The Animal Mind: A Textbook of Comparative Psychology

of discrimination is chemical. Lacrymaria, another ciliate, tests with its ‘head’ ‘‘every object within reach and rejects all those which cannot serve as food. It does not swallow inorganic substances, carmine, or ink particles and the like. This protozodn unquestionably exercises selection in feeding’’ (469, p. 243), but the basis of the selection is not determined. Didinium is a ciliate which has a peculiarly modified seizing organ, but the only selection of food which it makes rests on the fact that this organ will adhere to the surface of some organisms and not to that of others (466). Two other protozoa, Actinobolus radians and Spathidium spathula, have each so far refined the process of selection of food that they swallow only one kind of organism. Actinobolus, an anchored form, awaits its destined prey, and Spathidium selects it in freely swimming about ; but as to whether the prey is recognized by chemical or by mechanical features we have no information (499).

The lowest of the Metazoa, or many-celled animals, are the ccelenterates. Although externally the forms of different families of ccelenterates differ widely, yet the general plan of structure is the same in all: the body of the typical ccelenterate is a hollow sac, whose walls consist of two layers of cells, food being taken into a mouth at one end of the sac, and the arrangement of cells being on the plan of circular symmetry. In the phylum of the ccelenterates are included sea-anemones, jellyfish, the little green or yellow Hydra, sponges, corals, and ctenophores.

Hydra (Fig. 6), one of the simplest ccelenterates, shows a food reaction distinct from the contact reaction. Me- chanical stimulation is followed by withdrawal of the tentacles and by contraction of the stem. This behavior may be called a negative or avoiding reaction, and no positive reaction to a mechanical stimulus has been observed. The food-taking reaction, on the other hand, consists in the seizing of the food by the tentacles. It seems to be given in response to a combination of chemical with mechanical stimulation, such as is offered by contact with a solid edible object (751 a). Shall we say that Hydra possesses, then, a _ food. sensation and a contact sensation that are distinguishable in its consciousness, provided such consciousness exists? It may be that the contrast between the two is more nearly - Fic. 6.— Hydra. mith, mouth; ¢, tentacle. analogous to that be- After Parker.

_ tween pleasantness and unpleasantness in our own experience, for the food-. taking reaction in Hydra is the only form of the positive reaction, and the response to mere contact is distinctly negative in character. The influence of physiological condition in Hydra’s reactions is shown by the fact that although ordinarily the food response is brought about only by contact with food, if the animal.is very hungry any chemical.stimulation, even quinine, will produce

the adaptive aspect that the starved animal can afford to lose no chances, and suggests the analogy from our own experience of the loss of intellectual discrimination in moments of intense emotion. For the emotion too repre- ‘sents a situation where the organism cannot afford to lose chances by hesitating in reaction long enough for nice discrimination. j In Tubularia crocea, a ccelenterate belonging to the family of hydroids which form colonies of many individuals on a common stem, food and contact stimuli do not produce different reactions, but have different degrees of efficiency in bringing about response. When a grain of sand was placed in contact with the tentacles on one side and a bit of meat in a corresponding position on the other side, the reaction was almost invariably in the direction of the meat. Filtered meat juice allowed to flow upon the distal tentacles produced a reaction 82 per cent. of the time, while ' carmine water was effective only 15 per cent. of the time. Further, if the distal tentacles were touched several times with a needle, they remained closed; but if the second stimulus used was a piece of meat, the tentacles opened out and waved about (564). Whether in such a case as this the possible conscious accompaniments of the responses are to be regarded as qualitatively different sensations, or only as different degrees of intensity of the same sensation, it is difficult to say. Another hydroid, Corymorpha palma, gives no response whatever to meat juice; only irritating chemicals produce reactions, whose character appears to be tactile (714).

