Beneath the surface of the water is a very active world. There are animals that never stop moving– swimming, floating, drifting, burrowing, walking, crawling and pulling themselves over rocks. And the structures they use for movement are as varied as the movements themselves: legs, fins, feet, paddles, suckers; tiny, beating hairs called cilia; and small, whip-like appendages called flagella.

Small animals glide over plants and rocks on cilia, while sea slugs slide forward by contracting the muscles that run up and down a flattened sole-like undersurface. Starfish have stalk-like tube feet lining the grooved underside of their arms. Ending in tiny suckers, these feet are in slow but constant motion, extending out searchingly and contracting again to move the animal along the bottom. Clams burrow through mud or sand by pushing a pointed foot into the soft bottom, whereupon the tip of the foot expands, anchoring the clam. The shellfish then contracts the extended foot and thereby pulls itself to a new position.

Other animals have bodies constructed to aid movement in equally remarkable ways. Spiny sea urchins use their spines as levers while pulling themselves along the bottom with tiny tube feet. Fish have streamlined bodies that offer little resistance to water, as well as fins moving through the water, changing direction and braking. Some, like the flying fish, sport expanding, wing-like fins that allow them to leave and glide over the water for short distances. Lobsters and prawns, heavier than water, must walk in the bottom however; they can also propel themselves quickly backward by suddenly flicking their abdomens. Squids can swim by using their fins, but like octopuses, they can shoot backward to escape enemies by a similar method of squirting a stream of water out of their bodies through a siphon. Jellyfishes move in a comparable manner, contracting the edge of their bell-shaped body to squirt out water in order to move.