The Animal Mind: A Textbook of Comparative Psychology

That the semicircular canals in fishes have a static? function has been shown by experiments to be described later. {Is the fish ear also an organ of hearing? Again authorities disagree, and it is probable that species differ) Kreidl got no response from goldfish when vibrating rods were placed éither in the water or in the air near the water. Only when the fish were made more sensitive by strychnin did they react, and only to noise, not to tone. They reacted quite as well, moreover, when the ears were removed; whence it was concluded that their sensitiveness to noise resided in the skin (408, 409). A similar negative conclusion regarding auditory sensation has been reached by F. S. Lee (416), by O. Korner as a result of experiments on twenty-five species (404), and by Marage (460 a), using

1The word “static” is here used to mean “relating to equilibrium” in general, not to static equilibrium as distinguished from dynamic equilibrium. vowel sounds sung on notes ranging from C2 to Gg, transmitted through rubber tubes, the tests being made on eight species. (On the other hand, Bigelow found that the goldfish on which he experimented were sensitive in their normal condition, but insensitive when the auditory nerves were cut, and thinks that Kreidl’s operation did not remove the whole of the fish’s ear (54).) Triplett thought both perch and goldfish were excited by the sound of whistling, which usually preceded their being fed (720).! Parker tested the killifish, a species of minnow, using the sustained slow vibrations (40 complete swings per second) of a bass viol string placed on dne side of the aquarium as a sounding board. The fish cage was suspended in the aquarium from an independent support. Normal fish responded to the vibrations, usually by movements of the fin, 96 per cent. of the time. Fish in which the nerves to the ears had been cut responded in 18 per cent. of the tests; those in which the skin had been made insensitive, but the ears left, in 94 per cent. Since causing the string to vibrate jarred the whole aquarium somewhat, these experiments were checked by others where the stimulus was produced by placing the stem of a vibrating tuning fork against the sounding board. The results were the same as in the first set of tests.

(Parker concludes that the ears of the minnow are certainly organs for the reception of sound; but as he obtained no such reactions from dogfish, he is inclined to think that different species vary (535, 536).} In later experiments (544) on the dogfish, Parker finds that individuals with the “auditory” or eighth nerve cut show diminished sensitiveness to the blow of a pendulum, the force of whose impact on the walls of the aquarium could be measured, while cutting the optic nerve or cocainizing the skin has no effect on the responses to these stimuli: (his conclusion is that the reactions are

auditory.) In the squeteague (542), he infers from the results of operation that one part of the ear, the utriculus, functions in the maintenance of equilibrium, while the other part, the sacculus, is the organ of hearing. The otoliths, or statoliths, in the ears of the squeteague and dogfish Parker thinks have actually an auditory function, contrary to what is known of their use in invertebrate animals; when they were removed from the ear of the dogfish, he reports, there was no disturbance of equilibrium, but a reduction in the reaction to blows on the aquarium wall, and when the large otolith in the sacculus of the squeteague was pinned down, a similar result was obtained. Most sounds made in the air are extremely faint under water, but to sounds really propagated through water, Parker thinks many fish are sensitive. Certain sounds may actually attract them: the squeteague, for instance, itself makes sounds which may serve to bring the sexes together. Tests by Zenneck on Leuciscus rutilus, L. dobula, and Alburnus lucidus also led to the conviction that these fish, at least, could hear. A bell was struck by electricity under water, and occasionally a piece of leather was placed upon it at the point where the clapper struck. In the latter case the mechanical vibrations produced were, it was held, the same as those occasioned by the actual ringing of the bell, but the sound vibrations were destroyed. The fish reacted by swimming instantly away from the neighborhood of the bell when it was rung, but not when the leather was used; hence, apparently, they reacted to sound (840). These experiments, however, have been repeated on trout and eels by Bernoulli (43) with negative results.

