Fig. 1. The herbivorous gastropods studied include the marine opisthobranchs Aplysia (10 cm long) and Pleurobranchaea (5 cm long, eating a nudibranch, Flabellina which is coloured purple and orange) and the pulmonates Helix (terrestrial, 2 cm long) and three freshwater genera (Planorbis, Helisoma and Lymnaea, all 1 cm long). The pictures are not to scale. We are grateful to Rhanor Gillette and Andy Bulloch for the pictures of Pleurobranchaea and Helisoma.