The olfactory system of the snail is functionally capable and structurally complex. The morphology of the olfactory epithelium and the glomeruli are similar to analogous structures in vertebrates. However, the snail system differs markedly from the vertebrate system in its lack of a mucus secretion and the apparent absence of spatial patterning. Such similarities and differences teach us about the limitations and options governing the evolution of olfactory systems. The comparative approach leads to the following conclusions, or ‘lessons’: (1) Death and replacement is normal for olfactory receptors. (2) Olfaction requires large numbers of receptors and other neurons. (3) Glomerular structures in the olfactory neuropil aid sensory processing. (4) Local interactions are important in the early stages of olfactory processing. (5) The role of mucus in olfaction is peculiar to the vertebrate nose. (6) The spatial patterning of odor responses is not necessary for effective odor processing.