Arthropods Animals, such as insects and crustaceans, that have jointed legs and an external skeleton. This exoskeleton is made of a light, strong material called chitin, which supports and protects the animal’s body. As an arthropod grows, it moults its exoskeleton and grows a new one.

Bivalves Soft-bodied molluscs with two shells joined by an elastic hinge. Bivalves include scallops, oysters and mussels.

Brachiopods Two-shelled marine invertebrates. Although they resemble bivalves, there is an important difference: the plane of symmetry in bivalves runs down the hinge between the shells (there is a left and right shell), whereas the plane of symmetry in brachiopods runs down the mid-line of each shell (there is bottom and top shell, hinged at the rear end). Brachiopods have a special organ called the lophophore, a crown of tentacles whose cilia (fine hairs) create a water current that enables them to filter food particles out of the water.

Cephalopods Molluscs with a large head and a beaked mouth surrounded by tentacles. Cephalopods include squid, octopuses, cuttlefish and nautiluses. They are very intelligent and can change colour to scare off predators or blend in with their surroundings.

Cnidarians Jelly-like invertebrates with stinging cells. Some, such as coral polyps and sea anemones, are anchored with their tentacles pointing upwards. Others, such as jellyfish, float with their tentacles hanging down in the water. Cnidarians use stinging cells in their tentacles to paralyse their prey.

Crab A crustacean with a round, flat body, four pairs of legs and a pair of pinching claws used for feeding and defence. Most crabs live on the seafloor but some may live in sandy burrows or even on land. Crabs eat the rotting bodies of dead animals and plants.

Crustaceans Arthropods such as crabs, lobsters, krill and barnacles that have two pairs of sense receptors called antennae on their heads.

Echinoderms Marine invertebrates with spiny skin and skeletons made of chalky calcium plates. Echinoderms include starfish, sea urchins and sea cucumbers.

Gastropods Soft-bodied molluscs, such as snails, slugs and whelks, that move about on one large foot. Some gastropods have hard, protective shells.

Horseshoe crabs A separate group of arthropods from the true crabs, more closely related to spiders and scorpions. Covered by a hard shell, horseshoe crabs each have four pairs of leg-like appendages and a long, spiky tail.

Jellyfish A cnidarian with a bell-shaped body and tentacles. Jellyfish are more than 90% water and have no heart, bones or brain. Some swim by jet propulsion, but most are carried by the ocean tides and currents. Their tentacles have stinging cells with which they paralyse their prey.

Jet propulsion A type of movement used by some jellyfish and cephalopods. The animal sucks water into the body sac, then squirts it out. As the water shoots out backward, the force of it pushes the animal forward.

Lobster A crustacean with a long body, four pairs of legs and a large pair of claws. Lobsters crawl on the seabed in shallow water, feeding on starfish, sea urchins, molluscs and crabs. One claw is used for crushing prey and the other is used for cutting it up.

Molluscs A group of invertebrates with soft bodies, often protected by hard shells. The main groups of molluscs are gastropods, bivalves and cephalopods.

Nudibranch A brightly coloured type of sea slug. Some nudibranches feed on sea anemones and store their stingers in growths on their backs as a weapon.

Octopus A cephalopod with a bag-shaped body and eight arms, used to fight, grab things and move about. The largest octopuses have an arm span of up to 9 metres (30 feet). Octopuses live on the ocean floor, feeding on crabs and lobsters. They are thought to be the most intelligent of all invertebrates.

Scallop A soft-bodied mollusc with two shells joined together at a hinge. Scallops move through the water by quickly closing their shells to force out water and push themselves forward.

Sea cucumber A soft-bodied echinoderm that lives on the ocean floor. It filters the sandy bottom for dead plant and animal material. It has five rows of feet and a mouth surrounded by tentacles.

Sea slug A soft-bodied sea creature that moves on one foot. Because sea slugs are not protected by a shell, some species are camouflaged and some are poisonous.

Sea snail A gastropod with a hard shell. Conches, whelks and winkles are all types of sea snail.

Sea urchin A small, round, spiny echinoderm that feeds on seaweeds and other plants. Its mouth is in its underside, so it crawls on top of things to eat them.

Sponge A type of invertebrate with no mouth, heart, brain or distinct body parts. Sponges live attached to solid surfaces. They feed by passing water through tiny pores in their bodies and filtering food from the water.

Squid A cephalopod with a long, pointed body, eight arms and two longer tentacles. Squids have an internal shell called a “pen” that protects their inner organs. They have large eyes and good eyesight.

Tentacle A flexible limb with no bones or joints, belonging to some animals, especially invertebrates, such as squid and jellyfish. Tentacles may be used for gripping or feeling objects, or for movement.

Worms A group of long, thin, soft-bodied creatures. Some species of worms live in the sea or in the sand of the seashore.

Consultant: Chris Jarvis