In the sea-anemones or actinians we find behavior in response to food stimulation as distinguished from contact stimulation varying in different representatives of the group. Generally speaking, the food reaction seems to be more marked than the contact reaction. W. H. Pollock a number of years ago reported his observation that certain unnamed sea-anemones opened out if food were suspended near them in the water, and referred the phenomenon to “‘a sense of smell” (609). Adamsia rondeleti winds its tentacles around bits of sardine meat and passes them from tentacle to tentacle toward the mouth. When balls of filter paper softened with sea water are substituted, the feeding reaction is wholly lacking. Either the tentacles fail to react at all; or the ball is “felt of” slowly with no attempt to seize it, or it is momentarily seized and then dropped. If the paper ball be soaked in fish juice, on the other hand, it is seized as eagerly as the fish meat. A

tentacles affected, may be produced by applying a bit of paper soaked in quinine solution or by the discharge of quinine solution from a pipette near the tentacles (427, 518). A peculiar form of negative reaction has been observed in Adamsia, and more strikingly in Cerianthus, when a paper ball soaked in fish juice has been passed from tentacle to tentacle till it has nearly reached the mouth. The process is suddenly reversed, and the ball is passed back from one tentacle to another till it reaches the outside edge and is dropped off. Nagel, the observer, thinks the. stimulus for this change of reaction is the gradual wearing off of the “‘sapid parts” of the ball during its passage toward the mouth — it might be the squeezing out of the meat juice — and calls special attention to the fact that the reaction whereby the paper is got rid of is wholly different from the ordinary reaction of a tentacle to mechanical stimulation, which, as we have seen, does not involve seizing the object at all. A tentacle touched by a bit of moistened filter paper ordinarily responds, if at all, by a mere

contraction without the winding seizure of the object. Touched by the same object “handed on” to it byatentacle nearer the mouth than itself, it seizes the paper and passes it on to the tentacle beyond it. The cause of this difference in behavior seems to lie in the processes that have been taking place just previously. Nagel does not hesitate to say that a psychic process must be involved, but its details are not easy to construct Another sea-anemone, Aiptasia, has but one ring of tentacles, and F's: 7-— like Tubularia crocea, instead of showing different responses to contact stimulation alone and to contact plus food stimulation, it merely reacts with greater emphasis to the latter. In both cases the tentacles wind around the object, contract, and direct themselves toward the mouth (521). Again the question arises whether the possible accompanying sensations differ in quality or only in intensity. One species of Aiptasia, A. annulata, however, does react differently to filter paper soaked in crab juice and to plain filter paper (374), showing that even within a genus the capacity for stimulus discrimination may differ. In like manner one sea-anemone, Actinia, will take filter paper soaked in acetic acid, while another, Tealia, rejects it (228).

Metridium, a common sea-anemone of our coasts, has its tentacles covered with cilia which have a continual waving motion toward the tip of the tentacle (Fig. 7). If particles of an inedible substance are dropped on a tentacle, no definite reaction occurs, but the particles are carried by the ordinary motion of the cilia out to the tentacle tip, where they drop off. When a bit of crab meat, or some meat juice, is dropped on a tentacle, the latter contracts and curls over with the tip directed toward the mouth. The ciliary movement continuing in its usual direction now of course carries the food toward the mouth. Applying food to the lips on either side of the mouth causes a different response. The cilia on these lips ordinarily wave outwards; when food is brought in contact with them their motion is reversed, and the food is thus passed into the mouth. In Metridium, then, there is no specific rejecting reaction for inedible substances (533).

Various instances of the effect of physiological condition upon response to food stimulation in sea-anemones have been noted. Adamsia loses the power to discriminate between edible and inedible substances when very hungry (521). Sagartia davisi will also swallow inedible substances if hungry enough (715). Stotachactis helianthus will give either a positive or a negative reaction to food according to its condition of hunger or satiety (374). The reaction of Metridium to food may vary decidedly with the degree of hunger (3), although it will continue taking food as long as the process is mechanically possible (378). Fa- tigue has also been shown to affect the food responses of Metridium and other sea-anemones; specimens that have

be fed meat and filter paper alternately will after a time refuse to take filter paper (374, 521, 533). This behavior was thought by Nagel to indicate that the animal had discovered the deception practised upon it; but according to Gee (256) the real cause is increased secretion of mucus, which lowers the responsiveness of the animal. This effect would naturally be felt first in response to weak stimuli. i, As regards the localization of the sensitive elements,

