Skeletons provide support and protection and can usually be moved by the muscles which are attached to it. This is investigated further in Casing the Joint. Most living organisms have some sort of a skeleton, but there are a great many different types.

Launch it ... Try to Build A Skeleton and see if you can find some similarities between the different skeleton types.

Generally, there are three types of skeleton:

Hydrostatic skeletons

Have you ever heard of a water skeleton? Well, now you have! Water is a very dense substance and it cannot be compressed, so it can provide a lot of support and therefore act as a type of skeleton.

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Discuss it ... Why do you think it is important not to get dehydrated?

An earthworm has body segments full of fluid that muscles act upon in order to make it move.

Discuss it ... Why do you think an earthworm has a hydrostatic skeleton?

Exoskeletons

This is a hard outer skeleton, such as the shell of a mollusc (a soft-bodied animal like a snail or a mussel). A lobster has an exoskeleton which it sheds as it grows, a bit like moulting.

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Discuss it ... Just how big could a lobster grow?

Insects, such as fleas, have exoskeletons that are made of a very tough material with a layer of wax on the outside to make it waterproof. The wings of an insect and all of its mouth parts are also made of the same material, making it possible for insects to live in very different environments and eat very different things.

If you think about them, insects are pretty agile really… take human fleas, for instance; they can pull 400 times their own body weight, and can jump up to 150 times the length of their bodies, about 7 inches high and 14 inches long. And they don’t even have wings. An equivalent jump for a human would be 250 foot high and 450 foot long! Pretty impressive…

Launch it ... See for yourself just how agile the flea can be: try to Flick the Flea!
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Endoskeletons

Animals that have endoskeletons are usually vertebrates, in other words they have a backbone. Humans, horses, giraffes...these all have endoskeletons. The skeleton is internal (which is what the endo- part means) and is made up of cartilage and bone and held together and supported by connective tissue.

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The human skeleton is an example of an endoskeleton, with its complex array of joints and bones, including a total of 33 bones in the spine alone. A whale has an endoskeleton: the blue whale is the largest living animal on earth, with the adult skeleton alone weighing 3 tonnes!

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But what is it that makes one species have one type of skeleton and another species have another? The answer might be in where the species lives, its habitat. Can you think why a crab would have an exoskeleton and not an endoskeleton? And why a whale cannot live out of water?

Discuss it ... How do you think different types of skeletons relate to habitat?
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Teachers' Notes ... For Teachers' Notes on this subject, click here