“The Chemical Sense”
In the simpler multicellular organisms, which develop by cell division and multiplication from a single cell, the cells differ from the original type and from each other in position, structure, and function. In the course of growth the organism originally spherical in shape becomes modified by irregular growth of cells, producing folds and prominences. Cells are crowded out of shape; some lie at the base of a depression protected from stimulation; others occupy positions which make them especially liable to be acted upon by such stimuli. In the course of these modifications some of the cells become especially adapted for receiving impressions, others for conducting or transmitting these impressions to various parts of the organism, others for producing movements of the organism. It is with the first type of cell that we are concerned, the receptor mechanisms. They are in the simpler organisms, adapted to receive two sorts of stimulation, mechanical and chemical. In fact, through the whole series of multicellular organisms such reactions to mechanical and chemical stimuli have been noted more or less definitely, although special sense organ structures have in many cases not been discovered. This is especially true for the reactions to chemical substances. It is customary to speak of the “chemical sense,” to signify these responses to chemical substances, without any attempt to differentiate between smell and taste. Obviously, in the case of organisms which live in a fluid environment, this chemical sense might be called taste, since it would correspond in a way to that sense in man, for which the adequate stimulus is a fluid. But since it is a “distance receptor,” in that objects at a distance can produce responses, probably by diffusion of substances in the fluid, it might also be looked upon as more nearly resembling the smell sense. In most cases structure offers no help in settling the matter.
In the medusa, or jellyfish, one of the earliest forms in which a nervous system and sense organs are found, the tentacles are especially sensitive to chemical stimuli, much less so to mechanical stimuli. To the former they respond by shortening and twisting themselves about the object. As for sense organs in these parts, there are small club-shaped papillæ in the neighborhood of the tentacles, differing somewhat in character in the different species. These papillæ contain a narrow canal lined with thick cylindrical cells. As far as both structure and function are concerned, they may be considered either as taste or smell organs.
In the flat worms, where a nervous system with a rudimentary brain is found, the reaction to chemical stimulation is not clear. This organism has specialized responses, among which is a movement toward food placed near it. But whether this is a reaction to chemical stimuli alone or combined with mechanical is not known. No taste organs have been found. Pits or depressions found on the lateral surface of the anterior end of the worm, and supplied with nerves from the brain, have been regarded as olfactory rather than as taste organs.
In the annelid group, of which the earthworm may be taken as an example, there are well-defined chemical reactions, which more nearly resemble taste reactions than the cases previously mentioned. Here a positive reaction to food substances seems to occur only when these substances come into contact with the body. For instance, the characteristic burrowing reactions of the earthworm are not aroused by placing filter paper soaked in manure near them, but only when the paper is actually in contact with the body. Negative reactions, however, to strong chemical stimulation may take place without contact. Attempts have been made by Parker and Metcalf to show specialized taste reactions to different chemical substances by measuring the latent time in the responses to various substances brought into contact with the body. From such evidence as this it would appear that earthworms have specialized reactions to the chlorides of sodium, potassium, lithium, and ammonium, which are indistinguishable to the human taste sense, with their common salt taste. These results are interpreted as indicating qualitatively different effects of the stimuli. In these organisms it has been possible to discover taste organs, distinct from the olfactory organs. They are described as cup-shaped organs, which may be either depressions or prominences. They occur in large numbers and are widely scattered over the body. They are said, however, to be especially numerous at the edges of the mouth and within the mouth cavity.
The crustacea, among which are the crabs and the lobsters, characterized by their hard shell-like covering, show certain specific reactions to chemical substances when these come into contact with the parts of the body near the mouth. Reactions to chemical stimuli applied to any part of the body of the crayfish have been reported by Bell. The positive reactions were such as to bring the substance toward the mouth and the negative reactions such as to remove the substance. Responses to such substances at a distance are uncertain. But it is difficult to differentiate between possible smell and taste reactions. The sense organs in these organisms are usually located upon the antennæ, or feelers, in the neighborhood of the mouth. Here there is a different kind of response to chemical and mechanical stimulation. No structures with a specific taste function have been described, although smell and tactile organs have been localized.
In the organisms described above, the chemical, or, more specifically, the taste, sense is a food sense,—edible and inedible substances causing reactions of different character. The reactions to stimuli within the edible group, however, show no variation. In the insects, especially the ants, bees, wasps, etc., there seem to be qualitative differences in the effect produced by chemical substances. It is by means of this chemical sense that bees and ants are able to find food at a distance, to return to their homes under all sorts of adverse conditions, and to distinguish nest mates from enemy intruders. But, since these are all reactions to stimuli at a distance, they must be attributed to the smell sense, rather than to the taste sense. But in the case of these organisms a sharp distinction between smell and taste seems possible. Forel and others have offered honey mixed with strychnine to ants, who seized it greedily, indicating an olfactory sensibility. But immediately after the honey had touched the mouth parts, avoiding reactions, such as to remove the substance, followed, indicating sensitiveness to the bitter substance. Wasps and bees will make the same sort of responses if distasteful substances which are inodorous are mixed with pleasant, odorous substances. The sensitivity to tastes varies considerably in different insects, being very great in bees and ants. From such experiments as the above it has been concluded that the smell organs are located on the antennæ and that the taste organs are located on the lips and in the mouth. Microscopical examination shows that in all insects the tongue and inside of the mouth are covered with minute pits, or depressions. In each pit there is a minute hair, or rod. Some observers say that this rod is hollow and perforated at the end, thus communicating with the nerve which ends at its base. Other observers say that there is no perforation upon the end of the hair. However this may be, there seems to be no doubt that these are the taste organs. The same type of structure has been reported on the proboscis of the bumblebee, the hive bee, and the common fly. They are said to resemble a hollow hair, the channel communicating with a nerve fiber at its base. In the insects, then, we find the earliest definitely specialized taste mechanism.
Chemical Sense in Fishes
In the fishes, again, the distinction between the senses of smell and taste becomes more difficult, on account of their fluid environment. But, disregarding the distinction between smell and taste, the general chemical sense plays a very important part in the life of the fish. Now, some observers have included all of this sensitivity to chemical substances within the sense of smell, while others have attributed a part of it to a taste mechanism. As representative of the latter, Herrick’s conclusions are of interest: “In fishes the gustatory system is much more extensively developed than in mammals, especially the vagal part which supplies the taste buds in the gill region. In some species of fishes, moreover, taste buds appear in great numbers on the outer skin, and these are in all cases innervated from the seventh cranial nerve. In the common horned-pouts, or catfishes, and in the carps and suckers these cutaneous taste buds are distributed over practically the entire body surface, and especially on the barblets.... These sense organs and their nerves are entirely independent of those of the lateral line system, and of the ordinary tactual system, though the gustatory and tactual systems have been shown experimentally to coöperate in the selection of food.”
Herrick determined by experiment that the sense organs thus generally distributed over the body of the catfish really had a taste function. Food placed at a distance from the fish produces only restless movements, indicating that the eyes do not direct them to it. But if food comes into contact with the mouth parts, or, in fact, any part of the body, it is immediately seized. To show that this reaction is not alone due to tactual stimulation, the tactual organs were first stimulated with cotton wool, which produced the characteristic seizing reaction. But after stimulation was continued for a while reaction no longer followed. If at this point the cotton wool be soaked with meat juice, the seizing reaction is again set up. Adaptation to tactual stimulation has taken place, leaving the taste organs to function alone. To show further that the responses did not depend on olfactory stimulation, the olfactory nerves of certain fishes were cut. When the experiment was performed, after recovery from the operation the responses were the same as in normal fish.