Siphons in Mollusca are tube-like structures in which water (or, more rarely, air) flows. The water flow is used for one or more purposes such as locomotion, feeding, respiration, and reproduction.
The siphon propels the squid through the water in the opposite direction to which the siphon is pointing, much like jet propulsion. To use this jet propulsion, the squid takes in water through the large opening and then closes off the opening.