(Widely distributed among fishes is a curious set of structures known as the lateral-line canals. Along each side of the fish, extending from head to tail, there is a row of pores opening into a long canal, which at the head divides into three branches, one going upward above the eye, a second below the eye, and a third down toward the lower jaw. The functions of these canals have given rise to much discussion among zoélogists, an exhaustive history of which will be found in Parker’s monograph entitled “The Function of the Lateral-line Organs in Fishes.” Parker first proved experimentally that the canals played no part in responses to the following stimuli: light, heat, salinity of the water, food, oxygen dissolved in the water, carbon dioxide, foulness of the water, hydrostatic pressure, steady currents flowing through the water, and sound. (When, however, the water in the aquarium was made to vibrate slowly, about six times per second, the fish made certain characteristic reactions, differing somewhat for the four or five species observed, but always failing to appear when the lateral-line nerve was cut.) Parker concludes that ‘the stimulus for the lateral-line organs (a water vibration of low frequency) is a physical stimulus intermediate in character between that effective for the skin (deforming pressure of solids, currents, etc.) and that for the ear (vibrations of high frequency), and indicates that these organs hold an intermediate place between the two sets of sense organs named” (539). The ear is thus regarded as actually derived from the lateral-line canal, as this in turn was derived from the skin. We may suppose that at least three different sensation qualities result from stimulation of the skin, the canals, and the ear, where hearing can be shown to exist{

Hofer (326 a) criticizes these experiments on the ground that when Parker cut the lateral-line nerves he also destroyed the nerves supplying the skin of the head, a particularly sensitive region to touch stimuli. It is, according to Hofer, the skin nerves that are affected by the slow vibrations which Parker thought to be the proper stimulus for the lateral-line organs, and in certain cases he demonstrated that such stimuli were responded to when the lateralline organs had been destroyed. E The true function of the lateral-line organs Hofer finds to be that of response to streaming movements in the water>) A skin sensitiveness to currents would be of the greatest practical value in guiding the fish’s migrations. -

Emergence from the water, on the part of adult Amphibia, is accompanied by disappearance of the _lateral-line canals, and consequently of whatever sensations these mediate. In the frog, the ear has a tympanic membrane lying at the surface of the head. A single bone, the columella, with one end against this membrane, lies across the middle ear. The internal ear is not essentially different in structure from that of the fish; there is no cochlea. Yerkes has made an interesting study of the reaction of frogs to sound. He found that they occasionally “straightened up and raised the head as if listening”? when other frogs croaked or made a splash by jumping into the water. To no other sound did he get any apparent response, nor was it possible to make frogs in their native habitat jump or show any uneasiness by producing any sort of noise, so long as the experimenter remained invisible. ‘Apparently,’ Yerkes says, ‘‘they depend almost entirely upon vision for the avoidance of dangers.” It is of course highly improbable that an organ should be adapted only to the reception of the croaking of other frogs and the splash of water, and not to noises made

in imitation of these; and Yerkes suggests that the frogs may hear many sounds to which they respond _by inhibiting movement as a measure of safety. This view is confirmed by the results of experiments where the breathing movements of the frog’s throat were registered by means of a lever resting against it and recording on smoked paper. Evidence from change of the breathing rate was obtained of the hearing of sounds ranging from fifty to one thousand single vibrations a second (807). Later, it was shown that sounds, although they did not, when given alone, cause the frogs to react, modified the responses to other stimuli, reinforcing or inhibiting them according to the interval between the sound and the other stimulus. This effect was noticed both when the frogs were in the air and when they were under water. It was more marked in the spring (the mating season) than in the winter. That it concerned the special auditory sense-apparatus, and hence may have been accompanied by true auditory sensations, was shown by the fact that it disappeared when the auditory nerves were cut. Sounds ranging from fifty to ten thousand single vibrations a second were effective (817, 815). This, of course, does not mean that the frog perceives such sounds as differing in pitch.

The reptilian.ear does not differ markedly from that of amphibians. The writer knows of no experiments upon the sense of hearing in reptiles. The cochlea,the organ of hearing in mammals, is still imperfectly developed in birds. But if we grant that animals which produce sounds are capable of hearing them, some birds at least must be able to make_pitch-discriminations of wide range and great acuteness. The powers of imitation so often evidenced in bird song are proof that this is the case. Craig (157,158) has carefully observed the social significance of a great variety of sounds made by pigeons, but gets little evidence that these birds learn new sounds by imitation.

