised for jumping may have:
- Long hind limbs.
- Powerful muscles.
- Strong joints.
- Elastic structures that store and release energy.
Frogs, for example, possess elongated hind limbs that help generate large forces against the ground.
This demonstrates how skeletal proportions can be closely related to locomotion.
Protection Versus Mobility
Support systems often involve trade-offs.
A very thick, rigid skeleton might provide excellent protection but could:
- Increase body mass.
- Reduce flexibility.
- Require more energy to move.
A highly flexible support system might improve manoeuvrability but provide less physical protection.
Evolution therefore often produces compromises between:
Protection + support + flexibility + mass + energy cost
Different animals show different combinations depending on their environments and lifestyles.
Worked Example: Earthworm
Consider an earthworm moving through soil.
It has no rigid bones.
Circular muscles contract, causing sections of the body to lengthen.
Longitudinal muscles contract, causing sections to shorten.
Fluid inside the body resists compression.
Setae grip the soil.
The combination produces forward movement.
Therefore:
Muscles + fluid pressure + grip → locomotion
This is a hydrostatic skeleton in action.
Worked Example: Grasshopper
A grasshopper has an exoskeleton.
Its muscles attach to the inner surfaces of this external skeleton.
When leg muscles contract:
- Forces are transferred to rigid exoskeletal sections.
- Sections rotate around joints.
- The hind legs push strongly against the ground.
The ground exerts a force on the grasshopper, accelerating it upward and forward.
The exoskeleton therefore provides both support and a mechanical system for movement.
Worked Example: Human Arm
A human arm uses an endoskeleton.
When the biceps contracts:
- The muscle produces tension.
- The tendon transfers the force to the forearm.
- The forearm bones act as levers.
- The elbow acts as a pivot.
- The forearm moves.
Therefore:
Muscle contraction → tendon force → bone movement → movement at joint
The skeleton does not produce the force itself. The muscles provide the force, while the skeleton provides the mechanical framework.
Worked Example: Moving from Water to Land
Imagine an animal lineage moving from a mainly aquatic environment to a terrestrial environment.
In water, buoyancy supports much of the animal's weight.
On land, this support disappears.
Gravity places greater demands on:
- Limbs.
- Joints.
- Vertebral column.
- Muscles.
A successful terrestrial animal therefore requires structures capable of supporting its body against gravity while still permitting movement.
This illustrates how environment can influence the evolution of support systems.
Body Size and Support
As an animal becomes larger, its mass increases rapidly.
Its support system must withstand increasing forces caused by gravity and movement.
Large terrestrial vertebrates often have:
- Thick limb bones.
- Strong joints.
- Large muscle attachment areas.
- Limb positions that efficiently support body mass.
An elephant, for example, has thick, relatively column-like limbs that support its large mass.
A tiny insect can use a very different structural arrangement.
Skeletons Can Have Additional Functions
Support systems can perform functions beyond support and locomotion.
For example, vertebrate bones can:
- Store calcium and phosphate.
- Contain bone marrow.
- Produce blood cells.
- Protect organs.
- Store energy in yellow bone marrow.
An exoskeleton can also:
- Reduce water loss.
- Provide camouflage.
- Carry sensory structures.
- Provide defensive structures such as spines.
A skeleton is therefore often a multifunctional system.
Support Systems and Evolution
Hydrostatic skeletons, exoskeletons, and endoskeletons represent different evolutionary solutions to similar problems.
All must allow an animal to:
- Maintain body shape.
- Resist external forces.
- Produce controlled movement.
- Protect important structures.
- Function effectively in its environment.
The details differ because animals have different:
- Body sizes.
- Habitats.
- Feeding strategies.
- Predators.
- Methods of locomotion.
- Evolutionary histories.
Common Mistakes
Thinking Every Skeleton Is Made of Bone
Hydrostatic skeletons use fluid, and arthropod exoskeletons are largely composed of materials including chitin.
Thinking an Exoskeleton Is Just Protective Armour
An exoskeleton also provides support and attachment surfaces for muscles and plays an important role in movement.
Saying Exoskeleton Muscles Are Outside the Skeleton
In arthropods, muscles are located inside the exoskeleton and attach to its internal surfaces.
Saying Hydrostatic Skeletons Have No Structural Support
The pressurised fluid provides the resistance against which muscles act.
