A squid moves using a system of jet propulsion, pulling water into its mantle cavity, contracting the muscles of the mantle wall and expelling the water through its siphon or funnel. Its fins aid maneuverability and also help with lift and motion while the squid travels at slow speeds.
A squid has one funnel system for intake and another for outflow. The mantle expands as the squid draws water into it. When it is full, the intake closes. The strong mantle muscles contract, the outflow funnel opens, and the pressure of the fluid in the mantle creates kinetic energy that propels the squid in a rapid burst of speed. Squid are able to reach 25 mph, making them the speediest marine invertebrates. When a squid is swimming steadily, it draws in and expels water in a pulsing pattern, drawing in more water per burst when it wants to increase speed.
By controlling the direction of the outflow funnel, a squid is able to control its swimming direction. It can go backward, forward, spin or hover. When fleeing predation, a squid faces its funnel forward and jets backward. When attacking prey, it faces its funnel backward so it can approach front-on. Researchers have studied squid locomotion when designing mechanical jet propulsion systems such as turbojets.