Extremely significant are Hunter’s (353, 354) experiments on the hearing ability of the white rat. Their net result is that these animals can hear only noises, not tones. None of the rats he tested was able to hear a tuning fork tone; the evidence is that they were unable, under the stimulus of both punishment and reward, to learn to turn to the right when the tone was sounded and to the left when it was not sounded. They could perfectly well acquire such a habit when the noise of clapping the hands was substituted for the tone. They could not form the habit when two forks of different pitch were sounded together as a signal to turn to the right. They acquired the habit of making the proper response when the tones of a whistle were substituted for the fork tones, but it was clear that they were really responding to the noise of the rush of air through the whistle, for they would react equally well when this noise was substituted for the actual blowing of the whistle. Moreover, they broke down in their choices when the whistle was sounded in another room, although they were not disturbed by the mere diminution of intensity in the sound of the whistle sounded near at hand; the natural inference is that removing the whistle to a distance made the noise accompanying its tone inaudible. In short, there seemed to be, for these rats, no difference between the sound of a pure tone and entire silence. Confirmatory results appear in

1 Interesting evidence of this power in a bird which might not have been supposed to possess it was obtained by Conradi, who found that English sparrows reared by canaries acquired recognizable bits of the canary song Barber’s (19) experiments on the white rat’s ability to localize sounds: noises, such as those made by tapping on wood, were localized within an average limit of error of from two to four inches, but tuning-fork and organ-pipe tones were wholly ignored.

The experiments of Johnson (384) on the dog’s ability to discriminate tones and noises gave results very similar to these of Hunter on the rat, and furnish an illuminating commentary on certain difficulties in experimentation on animals. Zeliony (839), working by the salivary reflex method described on page 57, had reached the conclusion that the dog can discriminate between tones whose pitch differs by only a ter of atone. Kalischer (388), whose interest lay in testing the work of Munk! on the localization of the central terminations of the sensory pathways for tone in the temporal region of the cortex, succeeded in training dogs, with and without temporal lobes, to snap for food when one tone was sounded and inhibit reaction when a tone of considerably different pitch was given. Rothmann (646 a) and Swift (699) also observed discrimination of tones in the dog, and Kalischer (388) claims for the dog memory of absolute pitch. But the experiments of all these investigators, including those who used Pawlow’s method, suffer from the fatal defect that the experimenter was in the room with the animal tested, and hence might have presented other clews, by making slight involuntary movements, which could act instead of the tones to guide the animal’s choices. How serious this objection is appears from Johnson’s own results. His dog subjects all learned to discriminate between a tone of 256 double vibrations and one of 384 double vibrations, an interval of a fifth, whether the tones were sounded on tuning forks or on a wind appara-

1 Munk, H., 1890. Ueber die Funktion der Grosshirnrinde. Berlin. tus (the Stern tone variator); and to discriminate between a chord containing one of these tones and a chord containing the other tone. This was when the experimenter remained in the room. Experiments by a much more accurate apparatus, where electrically driven forks were sounded from another room, while the experimenter observed the dog also from an adjoining room, the tones being sounded with exactly the same duration, gave precisely opposite: results : the dogs could not discriminate. (The test of discrimination was learning to turn to the right when one tone was sounded and to the left when the other tone was sounded.) Moreover, they could not even learn to take one turning when a tone was given and the other when no tone at all was given; apparently if they heard the tone at all they paid no attention to it. On the other hand, the noises of two electric buzzers, of different intensity, pitch, and timbre, were readily discriminated and localized by the dogs. The same objection, that secondary clews derived from the presence of the operator may account for the seeming discrimination of sounds, applies to the work of Shepherd on cats (677, 678) and raccoons (676). The apparent fact that certain mammals are deaf to tones, while perfectly able to hear noises may, as Johnson suggests, be connected with the fact that even human beings cannot localize pure tones with any accuracy: a sound stimulus, to have practical significance, must be capable of being localized. Zeliony (839) trained a cat to come from one room into another when a C’ whistle was blown, and thought he had evidence of the cat’s ability to distinguish the sound of this whistle from that of others differing not more than a half-tone; but the difference reacted to may have been in the accompanying noises. Hahn (282) finds the bat very sensitive to high-pitched sounds, but not to low ones.

In this chapter we shall omit all references to the function of vision as a spatial sense, that is, as giving rise to perceptions of form, size, distances, and direction. It appears that light may act upon living beings either as a continuous or as an interrupted stimulus. That is, light maintained steadily at a constant intensity produces responses in organisms, and there are also reactions when the intensity of the light is suddenly altered, in the direction either of brightening or of darkening. Most physical forces act as stimuli only when they change in some way: an unchanging environment fails to call forth response. We may briefly survey the facts which point to the existence in animals of reactions to changes in light intensity.

