In the experiments on the ciliates and flagellates set forth in the present paper the precise method of reaction was determined by obser- vation. It is not in accordance with the tropism theory above set forth. This has been emphasized in detail in the account of the reactions of Stentor, so that it need not be reiterated here. The reac- tion to light is of the same character as that to other stimuli, and takes the form of a motor reaction in which the organism performs a definite set of actions. It first usually stops or swims backward, then turns toward a structurally defined side, then continues forward. The result is to change the course of the organism. As a result of the con- tinual rotation on the long axis, together with the swerving toward a certain side, the organism comes to be pointed successively in every direction. In continues to swim forward in that direction which does not induce a stimulus to further swerving. The whole reaction is a strongly marked example of the type of behavior which may be called the *' method of trial and error." REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 65 NATURE OF AGENT CAUSING THE REACTION. (i) The primary and essential cause of the reaction is a change of illumination. The change of illumination must take place with some suddenness, but need not be very great in amount. The change in illumination acts as an effective stimulus even though the degree of illumination preceding the change and that following it would, when acting continuously, produce no such result. This is shown by the experiments on Euglena, in which the light coming from one side was decreased a certain amount. The orientation of the organisms and their direction of movement was the same before and after the change, but at the moment the change occurred there was a marked reaction. Other experiments detailed above demonstrate the same thing. Further, the change in illumination acts independently of the direction of the rays of light. This is shown by the experiment just cited, in which the effective direction of the rays of light was the same before and after the reaction ; it is also shown in the reaction caused when the light is decreased from below, in the case of Euglenae swimming toward a window (p. 50), and in the reaction of Stentor on passing from a shadow to a lighted region even when the animal is oriented with anterior end away from the light (p. 39). The change in illumination acts equally whether it affects the entire organism or only the anterior end. The evidence indicates that in all cases it is really the change at the anterior end which induces the reaction. (2) The absolute intensity of the light affects the reaction by deter- mining in a given case whether a reaction shall be caused by an increase or a decrease in illumination. Through this action it also determines, in the way to be mentioned in the next paragraph, whether in a continuous light the sensitive anterior end shall be directed toward or away from the source of light ; that is, whether the response shall be ''positive" or "negative." (3) Indirectly, and through the factor set forth in paragraph (i), the direction from which the light comes is a determining factor in the reactions. Through the spiral course in which the organisms swim such conditions are furnished that in a field continuously lighted from one side the sensitive anterior end of the unoriented organism is subjected to repeated changes in the intensity of illumination. As a result, organisms which respond by the motor reaction to an increase in illu- mination at the anterior end must become oriented with anterior end directed away from the light ; organisms which react to a decrease in illumination must become oriented with anterior end directed toward the light. (Details in the account of Euglena, pp. 60, 61, and Figs. 23, 24.) 66 THE BEHAVIOR OF LOWER ORGANISMS. The results of this method of reacting may be stated correctly, though not completely, as follows : In a negative organism light falling upon the sensitive anterior end causes a reaction by which the anterior end is pointed in many different directions ; the reaction ceases as soon as a direction is reached in which the anterior end is pointed away from the light. In a positive organism the shading of the sensitive anterior end produces the reaction by which the anterior end is pointed in many different directions ; the reaction ceases as soon as the anterior end is no longer shaded.