Studies on the respiratory and hydrostatic functions of the mantle cavity in two freshwater pulmonate snails

1. A study has been made of the function of the gas bubble in the mantle cavities of Lymnaea stagnalis and Planorbarius corneus by various techniques including gas analysis and underwater weighing. 2. The volumes of enclosed gas are variable, but under laboratory conditions, are always sufficient to make snails of both species buoyant immediately after they have ventilated at the surface. 3. P. corneus, with a proportionately heavier shell, is less buoyant than L. stagnalis under the same conditions. 4. Both species lose their buoyancy rapidly over the first three hours of submersion by a process reducing the gas volume. Gas analysis has shown this to be due, in part, to the extraction of oxygen for respiratory purposes. There is no evidence of significant accumulation of carbon dioxide in the cavity of either species. 5. Fluctuations in gas pressure within the cavity are too small to produce variations in any snail's buoyancy. 6. The use of the cavity as a physical gill under certain conditions has been confirmed by experiment and observation. 7. It appears that the buoyancy of the shell and visceral mass determines the amount of gas-uptake during ventilation at the surface. 8. Any prolongation of dive, and any interference with the snails, leads to an increased uptake of gas at the next surfacing. 9. Decreasing buoyancy need not determine the onset of the surfacing pattern in L. stagnalis. 10. Both species have an equal capacity for lengthy periods of submersion. Great variation in the duration of dives is shown under laboratory conditions, and they can be much longer than has previously been suggested. 11. It is concluded that the stimulus evoking the surfacing behaviour to end a dive is a hydrostatic one, but the expression of this is dependent on its interaction with other factors, including the pallial and cutaneous respiratory supplies