Saying Endoskeletons Do Not Protect the Body
Although they are internal, structures such as the skull, vertebral column, and rib cage provide substantial protection.
Thinking Skeletons Produce Movement by Themselves
Muscles produce force. Skeletons provide structures against which those forces act.
Saying Exoskeletons Cannot Move
Exoskeletons contain joints and flexible regions that allow movement.
Thinking an Exoskeleton Grows Continuously
In arthropods, the rigid exoskeleton must periodically be shed during molting.
Assuming One Skeleton Type Is Always Better
Each system has advantages and limitations. Its effectiveness depends on the animal's body form, size, environment, and lifestyle.
Check Your Understanding
1. Give four important functions of an animal skeleton.
2. Define a hydrostatic skeleton.
3. Explain how an earthworm uses its hydrostatic skeleton to move.
4. Give two advantages and two limitations of hydrostatic skeletons.
5. Define an exoskeleton and name two groups of animals that possess one.
6. Explain how muscles produce movement in an animal with an exoskeleton.
7. Why must arthropods molt?
8. Explain one disadvantage of molting.
9. Define an endoskeleton.
10. Explain how an endoskeleton works with muscles to produce movement.
11. Compare the growth of an exoskeleton with the growth of a vertebrate endoskeleton.
12. Explain why an exoskeleton can be useful for a small terrestrial animal.
13. Describe two skeletal adaptations for flight, running, swimming, or jumping.
14. Explain why large terrestrial animals require particularly strong support structures.
15. An unknown animal has a soft flexible body, no rigid skeleton, and muscles surrounding a fluid-filled cavity. Identify its type of support system and explain how it could use that system for locomotion.
Key Terms
- Skeleton – structural system that supports an animal and helps muscles produce controlled movement.
- Support system – structures or mechanisms that maintain body shape and resist forces.
- Hydrostatic skeleton – support system in which muscles act against pressurised, relatively incompressible fluid.
- Exoskeleton – rigid supporting structure located outside the body.
- Endoskeleton – internal supporting framework.
- Chitin – structural material forming an important part of arthropod exoskeletons.
- Molting – shedding of an old exoskeleton to allow growth.
- Ecdysis – another term for molting.
- Bone – strong living connective tissue forming much of the vertebrate skeleton.
- Cartilage – flexible supportive connective tissue.
- Joint – location where skeletal structures meet and, in many cases, permit movement.
- Tendon – connective tissue attaching muscle to skeletal structures.
- Lever – rigid structure that rotates around a pivot when a force is applied.
- Locomotion – movement of an organism from one location to another.
- Buoyancy – upward force exerted by a fluid that helps support objects immersed in it.
- Setae – small bristles that help organisms such as earthworms grip surfaces.
- Antagonistic muscles – muscles that produce opposing movements.
- Keel – projection of the sternum in many birds that provides attachment for major flight muscles.
Key Takeaways
- Animals require support systems to maintain shape, protect organs, and produce effective movement.
- The three major support systems are hydrostatic skeletons, exoskeletons, and endoskeletons.
- Hydrostatic skeletons use pressurised fluid as a supporting structure.
- Earthworms use circular and longitudinal muscles acting against fluid to produce locomotion.
- Hydrostatic skeletons provide excellent flexibility but relatively little rigid protection.
- Exoskeletons form rigid structures around the outside of the body.
- Arthropod exoskeletons provide support, protection, muscle attachment, and reduced water loss.
- Joints allow animals with exoskeletons to move despite their rigid outer covering.
- Arthropods must periodically molt because their rigid exoskeleton cannot continuously expand.
- Endoskeletons are internal support structures that can grow with the animal.
- Vertebrate endoskeletons consist largely of bone and cartilage.
- Bones protect important organs and provide attachment points for muscles.
- Muscles produce force while skeletons provide the structures against which those forces act.
- Bones and rigid exoskeletal sections can function as levers during movement.
- Aquatic animals benefit from buoyancy, while terrestrial animals require greater structural support against gravity.
- Skeletal adaptations can support specialised forms of locomotion such as running, flying, swimming, jumping, and burrowing.
- Support systems involve trade-offs between strength, protection, flexibility, mass, and energy cost.
- The structure of an animal's support system is closely related to its size, environment, lifestyle, and method of locomotion.