* The reaction is thus by the method of trial and error ; when stimulated the organism tries many different positions, till one is found in which there is no further stimulation. Consideration will show, I think, that the factors producing reaction to light in these lowest organisms are essentially the same as in higher ones, if man may be taken as a type of the latter. The factors are, as we have seen, variations in intensity of illumination, and, indirectly, the direction from which the light comes. It is possible that in man the latter factor works more directly than in the infusoria ; leaving this question out of consideration, the two factors are present in both cases. Consider a human being who reacts to light as a purely physical agent, not with regard to the associations which it brings up. In a dark space a gleam of light is pleasant and induces movement toward it. There is then a positive reaction with orientation, but the orientation is not due to the difference in intensity of light on different parts of the body, nor to its direct effect on the motor organs. The orientation is such as to keep the light shining on the more sensitive part of the body, the eyes. An excessively powerful light is unpleasant and induces a nega- tive reaction just as happens in Euglena ; the orientation is then such as to keep the more sensitive part of the body, the eyes, away from the light. Further, man is sensitive to a sudden change in illumination. A strong light bursting from the darkness, or sudden darkness in the midst of bright light, induces a marked motor reaction, and less striking differences may produce a response. Both in man and in Euglena the reaction likewise depends upon color ; but with this phase of the matter we are not at present concerned. When the factors above set forth are taken into consideration certain peculiar experimental results that have given rise to much discussion become clearly intelligible. I refer particularly to the experiments in which the direction of the light and the decrease in intensity of illumi- nation do not show the usual relations. Under ordinary conditions movement away from a source of light is movement into a region of less * This statement is incomplete in that it does not bring out the fact that it is a change from light to shade or vice versa that induces the reaction ; if this be understood, the statement is correct. REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 67 intensity ; movement toward the light into a region of greater intensity. In the well-known experiments of Strasburger (1878) and others, this condition is modified by passing the light through a wedge-shaped prism filled with a solution that cuts out part of the light. When a drop of water or a culture dish is placed beneath such a prism, and the latter is so situated that its surface is perpendicular to the light rays, the intensity of the illumination is greatest behind the thin edge of the prism, and thence decreases gradually toward the opposite end, while the rays of light all come directly from above. Under these conditions Strasburger (1878, p. 36) found that the positive swarm-spores remained equally distributed throughout the drop, not collecting at the lighter end. Now, the only difference between this experiment and the one illustrated in Fig. 11 of the present paper is that in Strasburger*s experiment the decrease in illumination is very gradual. We have seen above (p. 52) that a very gradual change in illumination produces no reaction. Hence the organisms may wander from one side of the drop to the other without reaction, the difference in illumination at two successive instants never rising to the necessary threshold of stimulation. If the relation of stimulus to reaction follows Weber's law, the result is just what we should expect, provided the change in illumination is sufficiently gradual. When the difference in illumination from above is great, Strasburger's own experiments (/. c, p. 33) show that the organisms do react On the other hand, Holt & Lee (1901), using a similar prism, found, under similar conditions, that the negative organism, Stentor, does, on the whole, tend to gather at the darker side of the drop. This shows that the difference in illumination between neighboring points in this particular experiment was not below the threshold of stimulation for the organism in question. If, as Holt & Lee sup- pose, a certain amount of light was reflected from the lighter end of the vessel, then the inclination to go to the darker side would be rein- forced by Stentor's tendency to turn when the light falls upon its anterior end (see p. 43). The fact that in Strasburger's experiments the organisms remained scattered throughout the drop seems to indi- cate that this reflected light played no part in his results. In another set of experiments Strasburger placed his prism over the swarm-spores in such a way that the light came obliquely from the direction of the thick end of the wedge. If the positive organisms now go toward the thicker end of the wedge, they pass toward the source of light, but into a region of decreased illumination ; if they go toward the thin end they pass away from the source of light, but into a region of higher illumination. Which will they choose.