Among the Protozoa, Amceba, when subjected to sudden changes in light intensity, checks its movement at the point where the light falls. This is just what happens when a mechanical stimulus is applied, and offers no evidence of a specific light sensation (378). Similarly, various ciliate and flagellate Protozoa give their ordinary negative or avoiding reaction to changes in light intensity; some of them make it on passing from a region of less to one of greater illumination, and thus “‘seek” the darker regions, while others give it when undergoing a change in the reverse direction, and thus tend to remain in lighter regions.

But if nothing distinguishes the negative reaction to photic stimuli from the negative reaction to any other stimulus, then nothing shows the existence of a sensation quality peculiar to the effect of light — unless a special receptive apparatus can be demonstrated. In a flagellate Protozoén called Euglena, a pigment spot exists near the anterior end. Now although pigment apparently is not, as Hesse (323) has emphasized, a necessary constituent of visual organs, yet its occurrence always suggests some relation to light, as it is essentially a kind of matter having the property of absorbing light. Euglena gives the negative reaction on entering a shadow. Is its pigment spot really an ‘‘eye spot” and concerned in this response? Ap- parently the reaction occurs before the pigment spot has entered the shadow, and as soon as the transparent tip lying in front of the pigment spot has been pushed into the shaded region (204). It is uncertain, then, what the function of the pigment spot is. But in another organism, which is structurally intermediate between the singlecelled and the many-celled forms, pigment spots do play a role in light reactions. This organism is called Volvox, and it is really a colony of globular flagellates, each with its flagellum turned outward, and each with an “‘eye spot.” Very weak light has no effect on the movements of Volvox; moderate light causes movement toward the source of light, and very strong light causes movement away from the source (332). Accurate observation of these movements indicates that the eye spots are essential to them; each individual responds to a change of illumination of its eye spot (464). This much evidence, then, we have that if Volvox possesses consciousness, changes of light intensity produce in it a specific sensation.

tensity is found, although in the hydroid colonies of Tu- bularia it appears to be wholly lacking (564). Many sea-anemones are wholly unaffected by light stimulation, Sagartia luciae and Metridium, for example (286). Many others have been observed to contract when the light intensity is increased (266, 374, 521). loactis producta expands its tentacles only in light of low intensity, taking about fifteen minutes to do so when covered with a hood, and retracting in five minutes when the light is restored. This retraction is decidedly slower than that produced by mechanical stimulation (286); thus we have some evidence that it is accompanied by a specific sensation quality. That the responses to light are more marked in animals which have been living in comparative darkness than in those taken from illuminated spots, has been shown both for sea~anemones and for Hydra (228).

Many Meduse or jellyfish also react to light more slowly than to other forms of stimulation. It is true that on Sarsia, a form tested by Romanes many years ago, light seemed to act as quickly as any other stimulus. If a flash of light were allowed to fall on the animal while it was moving about, “prolonged swimming movements”’ ensued; if it was at rest, it gave only a single contraction — another instance of the effect of physiological condition upon reaction. Sudden darkening produced no reaction, whence Romanes concluded that “‘it is the light per se and not the sudden nature of the transition from darkness to light which in the former experiment acted as the stimulus.” There are, however, as we shall see, other animals in which an increase of illumination brings about response where a decrease fails, and vice versa. When a beam of light was thrown into a bell-jar containing many Sarsia and placed in a dark room, ‘they crowded into the path of the beam

and were most numerous at that side of the jar which was nearest the light.” ‘‘There can thus,” concludes Romanes, “be no doubt about Sarsia possessing a visual sense”’ (641, p. 41). But as these reactions are not differentiated in any way, they cannot be taken as evidence of a specific sense, unless indeed they depend on a specialized sensory structure. This latter Romanes found to be the case; Sarsia has pigment spots on the margin of its bell, and its response to light ceased when these were destroyed. Tiaropsis, another jellyfish studied by the same observer, gave further evidence of “‘a visual sense” in the fact that it responded to light more slowly than to mechanical stimulation. In Gonionemus, both difference in reaction time and dependence of response on a special organ indicate that light may produce a specific sensation, always granting the presence of consciousness. Yerkes found that this jellyfish, unlike Sarsia, reacts in the same manner in passing either from sunlight to shadow or the reverse. In both cases it stops swimming and sinks to the bottom. A _ sudden change of illumination, therefore, checks its activity. On the other hand, if when the light falls upon it the animal is at rest, it becomes active again; but sudden decrease of illumination has no effect upon the resting animal. The inhibitory effect of strong light falling upon the jellyfish while in motion Yerkes explains as a special adaptation. For one case of such increase of illumination occurs when the animal swims, bell upward, to the surface on being disturbed; the light of the surface is of course normally stronger than that in the lower regions. The inhibition of activity resulting causes the animal, after turning over, to sink slowly, bell downward, with expanded tentacles.

