After arthropods, such as insects, molluscs are the largest of all groups in the animal kingdom. Most tend to have hard shells and soft bodies, and most live in water, especially the sea. The cephalopods are the most complex of all animals without backbones. They are also the biggest. Giant squids can have bodies up to 6 metres long, with tentacles adding another 10m.
Common cuttlefish have no shell but an internal skeleton of chalky material.
When threatened, they eject a dark ink to hide them from predators, as do all cephalopods. Sea urchins and sea cucumbers Are like starfish, echinoderms. Sea urchins live near the shore, and fragments of their shells are often found on beaches. When alive, they bristle with spines. These protect them and help them move, rather like stilts.