Olfaction is generally defined as the sensory system that allows an organism's nervous system to [Page 665]detect chemicals coming from a distant source. Senses that require contact with the chemical source, such as when we taste food on our tongue, are not considered olfaction; neither is the kind of chemoreception present in individual cells or in single-celled animals. This definition is important to keep in mind when trying to understand the evolutionary history of the olfactory system. For example, lobsters can have chemosensory hairs on their antennae, antennules, legs, mouth parts, and carapace. To be able to make inferences about evolution, we need to know which of these groups of hairs constitutes the olfactory system. Thus, we need behavioral or physiological data demonstrating which hairs ...