^ Strasburger found that the positive swarm-spores pass toward the 6S THE BEHAVIOR OF LOWER ORGANISMS. source of light, into the region of less illumination. But is not this exactly what we must expect? His former experiment showed us that under the prism the change from light to darkness was so gradual that it produced no effect on the organisms. Hence the direction from which the rays come is left to produce its effect alone, and it produces the usual effect. The organism reacts in the usual " trial and error'* way until the anterior end is directed toward the light ; then it moves in that direction. Incidentally it comes into a region of less intensity of light, though the decrease is so slight as to produce no effect on the organism. Parallel considerations hold for the negative organism. Under similar circumstances, if the variation in illumination is very gradual, it directs its sensitive anterior end away from the source of light (by the method of "trial and error") and swims to the opposite side of the drop, incidentall}- moving into a region of slightly greater (but " unperceived ") intensity of illumination. Under similar conditions, as we have seen in the experiment described on p. 39, if the decrease in illumination is marked, the animal swims back into the shadow, though in so doing it passes toward the source of light. Thus in Strasburger's experiments with the prism the difference in the intensity of light between neighboring regions has been made so slight that they are unmarked by the organism and have no effect upon it. We need not be surprised, therefore, that it reacts as if these differ- ences did not exist ; for the organism they do not exist. The reaction is in this case just what it would be in a higher organ- ism under similar conditions. Let us suppose that the light stimulates strongly the sensitive anterior end, the eyes, of a higher animal or man ; it causes pain in the case of man. There will be a tendency (i) to move into less illuminated regions ; (2) to turn the eyes away from the light. Suppose that the man is enclosed in a space into which the sun shines obliquely from above, and that the end from which it shines is a little less illuminated than the opposite end, owing to causes similar to those in Strasburger's experiment on the swarm-spores. Suppose that the man is at the end next the sun. He cannot know that the other end is more illuminated, for the only way this would be possible would be for the greater number of rays of light to meet his eye coming from that direction, while by hypothesis all, or a much larger number, of the rays are coming from the opposite direction. He will, therefore, turn his eyes away from the sun, and if he moves will move toward the end away from the sun. After having traversed some distance he may observe, if he is very discriminating, that he is as a matter of fact getting into a region of somewhat greater illumination, and may perhaps reason that the best thing he can do under the circumstances is to keep his eyes turned away from the source of light and move backward to the REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 69 less illuminated end. But this involves the capability of making fine distinctions, and a considerable degree of intelligence in deciding what to do under the peculiar circumstances. The experiment w^ith the swarm- spores shows that they are incapable of such fine discrimination, or that they are not sufficiently intelligent to know what to do under the circum- stances. They give no indication that they notice the greater illumina- tion after having passed to the end away from the light. Their action may be considered perhaps in a certain sense as a " mistake," but it is a mistake which even the highest organism would make, and which could be corrected only after experience of its results. The results of our study of the light reaction in ciliates and flagel- lates lead to conclusions which stand in sharp contrast with certain general conclusions in Radl's recent extensive and interesting paper on Phototropism (Radl, 1903). Radl reaches the somewhat extraordinary conclusion that light orients organisms by exercising an actual mechanical pressure upon them. This pressure necessarily disturbs the equilibrium of the body, which is then compelled to change posi- tion until equilibrium is restored ; the organism is then oriented. The orientation is a consequence of the interplay of two sets of forces, inner and outer ; these cannot be in equilibrium until the body has taken a certain position with reference to the pressure exercised by the light (/. c, pp. 151 fT.) The actual turning which induces orientation must be due to the action of a pair of forces (/. c, p. 148). One of these forces is the pressure produced by the light. Orientation produced in the manner described in the present paper for the reaction of ciliates and flagellates to light, and in the preceding paper for the reaction to heat, could of course not be brought about in the manner supposed by Radl. One of Radl's chief arguments for his view is that ''no observation thus far shows that the final orientation is attained by a trial or after an oscillation, but it takes place auto- matically"* (/. c, p. 141). The observations on ciliates and flagellates given in the present paper show conclusively that the orientation in these cases is brought about through repeated trials. In the statement quoted above Radl has over- looked certain other cases. Thus Strasburger, as we have seen (p. 59), states that after the direction of the light is changed Haematococcus becomes reoriented " nach verschiedenen Schwankungen" (Strasbur- ger, 1878, p. 24). Radl himself refers on a previous page (p. loo) to Strasburger's observation of the oscillating movement of swarm-spores under the influence of a variation in light intensity; Rothert (190J, *"Keine bisherige Beobachtung zeigt ferner, dass die schliessliche Orientie- rung etwa durch eine Priifung oder nach einem Schwanken erzielt wurde, sondern sie folgt automatisch." 