What is a body covering?
Every animal has an outer cover. It is the first thing you see when you look at an animal. In most animals this cover is the skin. In some animals it is a hard shell or a hard outer skin made by the body.
The cover has four big jobs. It protects the inside of the body. It stops water loss. It helps to control body heat. It senses touch, heat and pain.
Layers of the skin
Skin in a mammal has three parts. Each part does a different job.
- Top layer (epidermis): thin and tough. It is made of dead flat cells that rub off. New cells keep coming from below. It carries the colour of the skin.
- Middle layer (dermis): thick. It has blood vessels, nerves, sweat glands and the roots of hair. Nerves here feel touch and pain.
- Fat layer: stores food energy and acts like a soft cushion that keeps heat in.
Fish, frogs and snakes have skin too, but the parts are a little different. A frog has thin wet skin that also helps it breathe.
Skin as defence
The top layer is like a wall. Germs cannot easily go through unbroken skin. Skin also makes oil and sweat that kill some germs. If the wall breaks (a cut), blood clots and closes the gap.
Skin defends in other ways too. Thick skin protects a rhino. Colour matching (camouflage) hides a chameleon. A porcupine's quills are changed hair that keep enemies away. A cobra's bright skin warns others.
Temperature control: a layer of hair or feathers traps air. Trapped air is a poor carrier of heat, so heat stays inside. Sweating cools the body when it is hot.
Skin derivatives: scales, feathers, hair, nails
A derivative is a part that grows out of the skin. Different animal groups have different ones.
- Scales: small flat plates. Fish have thin bony scales. Snakes and lizards have dry horny scales that stop water loss.
- Feathers: only birds have feathers. Fluffy feathers keep birds warm and large flight feathers help them fly.
- Hair or fur: only mammals have hair. It keeps the body warm and feels things (whiskers).
- Nails, claws and hooves: hard parts made of the same stuff as hair. They help to dig, climb, hold and run.
- Horns: hard covers over bone, as in cows and goats.
Coverings of other animals
Insects have a hard outer skin called the cuticle. It cannot grow, so the insect sheds it (moulting) as it gets bigger. Snails and clams have a hard shell made by the body. A starfish has a spiny skin. A jellyfish has a very thin soft skin.
Try it
In the 3D: press Hair, Scales, Feathers and Claws. Say which animal group each belongs to. Then press Drop germs.
At home: put one hand in a woollen glove and one hand bare in cold air for two minutes. Which hand feels warmer? The glove traps air, just like hair does. Guess first, then check.
Key formulas and definitions
- Skin layers: top layer (epidermis) + middle layer (dermis) + fat layer
- Skin jobs: protect, stop water loss, control heat, sense touch
- Fish: scales. Birds: feathers. Mammals: hair. Reptiles: dry scales. Insects: cuticle. Snails: shell.
- Trapped air in hair or feathers = warmth
Worked examples
1. A bird puffs up its feathers on a cold day. Why does this help?
Puffed feathers trap more air between them. Air carries heat badly, so less body heat escapes and the bird stays warm.
2. Name the body covering of: (a) rohu fish, (b) crow, (c) goat, (d) cockroach.
(a) scales, (b) feathers, (c) hair (and horns and hooves), (d) hard cuticle.
3. A cut on your arm stops bleeding after a few minutes. Which job of skin has broken and then been repaired?
The defence job. A cut breaks the skin wall. Blood clots to close it and new skin cells grow to repair it.
Common mistakes
- Thinking scales, feathers and hair are the skin itself. They are derivatives that grow out of the skin.
- Saying only mammals have skin. All animals with a backbone have skin; insects have a cuticle instead.
- Saying fur keeps animals warm because it makes heat. It keeps warm by trapping air, not by making heat.
- Mixing up the layers: the fat layer is the lowest layer, the thin tough layer is on top.