Biological ScienceLife ProcessesYear 4 Science

Skeletons, shells and stems: how living things are held up

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What this lesson covers

Vertebrates have a backbone; invertebrates do not
Most animals are small invertebrates, and water holds up some sea animals
Skeletons and muscles work together to protect organs and to move
The human skeleton: skull, ribcage, spine, pelvis, and arms and legs built on one pattern
Exoskeletons, shells, and soft bodies with no hard skeleton
How trunks and stems hold plants up

Key idea

Backbone or no backbone

A vertebrate is an animal with a backbone, also called a spine. The backbone is a chain of small bones called vertebrae.
Fish, frogs, reptiles, birds and mammals are vertebrates.
An invertebrate is an animal with no backbone. Insects, spiders, worms, snails and jellyfish are invertebrates.
Most kinds of animals on Earth are invertebrates, and most of them are small.

Key idea

Try it: backbone or not?

Key idea

Held up by water

Some sea invertebrates have soft bodies that the water holds up.
A jellyfish floats open and wide in the sea.
Washed up on the beach, with no water around it, it slumps into a flat blob.

Key idea

Skeletons and muscles work together

A vertebrate's skeleton is inside its body. It is an internal skeleton.
It holds the body up, protects soft organs, and gives muscles something to pull on.
Muscles are joined to bones. A muscle can only pull, by getting shorter.
So muscles work in pairs. To bend your arm, the front muscle pulls while the back one relaxes. To straighten it, they swap.

An arm bent at the elbow: the upper arm bone, the elbow joint and two forearm bones, with the front muscle pulling to lift the forearm while the back muscle relaxes

Key idea

Your skeleton

Your skull protects your brain.
Your ribcage protects your heart and lungs.
Your spine holds you upright and protects the spinal cord, the main nerve running down your back.
Your pelvis holds up your body and joins your legs to it.

Worked example

Example: arms and legs share a pattern

Compare the bones of your arm and your leg, from the top down. What pattern do they share?

Part	Arm	Leg
Top	Upper arm: 1 bone	Thigh: 1 bone
Middle	Forearm: 2 bones	Lower leg: 2 bones
Joint	Wrist: small bones	Ankle: small bones
End	Hand: 5 fingers	Foot: 5 toes

An arm and a leg side by side, each with one long bone, then two bones, then a group of small bones, then five fingers or toes

Arm: 1 bone, then 2 bones, then small bones, then 5 fingers
Leg: 1 bone, then 2 bones, then small bones, then 5 toes
Both follow the same pattern: 1 bone, 2 bones, small bones, then 5 digits

Key idea

Skeletons on the outside, or none

Some invertebrates have a hard skeleton on the outside, called an exoskeleton. Wētā, beetles and crabs have one.
An exoskeleton protects the animal like armour, and its muscles pull on it from the inside.
An exoskeleton cannot grow. A growing wētā sheds its old one and grows a bigger one. This is called moulting.
Snails and pāua grow a shell that gets bigger as they do.
Slugs, worms and jellyfish have no hard skeleton at all.

Worked example

Example: sort by skeleton

Sort these animals by whether they have a backbone, and what kind of support they have.

Animal	Backbone?	Support
Kererū	Yes	Internal skeleton
Tuna (eel)	Yes	Internal skeleton
Wētā	No	Exoskeleton
Pāua	No	Shell
Slug	No	No hard skeleton
Jellyfish	No	Held up by water

Backbone means vertebrate: kererū and tuna
No backbone means invertebrate: wētā, pāua, slug, jellyfish
Vertebrates have internal skeletons; invertebrates have exoskeletons, shells, or no hard skeleton

Key idea

Try it: what holds it up?

Key idea

How plants stand up

Plants have no bones, but they still need support.
Woody plants have stiff wood in their trunks and branches, so they can grow very tall, like kauri and tōtara.
Non-woody plants have soft, green stems. They stay small or lean on other plants.
Water inside a soft stem pushes outwards and keeps it firm. Without enough water, the stem droops.

Common mistakes

Insects and spiders have backbones.
Insects and spiders are invertebrates with no backbone; their support is an exoskeleton on the outside.
Muscles push bones to move them.
Muscles can only pull, so they work in pairs: one pulls to bend a joint and the other pulls to straighten it.
A wētā's skeleton grows with it, like ours.
An exoskeleton cannot grow, so a wētā moults: it sheds the old one and grows a bigger one.

Try it at home

The floppy stem test

You need: 2 sticks of celery with leaves, 2 glasses, water, a ruler

Put one celery stick in a glass of water. Put the other in an empty glass.
Leave them both for one day.
Hold each stick flat by its bottom end, over the table. Measure how far the leafy end droops down.
Now put the floppy stick in water too, and test it again after a few hours.
Look for this: the stick with water stays firm and straight, and the dry one droops. Water inside a soft stem is what holds it up.

Safety: Ask an adult before taking celery from the kitchen, and put the glasses where they will not get knocked over.

Your turn

A tuatara has a backbone. Is it a vertebrate or an invertebrate?

Which part of your skeleton protects your brain?

How many bones are in your forearm?

What kind of skeleton does a crab have?

Why does a jellyfish slump on the beach?

Why can a kauri grow much taller than a daisy?

Show answer

1. A vertebrate. 2. The skull. 3. 2 bones. 4. An exoskeleton. 5. There is no water to hold its soft body up. 6. Its trunk is made of stiff wood, which can hold up a tall plant.

Summary

Recap

Vertebrates have a backbone; invertebrates do not, and most animals are invertebrates.
Internal skeletons hold up, protect and, with muscles, move the body.
Muscles only pull, so they work in pairs.
Arms and legs share one pattern: 1 bone, 2 bones, small bones, 5 digits.
Some invertebrates have exoskeletons or shells; some have no hard skeleton.
Wood holds up tall plants; water keeps soft stems firm.

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