SMELL AND TASTE, SENSES OF.—The senses of smell and taste are distinguished from the rest as "chemical senses," because their sensory cells respond to chemical stimuli. In verte brates the two senses are completely separated. See SMELL AND TASTE above.
In invertebrates a similarly sharp distinction between the sense of smell and that of taste is, in most cases, impossible. Insects alone possess separate sense organs which react to different chemi cal qualities. According to Schaller's investigations, this applies also to aquatic insects (Dytiscus). This is important as, formerly, it was denied that aquatic insects, in general, have a sense of smell. In gastropods, crustaceans, cuttlefishes and worms tastes and odours are perceived by the same sense organ, so that in these cases there is neither an anatomical nor a chemical differentiation. Under these circumstances, most naturalists consider it more correct to speak of only one unified chemical sense in such animals.
It has an olfactory and a gustatory component. It is uncertain, and will probably always remain undecided, whether two qualitatively different kinds of sensory cells share in the constitution of such a sense, so that special olfactory and special gustatory cells are situated side by side in the same organ, or whether one and the same cell responds to both tastes and odours.
Biologically speaking, in all such cases the olfactory component naturally appears as the more important, and is much more in the foreground. It is used in the seeking out of food and mates, in the recognition of enemies, and so on, while the taste component is used only in testing the food. The use of the term "olfactory sense" in such cases is, therefore, not to be altogether rejected, as will appear from the following.