authorities disagree, and probably species differ. Nagel finds the tentacles most sensitive (521); Loeb observed that the stump of the animal has discriminative reactions (427), while Fleure and Walton state that in the species tested by them the mouth-region is most responsive to chemical stimulation (228). A certain amount of discrimination between mechanical stimuli is ascribed to these animals by Romanes. “I have observed,” he says, ‘‘that if a sea-anemone is placed in an aquarium tank and allowed to fasten upon one side of the tank near the surface of the water, and if a jet of sea water is made to play continuously and forcibly upon the anemone from above, the result of course is that the animal becomes surrounded with a turmoil of water and air bubbles. Yet after a short time it becomes so accustomed to this turmoil that it will expand its tentacles in search of food, just as it does when placed in calm water. If now one of the expanded tentacles is gently touched with a solid body, all the others close around that body in just the same way as they would were they expanded in calm water” (442, p. 48), although the solid stimulus is decidedly less intense than that offered by the bubbles. Similarly, Fleure and Walton find that certain species show little reaction to accidental contact with a pebble that is moved, but react quickly to a finger (228).

The body of a typical medusa or jellyfish consists of a bell-shaped “umbrella” from the edge of which tentacles ’ depend. Hanging from the middle like the clapper of the bell or the handle of the umbrella is the manubrium, at the end of which is the mouth. In the medusa Carmarina hastata no differentiation in reaction to contact and food stimulation appears, merely a readier response of the tentacles to the latter; but we do find whatever evidence for

the existence of a specific sensation quality is furnished by localized sensitiveness, for the skin of the under side of the umbrella, and of the manubrium, is very sensitive to mechanical stimulation, and wholly insensitive to chemical stimulation, while the tentacles, as has just been stated, react, by shortening and twisting themselves about the object, more readily to chemical than to mechanical stimulation. A mechanical stimulus applied to any part of the under edge of the umbrella produces after from one to three seconds a movement of the manubrium tip toward the point stimulated (519,

The little medusa Gonionemus murbachit (Fig. 8) shows, on the other hand, two well-defined different responses to special stimulation: motor reactions and food-taking reactions. The motor or swimming reactions are given in response to mechanical stimulation and to the presence of food near the animal in the water; but the food-taking reaction occurs only in response to food (solution of fish meat); very rarely a weak inorganic chemical stimulus will produce the beginning of the response. An important exception to the usual inefficacy of mechanical stimuli in bringing about the feeding reaction occurs when a moving mechanical stimulus is used; this very quickly produces the early stages of the food-taking response. Special reactions to stimuli in motion are widespread throughout the animal kingdom; their significance will be discussed in the chapter on Space Perception. The food-taking re-

sponse in Gonionemus shows a marked coérdination of movements; if the food touches one or more tentacles, these contract and twist about it; they then bend toward the manubrium, and the margin of the bell also bends in; the manubrium swings over toward the bell and envelops the food with its lips (802). Another ccelenterate whose reactions to chemical stimulations have been observed is the ctenophore Beroe ovata. Its body is an elongated oval, with longitudinal ciliated ridges, having the mouth slit at the end which is normally uppermost when the animal is at the surface of the water, and at the opposite end an otolith or statolith organ lying between two flattened “‘polar plates.” The significance of this organ will be considered later. The aboral region is far more sensitive than any other to mechanical stimulation; the slightest touch on one of the polar plates causes the animal to shorten itself and fold in the plates. The aboral end, being the hind end of the creature, is not usually brought into contact with objects. Nagel, who studied the animal, suggests that this region, being sensitive to changes in pressure, may enable the animal to right itself when it rises to the surface with the aboral end up, as the change from water to air pressure could not fail to stimulate the polar plates. Nagel apparently made no experiments on the behavior of Beroe with reference to food stimuli; for chemical stimulation he used picric acid, dilute hydrochloric acid, quinin, strychnin, saccharin, coumarin, vanillin, and naphthalin. To all these unwonted stimuli the animal responded by some form of negative reaction, indicating possible unpleasant feeling. The edges of the mouth, where the nerves end in bulb-like structures, reacted to quinin, vanillin, and coumarin by stretching the mouth into a circular form instead of its

