Hyponome

The hyponome or siphon is the organ used by cephalopods for locomotion. It is a muscular funnel or siphon into which water is drawn. The hyponome expands and then contracts, sending a jet of water rushing out. This jet propels the cephalopod backward, but it can control its direction of movement by pointing the hyponome in different directions.

The hyponome of the nautilus differs from other cephalopods in that it is a one-piece flap that is folded over. In other cephalopods, such as octopus, squid, and cuttlefish, the hyponome is a muscular tube. The flap-like hyponome may be an adaptation to confinement in a shell that both nautiloids and ammonites shared.