Fig. 1. Lymnaea, the pneumostome and training. (A) Cartoon of the training apparatus used to operantly condition aerial respiratory behaviour in Lymnaea. A beaker is filled with pond-water and then N₂ is bubbled through it for 20 min in order to make the pond water hypoxic. Snails are then added, given a 10-min acclimatization period and then training begins. N₂ is continuously bubbled throughout the training period. Each time the snail attempts to open its pneumostome, the respiratory orifice, it receives a tactile stimulus to the pneumostome area, which causes the pneumostome to close. The tactile stimulus is delivered by means of a sharpened wooden stick, the `digital stimulator'. (B) Photograph of a 2.5 cm (i.e. adult) snail. The pneumostome (arrow) is open. The pneumostome only opens when the snail is at the air–water interface.