usual slit-like shape, suggesting an effort to get rid of the stimulus. Precisely similar reactions were produced by stimulation with lukewarm water. Nagel concludes that the organs for chemical and thermal stimulation are idenical; whether the sensation qualities are different is, he thinks, an open question. There is at least no evidence that they are different (519, 521). Next to the ccelenterates zodlogists place the phylum of the Platyhelminthes or flatworms, which possess a bilaterally instead of a radially symmetrical structure. Many representatives of the group are parasitic, and so far as the writer is aware, no extended study of the reactions of these forms to stimulation has been made. Most of our knowledge in regard to the sensory life of the flatworms in confined to the class Turbellaria, including the common freshwater and marine planarians. These are small slow-moving creatures which crawl about on solid objects under water or on films covering the surface. The mouth is situated on the ventral side of the body, sometimes quite far removed from the head end (Fig. 9). One chief interest of planarians to physiologists has lain in their remarkable power to regenerate parts lost by mutilation.

Planaria maculata, a common freshwater planarian, responds to stimulation by two forms of negative reaction, a positive reaction, and a feeding reaction. The negative and positive responses are given either to mechanical or to chemical stimuli, the former being produced by strong, the latter by weak stimulation. Hence they do not suggest correlation with qualitatively different sensation contents, but rather with unpleasantness and pleasantness.

The two forms of negative reaction correspond to differences in the location of the stimulus. If the head end of the body is stimulated strongly on one side, the head is turned away from that side. If the posterior part of the body is strongly stimulated, the animal makes powerful forward crawling movements. The significance of local differences in stimulation for response and for possible consciousness, again, will more properly be discussed in a later chapter. As has just been said, both weak chemical and weak mechanical stimulation cause Planaria maculata to give a positive reaction by turning its head in the direction of the stimulus, which need not be in actual contact with the body (561). A planarian will follow an object such as the point of a pin moved in front of it, and one planarian will follow the trail of another that happens to come within the proper distance. Similarly, the neighborhood of food will cause the animal to turn toward it. Bardeen has suggested that the so-called ‘“‘auricular appendages,” two small movable prominences on the

Fic. 9. — Plaanimal’s back near the head end, which are _narian, dorsal specially sensitive to touch, may be “delieae cate organs capable of stimulation by slight currents in the water set up by the minute organisms that prey” upon the animal’s food; so that the positive reaction when given to food may be really a response to mechanical stimulation (20). As Pearl, however, found that chemicals, diffused in the water, would produce positive responses (561), it is probable that Plonaria

maculata is directly sensitive to chemical stimulation, though it responds thereto in the same way as to mechanical stimulation. A land planarian, Geodesimus bilineatus, is reported by Lehnert to perceive food at distances from four to five times the length of its body, and he does not describe the positive reaction as given in response to any other than food stimulation (417). _@ The food-taking reaction in Planaria maculata is made junder the influence of combined mechanical and chemical ism in contact with the pharynx or the ventral side of the animal. When an object which has occasioned the positive reaction is reached, the head folds over it and grips it, contracting so as to squeeze it. The substance being thus brought into contact with the pharynx, swallowing movements are produced if the proper stimulus is given. In Microstoma caudatum the organ of the chemical sense has been held to be sensory epithelium in the floor of the pharynx (398). Bardeen was inclined to think that contact with a soft substance constituted the proper stimulus, as he found that hard particles placed on the pharynx were not swallowed (20). Pearl, however, believes that mechanical and chemical stimulation must combine. The former alone does not suffice, for swallowing movements are not evoked when one planarian crawls over another; the latter alone is insufficient, for placing the animal in a sugar solution has no effect. If chemical and mechanical stimulation are united, the reaction is given whether the chemical is edible or not; Pearl found it occurring in response to sodium carbonate (561). Evidence of the influence of physiological condition upon the reactions of planarians is furnished by the fact that the resting planarian shows a decidedly lowered susceptibility to stimulation. Bardeen found that if the animal was not