7© THE BEHAVIOR OF LOWER ORGANISMS. p. 397) has called particular attention to this as a possible factor in the so-called phototropism. Radl also refers (p. 99) to Exner's view that in Copilia the movements of the eyes are in the nature of a trial (" abtasten") of the surroundings. RadPs statement, quoted above, can then hardly be considered strictly accurate, even leaving out of consideration the results set forth in the present paper. In many organisms, doubtless, the reaction to light is of that direct character assumed to be general by Radl. But it may be strongly doubted whether this is what we may call a primitive condition ; in other words, whether it does not involve more complicated internal processes than the reaction by " trial and error." In any case, I am convinced that a similar reaction to light by the method of " trial and error " will be shown to exist in many other organisms ; it is demonstrated, for example, in Rotifera, in the paper which follows the present one. Recourse will doubtless be taken to the usual refuge when a sharp concept has been defined to which the phenomena are not found to correspond ; the reactions of the ciliates and flagellates will be simply excluded from the tropisms and the definition of the latter maintained in all its pristine purity. Indeed, it may be questioned whether the reactions of infusoria (and Rotatoria) to light are not excluded from phototropism through the definition given by Radl on p. 140, what- ever the method by which they are produced. Radl says " Unter phototropischer Orientierung ist die Fahigkeit der Organismen zu velstehen, eine feste Einstellung der Achsen des gesamten Korpers in dem Lichtfelde einzunehmen." Since the ciliate or flagellate (or rotifer) revolves continually on its long axis, and swerves continually toward a certain side, it can hardly be said that the body axes have a " feste Einstellung" with reference to the light. In an explanatory paragraph Radl says that in orientation ''immer geht dann der Lichtstrahl durch die (morphologische) Symmetrieebene des Korpers" (/. c, p. 140) . This is certainly not true for the ciliate or flagellate (or rotifer), even leaving out of consideration the fact that in the former two groups the animals are usually unsymmetrical. If it be proposed, then, to exclude the light reactions of ciliates, flagellates, and rotifers from the concept of *'Phototropismus," one can only agree that this is necessary, in view of the definitions of that concept. But what is the value of a definition which excludes some of the chief phenomena on which the concept that we are attempting to define is based ? And what is the value of a theory that depends on such a definition and that can only be correct so long as we hold to this definition.? The phenomena themselves are, after all, the final refer- ence for testing the correctness of any definition or theory ; it is the observed phenomena that we are attempting to formulate and explain. REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 7 1 What we desire in the study of animal behavior is (i) a correct description of what occurs ; (2) an understanding of the relation of what occurs here to other phenomena, this constituting their *' explana- tion" so far as an explanation is possible. Whether the phenomena when correctly described and understood are found to fall under some- one's definition of a tropism is comparatively unimportant ; it is only after such correct description and understanding that final definitions can be made. I question much if there has not been undue haste in framing precise definitions for the phenomena of animal behavior, when we know so little about the phenomena in any thorough way. Radl, I believe, makes a fundamental error in attempting to separate " Pho- totropismus" rigidly from other reactions to light. Thus, he repeat- edly cites Euglena as an example of an organism that shows undoubted phototropism. On page 114 he further cites the motor reaction of Euglena when suddenly shaded * as a reaction that has nothing to do with phototropism. As I have shown above, the two are really closely bound up together ; the orientation in the "phototropism" is produced through this motor reaction. When the reactions of organ- isms to light are known in detail, I believe that many other reactions which Radl (p. 1 14) attempts to separate sharply from " phototropism " will be found closely connected with the reactions that go under that name. I had occasion to point out, in the paper preceding this, on the reactions to heat, that if everything which the organisms do, except the orientation itself, is left out of consideration, the orientation can be accounted for by any theory desired. A thorough study of precisely this