This is a position that gives it a better chance of catching food and carrying it to the lips than is offered by the right-side-up posture, where food would have to be carried downward against the upward current occasioned by the sinking of the animal. Light is not the only factor in producing the inversion at the surface, however, for it will occur in darkness. When swimming, Gonionemus moves toward the light if the latter is fairly intense, but comes to rest in the shaded portions of the vessel containing it. The reaction time to light is much slower than that to other stimuli, but the animal responds most promptly when certain pigmented bodies at the base of the tentacles are exposed to the stimulus. If the margin of the bell containing these bodies is cut off, no reaction to light can be obtained (802, 809, 825). A great variety of structures apparently sensory in function is found on the bell margin of different genera and species of Meduse. Some of them are statocysts. Others suggest a visual function, and in the Cubomedusz there are fairly well developed eyes. Various annelids show response to changes in light intensity, the leech Clepsine, for example: the slightest shadow cast on the surface of the water in a dish where these animals are resting quietly will cause them to reach up and sway from side to side in an apparent search for prey (785). On the other hand Gee (256) says of the leech Dina microstoma that the casting of a shadow on it makes it contract. This is apparently the more primitive and the more common type of response to a change in light intensity. Dinacontracts in just the same way when mechanically jarred, but a difference in the physiological process_involved is indicated by the fact that these leeches get used to repeated shadows, and cease to respond, much more quickly than they get used to repeated jars. When the earthworm has partially emerged from its burrow, and has its tail still inserted, a flash of light will produce

quick withdrawal into the burrow (171, 327), but the reaction time to light is much longer than that to mechanical stimulation. The part of the earthworm’s body affected by the light also influences the reaction. Darwin indeed reported that the worms withdrew into their burrows only when light fell on the head end (171), but decapitated worms were found by Graber to respond to light like normal ones, only less strikingly (266), and Yung (833) obtained evidence that sensitiveness to light is distributed over the body. According to Hesse the anterior end of the worm is most sensitive, the tail next, and the middle region least (316). Not only the region, but the amount of body surface affected, makes a difference. When the whole length of the worm was illuminated, the percentage of reactions was to that obtained where the front third only was involved as 26 to 10.2, while the relative occurrence of responses where the middle third and the posterior third alone were stimulated is represented by the figures 2.4 and 1 respectively (552).

In many of the marine worms well-developed eyes exist, although not such as are capable of giving clear images. Their function seems to be chiefly that of receiving stimuli from shadows. Many tube-dwelling worms will withdraw into their tubes if a shadow is cast upon them (285, Turning to the molluscs, we find that the siphons of the Acephala, which are projected from the shell to take in currents of water containing nourishment, are withdrawn in response to sudden darkening in some cases, to sudden illumination in others, and in still other instances to either (195, 520, 650). The danger of arguing the existence of sensory discrimination from structure alone is well shown in the case of snails, for although many of them have eyes

of some degree of development, these very species have been shown to be devoid of sensitiveness to light (836, 837). “‘Skioptic” reactions, or reactions to shadows, appear among various echinoderms. The sea-urchin Centrostephanus longispinus, for instance, which lacks even a rudimentary eye spot, will when a sudden shadow falls upon it direct its spines towards the shaded side. The reaction time involved is decidedly longer than that to mechanical stimulation, and moreover, although pieces of the animal will react to the latter, responses to shadows depend on keeping the system of radial nerves intact. (This observation, according to Cowles (155), does not hold for the sea-urchin Toxopneustes.) Hence Von Uexkiill, who made the above observations, concluded that a special set of nerve fibres is concerned in photic reactions (735).

