Sensitivity of the Unicellular Organisms

The study of the sense organ of taste in adult human beings consists of an examination of the taste sensations resulting from controlled stimulation of limited parts, supported by the microscopical examination of the structures found in the regions in which these taste sensations can be aroused. The relation of cause and effect is then assumed. Neither method taken alone will suffice, as there always remains the possibility of function in the absence of definitely recognized taste structures and also the possibility of the presence of functionless structures. The difficulties and uncertainties arising in this combined study of structure and function have been discussed in earlier chapters.

But if the determination of the taste organs and their localization offers difficulties in the adult human being, these are multiplied many times when the study is carried to the lower animals. The method of stimulation is greatly limited, to what extent depending upon the kind of animal studied, because the results of the application of stimuli must be interpreted from the forms of behavior exceedingly crude, compared with the language behavior of man. This handicap in the study of taste is great, as compared with sight and hearing, at least, on account of the extremely close relation between the taste and smell organs in position and in the nature of their appropriate stimuli. In case of certain of the lower animals the assumption of distinct states of consciousness corresponding to our experiences of taste and smell is unwarranted, as is even the assumption of any consciousness at all.

The study of taste structures reduces itself largely to the search for sense organs resembling those of man, and in the same neighborhood as they are found in man. Here again the difficulty is especially great in taste, because the taste organs even in man are not very highly differentiated from other structures, and the really essential part of the organ is not definitely known. (See Chapter VI.) In the search for taste in lower animals one must rely much upon the expectation of finding the taste mechanism in the mouth or its immediate neighborhood. Structures found here and not known to function otherwise are likely to be looked upon as taste organs. These assumptions from location are then tested by stimulation of the parts with sapid substances and looking for characteristic responses, and further by extirpating these organs and noting the effect upon behavior. Other criteria of sensitivity, which can be used especially in the study of sight and hearing, such as rate of fatigue, reaction time to stimulation, and the like, are of little use on account of the above-mentioned close relation between taste and smell.

When looked at from the evolutionary point of view, all of the senses are seen to have developed through modification of the sensitivity of a single structure, the cell, with its additional properties of conductivity and motility. In the simplest living organisms, for instance, one finds sensitivity to consist in the irritability common to all living cells, and the sense organ to be represented by the whole cell. Still, the amoeba, one of these unicellular organisms, reacts differently to the contact of food substances and to purely mechanical stimulation. And the white corpuscles of the blood in the human body are said to adjust their behavior according to the chemical composition of their surroundings. So, even in this earliest stage of evolution, before any differentiation of structure appears, one sees a reaction analogous to the taste reactions of the higher animals.

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.

The experiments of Parker show further that the mouth and external surface of the body of certain fishes are sensitive to sour, salt, and alkaline solutions. Sheldon obtained about the same results. The external skin covering is not sensitive to sugars. The tongue of fishes presents a smooth, gray, dorsal surface, devoid of elevations or papillÆ, which characterize the tongues of many other organisms. Nor is it a mobile organ in comparison with other species. On the whole, the tongue itself seems little adapted for arousing taste sensations.

The system of “lateral line” organs of fishes have at times been thought to be concerned with the chemical sense. This is probably not the case, although their exact function is a question still under dispute.

In a general way, the taste buds, or sense organs of taste of fishes, resemble those of the human being. They are either flask- or cup-shaped, and are composed of two types of cells, called supporting cells and taste cells. The latter cells end peripherally in a hair or bristle, just as the same kind of cell in the human taste bud.

Land-Dwelling Animals

There seems to be no experimental evidence for a specific sense of taste in amphibia, or reptiles. But sense organ structures have been described upon the tongue and soft palate of the frog, where they are said to occur in hundreds. They are disc-shaped structures, made up of several kinds of cells, which correspond to the real taste cells and supporting cells of the human sense organ. The taste cells end peripherally in several hairs or bristles, and at their central end make connection with nerve fibers. In the reptile group there is neither experimental evidence of taste sensitivity nor anatomical evidence of the presence of taste corpuscles on the tongue or in the mouth cavity.

The experimental evidence for the taste sense in birds is slight. It certainly is greatly overshadowed by the keen senses of sight and hearing. Birds seem to represent one case, however, in which taste is more important than smell. Taste sensitivity for different chemical substances, in the case of young chickens, at least, seems clear from certain studies of instinct and learning, in which they accept certain kinds of food and reject others after tasting them. In considering the sense of taste in birds it must be remembered that most of them swallow their food without chewing it or without having it reduced to liquid form through mixture with saliva. The tongue, which varies in character considerably in different types of birds, is in most cases covered with a horny coat. Numerous hard papillÆ are found upon its surface. Microscopical examination of these papillÆ shows nothing which can correspond to taste buds or to gustatory cells. The parrot is said to form an exception to most birds, in that it has a relatively soft and fleshy tongue, with numerous papillÆ, and also in that it chews its food.

In the duck, which has a large tongue, there are certain portions which lack the hard covering common to birds’ tongues. Here, in addition to a large number of tactile corpuscles, there are groups of cells which resemble somewhat true taste corpuscles. The peripheral ends of their cells reach the surface of the mucous covering of the tongue. The cells do not end in the bristle, or hair-like, formation, as those of the human taste cells, but in a pointed elongation of the protoplasm. Experimental evidence of the function of these structures is lacking.

Taste sensitivity and the structure of the taste organs differ greatly in the mammals, but there seem to be two characteristics in common, namely, the localization of the taste corpuscles within the mouth and the importance of the tongue in arousing taste sensations. The character of the mucous lining of the mouth also shows great variation in the number of papillÆ and the taste buds which they contain. The number of papillÆ varies from two or three in the marsupials and four in the elephant to an extremely large number in rodents, e.g., the rat. The papillÆ are in general quite similar to the three most common forms in the human taste organs, the circumvallate, the fungiform, and the filiform, and have about the same location in relation to each other. The greatest difference is in the prominence of the fourth type, the foliate papillÆ in certain mammals, as compared with man. These are seen best in the rabbit, as folds directed downward and forward on the sides of the tongue in its posterior portion. They have been considered to result from the great number of papillÆ which throw the mucus into folds. Each foliate papilla is composed of a number of parallel ridges, each ridge in turn being composed of papillÆ of the fungiform type. Between the ridges there are narrow ditches. It is in the side walls of these that the taste corpuscles are found in greatest numbers. Thus, these ditches are analogous, in function at least, to those of the circumvallate papillÆ. Their origin, however, seems to be different from that of the circumvallate. In the monkey one finds less prominent folds on the sides of the tongue, rich in taste corpuscles, which represent the foliate papillÆ.

The taste corpuscles themselves have about the same characteristics in all mammals as in man. There are differences in size, to be sure, but their structure is the same, and the supporting cells, gustatory cells, and nerve fibers are present in them all.

This survey of taste in the animal kingdom suggests the conclusion that the taste organs represent a modification of the original skin sensitivity or touch sense, and surely a slight modification when compared with the senses of sight and hearing. A certain resemblance has been remarked by Wundt and others between the touch corpuscles and the gustatory corpuscles. His interpretation is that the whole body was originally endowed with the touch sense, while certain parts being affected continually by specific sorts of stimuli, became adapted to them by undergoing modifications of structure. The head or mouth end of the animal was more subject to chemical stimulation, and the adaptation of the tactile organs to this particular form of stimulation resulted in the development of the senses of taste and smell. To consider taste as one of the lower senses, in the sense of being least highly developed and the earliest to appear, is justified from this survey of the evolution of the taste sense, if from no other point of view.