Echinoderms (Echinodermata) are a type of animal living in the sea. They are a major phylum - this means that they are one of the main categories into which animal life is divided. The phylum includes starfish and sea urchins, as well as some animals which are not familiar to most of us because they only live at great depths in the ocean.

The word 'echinoderm' means literally 'spiny skin' and this is one of the characteristic features of this phylum, although they don't all have spiny skin. Their main characteristics are:

1. They live in the seas and oceans of the world, on the sea bottom. Adult echinoderms don't swim, but many of them crawl around on the sea floor. Others attach themselves to the rocks and wait for food to come to them, grabbing it as it floats past. Some species have a floating larval stage in their life-cycle, when they join the plankton (the floating mass of tiny creatures of all sorts) and are carried around the world by currents.

2. They are covered in spines - not all of them are, but enough of them are to give the phylum its name.

3. They are hard on the outside - they may be tough and leathery or may be actually solid like a shell.

4. The spines, if they have any, are controlled by a hydraulic system which pumps water in and out of them to make them bend and straighten. They also have modified spines on the underside, known as tube feet, controlled by the same hydraulic system, which they use to walk around.

5. They are radially symmetric, usually in five parts like a star.

6. They have no eyes, brains or hearts.

Since echinoderms mainly live in the deep ocean, most of them are not familiar to us - the only ones you are likely to find around the shore are starfish and sea urchins.

The Main Types

The phylum of echinoderms is divided into six types or 'classes':

• Crinoidea - sea lilies and feather stars; these are attached to the bottom and look like flowers.

• Asteroidea - starfish¹; these are probably the best known of the echinoderms. They have five long legs and live by walking around eating any small creatures they find.

• Ophiuroidea - basket stars, serpent stars and brittle stars; these have a central body with five thin legs.

• Holothuroidea - sea cucumbers; they can grow very large and are the best at coping with extreme depths - they have been found at depths of up to 10,000m.

• Echinoidea - sea urchins; globular shells covered in spines. They have a hole in the middle at the bottom which acts as both mouth and anus.

• Concentricycloidea - sea daisies; a little-known deep sea creature like a small starfish with no arms. Only discovered in 1986, it's not clear whether these are a separate class or a type of starfish.

As echinoderms can live at very great depths, they have colonised the continental slope, which is the sloping bit of the ocean between the continental shelf and the abyssal plains. There is a huge amount of continental slope beneath the Earth's oceans, so there are a huge number of echinoderms crawling around down there, quite possibly more than any other type of animal on Earth.

There are about 6,000 species in existence, but another 13,000 species have existed in the past, dating back as far as 560 million years ago; these creatures fossilise well due to the hard bits, so we have a very good record of previous species.

Reproduction

Some species reproduce by having males and females which get together in the usual way to produce babies. Others reproduce asexually by splitting themselves in two and each half growing new bits to replace the missing ones.

Sexual reproduction is a primitive process which does not involve any sort of courtship. The males and females congregate in a crowd and then release their eggs and sperm into the water. They then proceed with the normal business of life - eating.