Animals without bones
Imagine that you were an alien the size of a currant. You are flying through space in a spaceship about the size of a shoebox. You see the Earth and decide to land on it. After skimming the surface of the seas you decide to put down your spacecraft on land. As most of the land appears to be covered by grass, you seek out the corner of a field where the grass is highest and make your landing.

When you climb out of your spacecraft the leaves tower above your head and grass stalks lying on the soil look like fallen logs. You scramble over the grass stalks and find some soil. It is much easier to walk on this but you have not travelled far before you see the soil in front of you move. In a moment the slimy head of a worm pokes out of the soil and the front part of its body slithers past you. It is so large compared to you that you can only just see over its back. The body is divided into sections, which swell and shrink like the bellows of a concertina. When a segment shrinks, bristles, which to you appear the size of broomsticks, push into the ground and help the worm grip the soil. In a moment the worm's jaws have gripped a leaf and the whole body quickly slithers back into its burrow.

A little further on you meet another creature with a slimy body. This has a higher back that you cannot see over but it does not have any segments. At the front of the animal are two large stalks. One turns towards you. As the end of the stalk is an eye. You cannot be sure if the animal has seen you so you stay still. You can see some rippling movements where the animal touches the ground. In a moment, slime oozes from below the animal and the rippling movements propel it slowly forwards. When it has left, you carefully avoid the slime and move towards a stone. There is a gap under the stone and you can see something moving in there in the dark. The animal comes out. It is taller than you are and three times as long. Its back is covered in a row of dark grey plates. They bend towards the ground and underneath them you can just see fourteen pairs of armour-plated legs. At the head end are two segmented stalks. The tips of the stalks bend down and the animal uses them to touch and feel the ground. The animal is a woodlouse. It does not stay out from its rock for long and soon disappears back into the dark.

You decide you have seen enough of life in the grass and turn towards your spacecraft. As you make your way back you hear a loud humming sound. It is coming from the sky. You look up and see six legs dangling above you. They are on the body of an animal with a pair of rapidly beating wings. The animal drops to the ground between you and your spacecraft. It has two huge eyes. Each one looks like the top of a microphone so it is difficult to see if it is looking at you. The front of the animal's face touches the ground and liquid flows from it. Moments later the animal sucks the liquid back into its head. There is no expression in the animal's face. It beats its wings and the push on the air nearly knocks you over. The animal is a fly. It rises into the air and disappears above the grass leaves.

You return to your spacecraft and send a message home about the slimy, armour-plated and winged animals you have seen. And none of them had a skeleton of bone.

What are animals called that do not have a skeleton of bone?
They are called invertebrates. This is a huge group of animals and is divided up into smaller groups. The main groups of invertebrates are the jellyfish, worms, slugs and snail group, insects, spiders, crab and lobster group, centipedes and millipedes and the starfish group.

Do jellyfish have a skeleton made of jelly?
No. They have a skeleton made of water. They take seawater into their body and use it to support themselves. There are other animals in the jellyfish group besides jellyfish. Sea anemones that you find in rock pools on the seashore are jellyfish and so are animals that make coral. The animals that make coral are like small sea anemones but they join together and live in a group. They can take calcium out of seawater and use it to make a stony protective case around themselves. This is the material that we normally call coral.

What animals are in the worm group?
The earthworm is the most familiar member of the worm group. The leech looks similar to an earthworm but it has a sucker on each end. It uses the sucker on its head to stick onto its prey while it feeds on its blood. It uses the other sucker to hold onto plants or stones. On the beach you may see wormcasts made by lugworms. At low tide you may find tubes of sand and shells made by fanworms. Worms have skeletons made of water.

Are there other animals in the same group as slugs and snails?
Yes, there are. The animal group to which slugs and snails belong is called the Molluscs. This word means soft bodied but many molluscs have shells. Limpets, whelks, periwinkles, snails and cowries have just one shell. Clams such as cockles, mussels, oysters and scallops have two. The octopus and squid have a plate made of horn inside their bodies but like all molluscs they use a water skeleton to help support their muscles. The scallop also uses its shells too. It has muscles, which make its shells clap together. When they do this, they push water out through two pipes made by their shells. The jets of water enable the scallop to rise and swim through the water.

Aren't insects and spiders similar?
Insects and spiders have one big difference. Insects have six legs and spiders have eight. They are similar in that they both have a suit of armour that serves as their skeleton. The muscles that move the skeleton are on the inside. The crab and lobster group, also known as the Crustacea, have a similar suit of armour and so do centipedes and millipedes.

Where is the skeleton on a starfish?
It is on the outside of the body and it is also inside. The hard plates of shell-like substance on the outside of the body is a skeleton. On the underside of the arms you can see rows of tube feet. These contain water, which supports the foot muscles. The water in the tube feet forms a water skeleton.

To which animal group does a woodlouse belong?
It belongs to the same group as the crab and lobster: the Crustacea.

How do flies feed?
They have an extension to their head, which has a sponge on it. When a fly lands on food, such as material in the soil, it produces a juice. It lets this juice flow onto its food. The juice digests the food then the fly sucks it up. The hard part of the skeleton in the fly's head gives support to the muscles, which move the juice and its digested food.

Do the tentacles on a slug's head really have eyes?
The two longest ones do. The two smaller tentacles are used for tasting the air. A snail also has an eye on the end of each long tentacle. The tentacles can wave about because the water inside them acts like a skeleton to support the muscles.