Nautiloid cephalopods. This group evolved the consummate pump gill, and probably because of this became the largest of all invertebrate animals in the sea, with forms capable of thriving in water too low in oxygen for most other animals. The path of water into the animal, across its gills, and then out the funnel beneath the body allowed respiration and locomotion to be superbly combined.

better model for the nautilus. In the grandiose Zeppelins of the early twentieth century, the balloon was kept at neutral and largely unchanging buoyancy in air and it rose or descended through its use of powered engines. So too with the nautilus. It was indeed discovered that the nautilus undertakes nightly vertical migration, but changes in buoyancy through trim of the shell are not involved. Instead, the strong swimming action of water jetted through a tube beneath the shell powers the animal upward and also pushes it back down to depth.

A second line of discovery dealt with the jet power itself. It is enormously strong, and as scientists made further measurements of the system, it was discovered that a large volume of water was constantly being pumped through the front of the shell, even when the animal is at rest and motionless. There is a simple reason for this—the propulsion system is an offshoot of the respiration system. All of this water that is eventually destined to jet through the locomotion tube, called the hyponome in the Nautilus and Allonautilus, first passes over two pairs of large and complex gills in the back of the mantle cavity. The design allows a dual function—respiration and locomotion—from the same energy expended to draw water in and then force it out.