3. Cephalopod: Jet Propulsion
Cephalopods, a group of animals that includes squids and octopuses, are the fastest invertebrates in the sea, in part because of their unusual ability to use jet propulsion. When water flows into the animal’s mantle cavity, it is held under pressure as long as its orifices are closed, except for an opening called the funnel. When the muscles of the mantle wall contract and squeeze the pressurized water back through the funnel, the animal is propelled through the water at a rate of up to 25 miles per hour. Cephalopods are also able to control the direction and speed of their movements, allowing them to move away from a threat or toward a prey animal as needed.