point — the relation of "phototropism" to the phenomena supposedly unconnected with it— would, I believe, have saved Radl from marring his otherwise most excellent and useful contribution to the study of light reactions by the proposal of so fantastic a theory to account for the reactions to light ; a theory that fairly produces a shock in the mind of the reader when it is reached, coming as it does after Radl's thorough and valuable objective study of many of the phenomena and his exceed- ingly sane, if somewhat sharp, criticism of other theories. The thorough knowledge of the phenomena of animal behavior required for this is far from existing at present. * Radl says when " beleuchtet " ; this is evidentlj a slip of the pen. THIRD PAPER REACTIONS TO STIMULI IN CERTAIN ROTIFERA. 73 REACTIONS TO STIMULI IN CERTAIN ROTIFERA, In my series of " Studies on Reactions to Stimuli in Unicellular Organisms " and in the foregoing papers I have set forth the reaction methods of many infusoria to varied stimuli. The result has been to show that the reaction method in these organisms is of a peculiar character, differing radically from that required by prevailing theories of the reactions of lower organisms. The essential nature of these reactions, with their implications as to the character of behavior in the lower organisms, will be discussed in the following papers. Before proceeding to this discussion it is important to determine whether the reaction method in the Infusoria differs radically in character from that of Metazoa. For this purpose it seems well to select a group of Metazoa whose habitat and mode of life are similar to those of the Infusoria. In this way differences due primarily to the different plan of structure of the two sets of organisms may perhaps be brought out without the complications arising from different modes of life. A group of Metazoa much resembling the Infusoria in their mode of life is found in the Rotatoria. As is well known, the members of these two groups are usually found mingled together. They are of about the same size, and both swim about by means of cilia. So great is the resemblance in general habit and in habitat that they were at first classed together, all being given the name of Infusoria. As we know now, however, they are really widely separated in relation- ship. While the Infusoria are unicellular, the Rotifera are multicellu- lar organisms of a high degree of complexity, possessing many systems of organs, each composed of many cells. In particular, they have a well-developed nervous system. A comparison of the behavior of these two groups of organisms should show us, therefore, whether there are types of reaction having a high degree of generality, such as is claimed for the theory of tropisms — types that may give a key to the behavior of groups so widely separated in relationship as the two under consideration, which are representatives of the Protozoa and of Metazoa of a fairly high degree of organization. In the present paper I can attempt to give an account of the behavior of only a few free-swimming species, and that not in an exhaustive manner. I hope to return to an extensive study of the behavior of this interesting group, so as to develop its implications for the theory of 75 76 THE BEHAVIOR OF I.OWER ORGANISMS. animal behavior in general. In the study here set forth observation — -^^ was directed primarily to the questions of how certain Rotifera react under the stimulus of the agencies which usually give rise to the so- called tropisms — light, chemicals, heat, elec- tricity, contact, etc. — and to these questions the present account will be devoted. The species whose reactions were exam- ined belong chiefly to the loricate group of free-swimming Rotifera, and include a num- ber of species of the Rattulidae, several species of Cathypnadae, two or three species of Euch- lanis, Plcesoma lenticulare^ Anurcea cochle- aris^ and Brachionus pala. These were studied as opportunity offered. In most cases the reactions of any one species were not determined with relation to more than two or three classes of stimuli. The behavior of AnurcEa cochlear is was examined most fully. This species will be used as a type in describ- ing the reactions. I have already given a brief account of the general reaction type in certain species of the Rattulidae in my mono- graph of that group (Jennings, 1903). METHOD OF LOCOMOTION. The free-swimming Rotifera progress through the water in the same manner as the ciliate infusoria. The cilia in the Rotifera are limited to the anterior end, as they are in the peritrichous infusoria. It is interesting to note that the same device is adopted in the one group as in the other, to compensate for irregularities in the form of the body, etc., Fig. 25.* which might result in swerving from the straight course. This is by revolution on the long axis, causing the path to become a spiral with a straight axis. In the Infusoria the ♦Fig. 25. — Spiral path followed in ordinary swimming by Anurcea cochlearis Gosse, showing different positions of body in different parts of the course; a, dorsal surface; ^, left side; c, ventral surface; d, right side. The animal revolves on its long axis over to the right, thus taking successively the positions «, b, c, d, a, etc. The large arrow indicates the general direction of the course followed; the smaller arrows show direction of progression in certain parts of the course. REACTIONS TO STIMULI IN CERTAIN ROTIFERA. 