Fig. 1. Illustration of pneumostome opening and the underlying circuitry in Lymnaea. (A) Schematic diagram of the three neuron CPG and motorneurons innervating the pneumostome muscles. RPeD1, right pedal dorsal 1 neuron; VD4, visceral dorsal 4; IP3, input 3 interneuron; VK, visceral K closer motorneurons (MN); VI/J, visceral I/J opener motorneurons; filled circles, inhibitory connections; open triangles, excitatory connections; grey triangles, biphasic connections (excitation followed by inhibition). (B) An example of pneumostome opening in a typical semi-intact preparation. The pneumostome is located on the bottom right of each panel and is closed, but reared upward in (i) and fully open in (ii). The two photographs were taken in quick succession (1 s apart) after lowering the saline level to expose the pneumostome to the water surface. Scale bar, 3 mm.