Dubois had suggested from studies on the mollusc Pholas dactylus, that in such cases the pigment changes which occur, under the influence of light, over the surface of the body, furnish the stimulus (195), but Von Uexkiill thinks this impossible, as the light reactions occur before the pigment changes do. This migratory pigment, he believes, acts merely as a screen; the source of excitation for the optic fibres may lie in another pigment which he has extracted and found very sensitive to light (735). Centrostephanus, according to Hess (313), shows pigment changes when the light is decreased by a very slight amount, just enough to be perceptible to the human eye.

Starfish have pigment or eye spots on the arm-tips. As a tule, they seek light: Romanes (641) and Tiedemann (710) report that the light reactions are abolished if the eye spots are removed. MacCurdy (453) finds, however, that in Asterias forbesii the light reactions are independent of eye spots: Cowles (156) has shown that Echinaster will react to light without eye spots, although some evidence of dependence on the sense organ is indicated by the fact that the response is slower; Plessner (607) holds that skin sensitiveness is responsible for reactions to light intensity, and that the eye spots enable the animal to respond to the direction of the light; Cowles (155), again, observes that when pieces of starfish and sea urchins are cut off, their tentacles and suckers still move in response to the casting of a shadow.

Among crustacea, which are provided with a peculiar visual organ, the compound eye, to be described later, the chief function of the eye seems to be that of responding to ’ shadows and movements. Bateson, watching shrimps and prawns, noted that they apparently could not see their food when it had been taken from them and lay near at hand, but quickly raised their antennae when an object was passed between them and the light (24). The little fairy shrimp, Branchipus, will stop swimming as soon as the edge of a shadow falls upon it. Skioptic reactions in the family of Cirripedia, to which the barnacles belong, ‘were noted by Pouchet and Joubert in 1875, as well as the fact that those individuals which were attached to rocks, where a sudden shadow might mean danger, reacted, while those attached to floating objects, and therefore normally exposed to light fluctuations, did not (615).

When we come to animals with well-developed eyes, the specialized response to changes in light intensity gives place to reactions involving the use of a more or less adequate image of the stimulus object. But as we have seen, the most primitive type of reaction to a sudden change of light intensity is the checking or inhibiting of the animal’s movements. This type of reaction is called by Loeb “sensibility to difference.” In many cases its relative

slowness, as compared with similar reactions to other stimuli, or its dependence on a special organ or region of the body, give evidence that it is accompanied by a specific sensation quality. When light stimulates not through its change of intensity, but through its action as a constant force, its effects are apparently of two kinds. One of these is the phenomenon which Loeb (433) calls the tropism, from the Greek word meaning ‘“‘to turn.” In the tropism, the organism takes up . a definite position with reference to the action of a force. The phenomenon is therefore connected rather with the ‘spatial < aspect of the sense of sight than with its qualitative aspect, and we shall consider it in a later chapter. The other type of effect caused by light as a constant stimulus is that it_stimulates or inhibits the general activities of the animal. Thus some animals are restless in strong light, others in darkness. As a result of this influence, the former _ tend to form collections in the dark, where they remain quietly; the latter in the light. This influence of a certain intensity of light to stimulate to activity we may call its kinetic effect, or photokinesis. The ccelenterate Hydra, for example, has a disposition to come to rest in the more illuminated parts of the vessel containing it (719, 791). Very strong light, however, makes it wander about until it happens to reach a more shaded region. Thus if the animal is subjected to light either above or below a certain ‘optimum ” of intensity, it is restless. A vague uneasiness is the kind of psychic accompaniment to this behavior most naturally suggested. Since repeated strong mechanical stimulation also will make the animal wander, nothing

points to the existence of a specific visual quality in this consciousness. The medusa Gonionemus is less active in darkness than in light, and comes to rest in darkened regions, where it thus tends to collect (802). Such collections are evidently not due to a definite choice on an animal’s part. On the planarian, the general effect of light stimulation is kinetic; it comes to rest in the shaded portions of a vessel (20, 425, 429, 317). Decapitated and hence eyeless planarians respond to light, but their reactions are delayed (432); thus there is a certain amount of dependence on the visual organ.

Photokinetic effects seem to be common among insects, many of which, the house fly, for example, and the mason wasp (728), are active in light and sluggish in darkness. These animals are naturally so much more active than Hy- dra and planarians that we do not find them forming collections in the regions where they can rest; they seem able to continue in rapid motion for long periods, and it is rather a pleasurable than an uneasy activity that is suggested by the aérial dances of insects in the sun.