In the chambered shell forms, buoyancy is regulated by shifts in gases in the compartments of the shell. The gas in the compartments diffuses from the blood and the amounts of liquid and gas change over time. Therefore the buoyancy also changes over time. Squid are probably the fastest. When radial muscles contract and circular muscles relax, the volume of the mantle cavity increases and water rushes in around the neck.
Fins are stabilizers.
Some squid reported to land on deck of ship 12 ft. above the water level. Digestion is entirely extracellular. Digestive gland secretions are released into the junction between the stomach and caecum. Absorption in cecal walls. Wastes egested through the anus into the mantle cavity.