A siphon is a long tube-like structure that is present in certain aquatic molluscs: Gastropods, bivalves, and cephalopods. The tube is used for the exchange of liquids, or air. This flow can have different purposes, the most common are breathing, locomotion, feeding and reproduction.
A single siphon occurs in some gastropods. In those bivalves which have siphons, they are paired. In cephalopods, there is a single siphon or funnel which is known as a hyponome.