Echinoderms
Echinoderms are a group of animals that include starfish, sea urchins and sea lilies (or crinoids). They don't have muscles, but move instead using pressurised water that they pump through special tubes to expand and contract different parts of their body.
Although they are strange creatures in lots of ways, they are actually more closely related to humans than most other invertebrates.
A fossil starfish
A fossil sea urchin
Echinoderms live in many different ways, but most crawl around on the sea-floor looking for food. Sea lilies, however, are stationary, and are attached to the sea floor with a long stem. At the end of the stem is their body, from which lots of arms wave around to catch food that floats past.
Fossil echinoderms are quite easy to spot, as they are made of chunky plates of a mineral called calcium carbonate. Most also have an obvious five-sided symmetry - for example most starfish have five arms.
A fossil sea lily
A fossil brittle star
Echinoderms first appeared around 540 million years ago. They are found as fossils in rocks of all ages, and are still common in oceans across the world. Whole echinoderm fossils are quite rare, but are amongst the most beautiful of fossils.
Echinoderms, like these sea lilies on the right, can be beautiful fossils
