As the shell grew, the back compartments were sealed with a semi-permeable membrane. A single tube, the "siphuncle", passed through the centre of each septum and connected the chambers The animal could add or withdraw gas as it needed for buoyancy. On the inside of the shell, the compartments are marked by elaborate "sutures". These can be seen easily on those fossils which are "internal moulds", as most are. Ammonites were active predators, and they themselves were often eaten by fish and marine reptiles. The fossils are almost always found with the outer compartment broken off, probably as a result of just such an attack.