The Encyclopedia Americana (1920)/Animals, Chemical Sense in
ANIMALS, Chemical Sense in. Almost all animals exhibit specific responses to chemical stimulation. In Amœba proteus food-particles are ingested by the formation of a sort of invagination that surrounds them, whereas this is not true of bits of inedible material. In many species of Cœlenterate we find that though contact with an inedible body produces a reaction of aversion, contact with food is followed by motions leading to its ingestion. In other cases, as with Carmarina hastata, though the reactions to chemical and mechanical stimulation are qualitatively similar, the local sensitivity to chemical stimulation is independent of that to mechanical stimulation. In planarians the general evidence for the chemical sense resembles that in Cœlenterates. According to Lehnert, a land planarian, Geodesimus bilineatus, is able to perceive the existence of food with which it is not in contact. This is the first place in the animal scale where we have evidence of any sense analogous to smell, which perceives chemical substances in the form of vapor, as apart from taste, the sensitivity of which is limited to substances in solution. This distinction, be it noted, has only meaning in land-animals. In most planarians, however, chemical stimulants only act on contact.
With the earthworm, the evidence for the existence of a specific chemical sense is very slight. Responses to chemical stimuli exist, and even to stimuli at a distance, but they are of the same nature as the responses to mechanical stimuli. The only reason for regarding the chemical sense as distinct is to be found in the different speeds of the reactions to different chemical excitants.
There is good evidence for the existence of a rather sensitive chemical sense in the echinoderms and in many molluscs. In the snail, smell is apparently most acute at the ends of the feelers, but is not confined to them. An analogous distribution of chemical sensitivity is found in many crustaceans, where the antennulæ and the mouth-parts constitute its chief, but not its sole, organ.
The first place where we find evidence of a varied and sensitive set of chemical reactions is in the insects, and more particularly in the higher hymenoptera. The marvelous powers which are possessed by bees and ants of finding their way to their hive or nest and to their food, of recognizing whether an individual belongs to their colony or not and of taking care of their young, are to be attributed in a large measure, at least, to the acuity of their sense of smell. Though the mechanism of all these actions is still a matter of the gravest uncertainty and dispute, there is evidence that in the ants of the genus Stenamma the perception of different sorts of odors is located in different segments of the antennæ, which are the principal olfactory organs in all insects. The individual odor of an ant is perceived by means of the tenth segment of the antennæ, the race odor by the eleventh and the nest odor by the twelfth.
In the lancelet, there is not much evidence for the separate existence of a specifically chemical sense. While fishes, reptiles and amphibians possess organs structurally and historically similar to the smell and taste organs of man, little is known about their function. The so-called terminal buds of fishes, resembling the taste-buds of man, are found not only in the mouth but sometimes over the whole surface of the body, but it is not completely established that they exercise a gustatory function. The existence of taste has been established in birds but little is known about their sense of smell. The taste and smell sensations of mammals would seem to bear a great resemblance to our own, though they are often far more sensitive, as in the dog. See Taste and Smell.