The polyps build crusty limestone cups. These attach to other polyps to make a protective shield. When the small animal dies, the cup is left. Slowly, millions of these cups build up into rock formations that make the reef. One way to describe coral reefs is 'flower gardens under the sea'.
Polyps provide the 'bricks' of the city which house thousands of other animals and plants.
Sponges Sponges are simple animals that grow in different colours and shapes. Some grow and coat rocks in bright crimson or orange. A sponge is a simple animal and if you break it into pieces, each bit will grow into a new one. Seastars come in many shapes and they have no front or back, head or tail. They bodies grow in a circular pattern.