The three support systems differ in where the support is and how the animal grows. An endoskeleton is inside and grows with the body, an exoskeleton is outside and must be shed, and a hydrostatic skeleton uses fluid.
This lesson is part of support, movement and growth. The next lesson, explaining muscle and joint roles, shows how a skeleton lets muscles move the body.
How do the three systems compare?
Compare them on the same features, row by row. A comparison needs both items in every sentence.
| Feature | Endoskeleton | Exoskeleton | Hydrostatic skeleton |
|---|---|---|---|
| Position | Inside the body | Outside the body | Fluid inside the body |
| Growth | Grows with the animal | Shed in a moult | Body stretches with growth |
| Protection | Protects some organs | Protects the whole body | Little protection |
| Example | Human, fish | Crab, grasshopper | Earthworm |
Worked example: a growing crab
The data is original. The width of a crab’s shell was measured each month.
| Month | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Shell width (cm) | 2.0 | 2.0 | 2.6 | 2.6 | 2.6 | 3.3 |
Reading. The width stays at 2.0 cm for two months, jumps to 2.6 cm in month 3, stays flat, then jumps to 3.3 cm in month 6. The increases are 0.6 cm and 0.7 cm.
Explanation. The hard exoskeleton cannot stretch, so the width only changes when the crab moults and a larger shell hardens. That is why growth shows in steps, not as a smooth line.
Comparison. A human grows steadily because the bones inside grow along with the body.
The mistake that costs marks
The common slip is to say that an exoskeleton grows with the animal. The shell is hard and does not grow the way a bone does, so the animal has to shed it.
| Step | Wrong | Right |
|---|---|---|
| Exoskeleton and growth | Grows with the animal | Does not stretch, so it is shed |
| Growth pattern | Smooth | In steps after each moult |
| Comparison | Lists each skeleton on its own | Names both in one sentence |
| Example | Earthworm has bones | Earthworm has a hydrostatic skeleton |
The fix is to write “but” or “whereas” in each comparison sentence and to name both items.
Check yourself
A lizard, a prawn and an earthworm are each given a support system. Name the type of support system in each animal and give one advantage of the prawn’s.
Answer
The lizard has an endoskeleton, the prawn has an exoskeleton, and the earthworm has a hydrostatic skeleton.
An advantage of the prawn’s exoskeleton is that the hard outer covering protects the whole body. A limit is that the prawn must moult to grow, and is soft and exposed for a short time afterwards.
What to study next
Continue with explaining muscle and joint roles, then test the whole topic with the support, movement and growth practice set.
If you want a teacher to check your comparison sentences with you, see online one-to-one Science tuition.