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. The spines also have suckers that enable them to cling to rocks. The mouth is a hole in the underside, with five large teeth. Sea cucumbers have no shell, but they have a leathery skin and a covering of chalky plates called spicules.