77 organism usually swerves from the straight line toward the aboral side ; in the Rotatoria it is usually toward the dorsal side. Well-ordered forward progression would therefore not take place, were it not for the revolution on the long axis, converting the circular course into a spiral one. In the Rotifera the revolution on the long axis is, so far as observed, always over to the right. These relations have been brought out in detail in a previous paper by the present author (Jennings, 1901). The spiral path thus followed by most of the free-swimming Roti- fera may be illustrated in Fig. 25, for Anurcea cochlear is Gosse. As will be seen from the figure, the path followed depends upon three factors : (i) the animal continually swerves toward its dorsal side ; (2) it progresses ; (3) it revolves on its long axis. The result of these three factors is the spiral course. In all these relations the rotifer agrees with the infusorian. REACTIONS TO STIMULI. The most general reaction to a stimulus in such a free-swimming rotifer is an accentuation of one of the factors in this course, namely, the swerving toward the dorsal side. The result is to produce a spiral of much greater width than previously existed. This may often be observed when the vessel containing the rotifers is jarred. It is evi- dent that this method of reaction is fitted to enable the rotifer to avoid a small obstacle lying in its path, that is, in the axis of the spiral. When the animal resumes its former method of swimming the axis of the spiral lies in a new direction ; the course has thus been slightly changed. With a stronger stimulus, as when the rotifer strikes against an object lying in its path, the swerving toward the dorsal side may be still more pronounced, while the revolution on the long axis nearly or quite ceases. The result is that the organism swings strongly toward its dorsal side, and when the usual forward swimming is resumed the axis of the spiral lies in a totally new direction (Fig. 26). It thus avoids the obstacle, if the latter is small ; if the first reaction does not avoid the obstacle completely the reaction is repeated until the course is sufficiently altered so that the rotifer no longer strikes against the source of stimulus. In some rotifers the increased swerving toward the dorsal side is preceded by swimming backward a short stretch. In all these points the reaction of the rotifer agrees even to details with that of the ciliate infusorian. There is a difference in the fact that the Infusoria are unsymmetrical and cannot therefore be said to swerve toward the dorsal side, as do the prevailingly symmetrical Rotifera. In the Rattulidas, however, we have asymmetry of a char- acter similar to that found in the Infusoria. 78 THE BEHAVIOR OF LOWER ORGANISMS. We have dealt thus far specifically only with reactions to simple mechanical stimuli, such as are presented by an obstacle in the path of Fig. 26.* the rotifer or by a simple mechanical jar. This type of reaction under such conditions I have observed for Diurella tigris Miiller, D. por- ♦ Fig. 26 is a diagram of the reaction of the rotifer Anuraea to a strong stimulus, as when it reaches a source of mechanical stimulus or a region where some chemical is dissolved in the water. From a to ^ the animal is unstimulated, hence it follows the usual spiral course. At b it reaches the stimulating region, whereupon it turns strongly toward the dorsal side, following the arc of a circle, from b to d. Here it resumes the usual spiral course {d to e). The large arrow X shows the general direction of progression before the stimulus was received; the arrow J shows the direction of progression after the reaction has taken place. REACTIONS TO STIMULI IN CERTAIN ROTIFERA. 79 cellus Gosse, D. gracilis Tessin, and a number of other species of Rattulidae ; Plcesoma lenticulare Herrick, Cathypna ungulata Gosse, Monostyla bulla Gosse, Brachionus pala Ehr., Anurcea cochlear is Gosse, and A. aculeata Ehr. This method by no means exhausts the possibilities of reaction even to a simple mechanical stimulus in these species. They may retract the head and cease swimming, may creep over the surface of the object vv^ith which they come in contact, or possibly may sometimes turn other- wise than to the dorsal side when stimulated. Of this latter point I am, however, by no means sure. It is certain that the typical reaction, occurring in the great majority of cases, is that described above. REACTION TO CHEMICALS. The reaction given when the organism comes in contact with an area containing a rather strong diffusing chemical was observed in Aletopidia lepadella^ Anurma coch- lear is .