already in motion, it gave no positive response to food in its neighborhood (20). Y § 18. The Chemical Sense in Annelids In our own experience, as has been said, the “food sense” is represented by the two senses, taste and smell, the stimulus for the one being fluid, and that for the other gaseous, so that the latter enables us to perceive objects at a distance. For water-dwelling animals, such as most of those whose behavior we have been describing, the distinction evidently cannot well be drawn. If such an animal per- “ ceives food at a distance, the stimulus is necessarily diffused through the water, and Lloyd Morgan has proposed the term “telesthetic taste” for the sense which makes such perception possible (504, p. 256). The term indicates that this sense corresponds to taste in an air-dwelling animal because the stimulus is fluid, but differs in that it allows perception of a distant object, as taste in the ordinary sense does not. In the most familiar representative of the Annelida or segmented worms, the common earthworm, as in the land planarian, a distinction analogous to that between smell and taste in our own sensory experience may be made; in the leeches and marine annelids it cannot. Gentle and continuous mechanical stimulation produces in the earthworm “positive thigmotaxis”’; that is, the animals have a tendency to crawl and lie along the surface of solids (686). That there is some discrimination of edible from inedible substances when in contact with the body Darwin thought probable from the apparent preference of the worm for certain kinds of food (171). In the earthworm Allolobophora fetida we find a differentiated response

in barnyard manure. When placed on scraps of shredded filter paper moistened with water they refuse to burrow; when the filter paper is wet with a decoction of the manure they burrow as soon as they come into contact with it. The adequate stimulus for burrowing is thus a combined mechanical and chemical one; the chemical stimulus alone is insufficient, for filter paper thus prepared has no effect on the worms unless they are actually in contact with it (686). Using the human terms, the case is one of taste rather than smell. Nagel suggests that the earthworm’s chief use for a chemical sense is to help it find the moisture which is necessary to its life (522); but curiously enough Allolobophora fetida seems to have no power of doing this from a distance. Smith found that a worm would crawl around a wet spot on paper until its skin dried, without crawling into it. If by accident it happened to touch the moist place, it would enter and remain there (686). Parker and Parshley (555) find that the head end of the worm is negatively stimulated by contact with a dry surface, and will withdraw soon after such contact. There seems no satisfactory evidence that worms respond. to chemical stimulation from a distance by positive reactions, although Darwin believed that they found buried food by ‘the sense of smell” (171). Chemical stimuli not in contact with the body do produce negative reactions (522), but these reactions do not differ from the responses to strong mechanical stimulation. They are of various forms — turning aside, withdrawing into the burrow if the tail is already inserted, squirming, and so on, the differences being correlated with differences in the intensity and location of the stimulus and in the excitability (physiological condition) of the animal. But nothing in the character of the response suggests that negative reaction

to a chemical stimulus has a different conscious accompaniment from that of negative response to a mechanical stimulus. The most natural interpretation of them all on the psychic side is that of unpleasantness, increasing in intensity as the reaction takes a more violent form.! The time occupied in reacting has, however, been made a basis for differentiating the response to different chemicals. It was found that if the worms were suspended by threads, and their anterior ends dipped into solutions of sodium, ammonium, lithium, and potassium chlorides, the animals reacted to these substances with diminishing promptness in the order just given. The differences in reaction time were marked. Nowall four of these substances produce in man nearly the same taste quality, salt, for which the common constituent chlorine is therefore held responsible. The sodium, lithium, ammonium, and potassium ions have apparently but little effect on the human taste organs. Since the earthworm reacts with decided time differences to the four, it may be that its taste organs are specifically affected by each, and that different taste qualities may be occasioned in its consciousness, supposing it to be conscious (554). Kribs (410) has obtained evidence of localized chemical sensibility in the annelid Z£olosoma; weak chemicals would produce a reaction only if applied to the sensory hairs of the head end. Leeches

