Life in Water: Invertebrates - Locomomotion
Many marine invertebrates move about by pushing their way through the water, much as fishes do. When fishes contract their side muscles, their rigid internal skeleton causes their tail to swing from side to side with great power. But because invertebrates have no bones, they cannot generate nearly as much forward thrust. Consequently, invertebrates have evolved many different ways of moving through water.
Some, such as scallops, squids and octopuses, move by jet propulsion, sucking in water and squirting it out again to generate thrust. Leeches swim by undulating their bodies vertically—unlike fishes, which undulate from side to side. Some ocean invertebrates crawl and burrow in the mud; others simply float or drift wherever the currents take them; and some anchor themselves to the ocean floor and do not move around at all.
Other Ways of Moving
Some microscopic dinoflagellates move by waving tiny, whiplike flagella. Others twirl their tiny flagella like the propeller of an outboard motor.
The Portuguese man-of-war drifts with the current, like a jellyfish, but it is actually a colony of separate individuals, including some that provide a gas-filled float and others that specialize in feeding, hunting and reproduction.
Starfish, sea urchins and sea cucumbers walk with hydraulic power. They grip hard surfaces by pumping water through tubes in their feet to create suction.
The nautilus adjusts the size of a gas bubble inside its shell-similar to a fish's gas bladder—to rise at night and descend to the deep by day. It also can move around by jet propulsion.
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