1W. W. Norman argued that the squirming reactions of worms, and the corresponding reactions of other animals to injurious stimulation, cannot be taken as evidence of an accompaniment of disagreeable consciousness, because of the fact that when the worm, for instance, is cut in two, the squirming movements are confined to the posterior piece, while the head end crawls away undisturbed. The head end, he urges, containing the cerebral ganglia, ought to be the part capable of suffering, but it gives no reaction (525). We cannot, however, conclude from the absence of a re-

action under abnormal conditions that when it occurs in the normal state it has no conscious accompaniment. seem to be excited to their feeding reaction by a combination of mechanical and chemical stimulation. They are very sensitive to slight water disturbances, and react by stopping the respiratory movements if a needle is touched to the surface of the water above them. Food juice diffused through the water makes them very active; while in this state they will attach themselves to a glass rod, but drop off at once. When they attach themselves to food substance, however, they hold on with traditional tenacity. Chemicals of various kinds produce withdrawing reactions, and Lihner (438) finds evidence that leeches experience “‘ taste compensation.” When we have to eat sour fruit we cancel the sour sensation by putting sugar on the fruit. A five per cent. sugar solution produced withdrawing reactions in a leech, but if the sugar solution was mixed with a nine per cent. salt solution, its strength had to be raised to seven and five tenths per cent. before the leech reacted to it.

Aa. ; ca § 19. The Chemical Sense in Mollusks In the case of the Mollusca there is little satisfactory evidence on the subject of the chemical sense. The Acephala, to which the clam, oyster, and scallop belong, do not take food by active movements; hence, of course, they can have no specific feeding reactions. Chemical sensibility, distributed over the surface of the body, has been observed in lamellibranchs, a branch of the Acephala (522). Gasteropods, including snails and slugs, have, owing to their active food taking, more use for a chemical sense; in marine snails it seems rather definitely localized in the feelers (522). Yung found in the snail Helix pomatia that smell was most acute at the end of the feelers, but that the animal even when deprived of its

feelers could distinguish perfume. Taste he found best developed near the lips, and touch sensibility distributed over the body, but especially toward the end of the feelers Of two freshwater snails, Physa and Lymnea, the latter, whose movements are slower, can sense food at a greater distance than the former. In Physa an interesting relation between the chemical and mechanical stimulation produced by contact with food is apparent. ‘‘If Physa,”’ says Dawson (177), ““was moving at a moderately rapid rate when it came in contact with the meat, it received a sufficiently strong stimulus to cause it to turn away, to pause and then turn back. It would seem that the mechanical stimulus was not only sensed first but obeyed, and then the chemical stimulus was in turn sensed and obeyed.” The limpets Patella and Calyptrea respond to the neighborhood of non-irritating oils by withdrawing reactions (583). Irritating chemicals, of course, are not proper olfactory stimuli, but one can hardly be sure that a stimulus which like oil of bergamot would be non-irritating to the human mucous membrane, is non-irritating also to

the body surface of an animal. Mollusks in general seem to have chemical sensitivity distributed all over the body surface, although certain regions are especially sensitive. -Piéron (585) finds in marine snails three modes of chemical excitability: an aerial distance excitability, on all parts of the body with predominance of the mouth, the anterior edge of the foot, and the siphon; a contact sensibility in both air and water, on the mouth, the horns, and probably elsewhere; and a delicate distance | sensibility in the water, located in the regions of the mouth, the horns, the anterior edge of the foot, and the osphradial region.

In the phylum of the echinoderms, under which are classed starfish and sea-urchins, the “circular symmetry” of body structure characteristic of the ccelenterates reappears. Starfish were found by Romanes many years ago to show, besides pronounced negative reactions to strong or injurious mechanical stimulation, what he called a sense of smell. Its manifestations depended on the physiological condition of the animal; that is, upon its degree of hunger. If kept several days without food, a starfish would immediately perceive its presence and crawl toward it. “ Moreover, if a small piece of the food were held in a pair of forceps and gently withdrawn as the starfish approached it, the animal could be led about the floor of the tank in any direction.”” By cutting off various parts of the rays, Romanes found that ‘‘the olfactory sense was equally distributed throughout their length”; and he also showed that the ventral and not the dorsal surface of the body was concerned, by varnishing the latter, which left the reactions unaffected, and by observing that when a bit of food was placed on the back it remained unnoticed (642, pp. 321-322). Preyer reported great individual differences in the responses of starfish to food stimulation; while certain specimens were unmoved by the neighborhood of food, an individual of another species came from more than six inches away and fell upon it (617). Whether the unlikeness of behavior was due to the species difference or to a difference in the degree of hunger does not appear. In the holothurian Thyone briareus feeding movements could not be produced by external stimuli, and apparently result from the internal state of hunger (565).