^ A. aculeata^ and Diurella gracilis. The method of experi- mentation was as follows : A drop of water contain- ing the rotifers was placed on a slide. Near this was placed a drop of N/8 NaCl, and the two drops were connected by a nar- row neck. The behavior of the organisms as they came into the region of the neck and thus in contact with the salt solution was observed with the Braus-Driiner microscope. In the species mentioned the reaction was by a sudden turn toward the dorsal side, by which the path of the animal was directed away from the chemical. The reaction is thus of the same character as occurs in the ciliate infusoria. This manner of reaction to chemicals is in both these groups of organisms just what might be expected when the currents caused by the cilia are taken into consideration. In the ciliate, as I have shown Fig. 27. ♦Fig. 27.— Diagram of currents in a nearly quiet Anuraea, showing how a diffusing chemical or an advancing region of warmer water (represented by shad- ing), is drawn out by the ciliary vortex, so as to reach the mouth and the ventral surface before affecting other parts of body. So THE BEHAVIOR OF LOWER ORGANISMS. in a previous paper (Jennings, 1902, a) , the cilia cause a current coming from the region in front of the organism to pass along the oral surface to the mouth ; in this way the oral surface comes in contact with the chemical before any other part is aflected. It is not surprising, there- fore, that the organism should turn toward the opposite (aboral) side. In the rotifera the conditions are parallel to those found in the ciliate. The cilia cause a current, which passes to the mouth, on the ventral surface of the body (Fig. 27). The solution thus reaches the ventral surface first, and the reaction is, as might be expected, a turn toward the dorsal side. It should be distinctly stated that this reaction method is not universal in rotifers even toward chemical stimuli. In some of the larger species, bearing auricles, or with the ciliary apparatus of a very complex character in other respects, varied reactions may occur, which I hope to analyze in another paper. REACTION TO HEAT. This was studied in detail only in Anurcea cochlearis. A large number of the rotifers were mounted in a shallow trough formed of a slide, as described on p. 12, and one end of the slide was warmed by means of the apparatus shown in Fig. 5. The reactions were then observed with the Braus-Driiner stereoscopic binocular. As soon as a portion of the slide has been warmed above the optimum, the rotifers in this region turn more strongly than usual toward the dorsal side, so that the course followed becomes a very wide spiral and the animals make little progress. If the heat is increased the revolu- tion on the long axis ceases, while the animals swerve still more strongly toward the dorsal side (Fig. 28), so that they swim in circles, the dorsal surface being directed toward the center of the circle. Usually after circling thus a short time the animals begin again to re- volve on the long axis, and dart forward. The direction of this dart forward seems purely random. If it carries the animal out of the heated region the forward movement is continued and the animal escapes. If it does not carry the animal out of the heated region the circling toward the dorsal side is quickly resumed, followed by another dart forward. This is continued either until the rotifer passes out of the heated region or until it is overcome by the heat. Usually, if it does not escape soon from the heated region the circling becomes more rapid and continuous and is kept up till the animal is destroyed by the heat. If one end of the slide is heated and the animal approaches the heated region from the opposite end the reaction is of the same character as that last described. As soon as the region is reached REACTIONS TO STIMULI IN CERTAIN ROTIFERA. 8l wliere the heat acts as an effective stimulus the animal swerves strongly toward the dorsal side, thus beginning to circle, as shown in Fig. 28. If this swerving should continue only till the animal had described a semicircle, then were followed by the forward dart, the animal would of course retrace its original course (or one parallel to it), and would thus escape from the heated region, as happens in the reaction to the electric current (Fig. 29). But the reaction to heat is less precise than f Fig. 28.* this. Usually the animal makes several com- plete circles before darting forward, and the direction in which it darts seems a random one ; sometimes it is toward the heated region, sometimes away from it, sometimes oblique to it. If the path followed leads the animal into the heated region the circling toward the dorsal side, followed by the dart forward, is repeated ; while if the path leads away from the heat no farther reaction is caused and the animal escapes. Thus when a large number of the animals swim toward the heated region a considerable number will be seen a little later to swim away again. But in many cases the dart forward carries the animal still farther into the heated region. These specimens then begin to circle again toward the dorsal side, and if the temperature is high they may * Fig. 28. — Diagram of a reaction to heat in Anuraea. The unstimulated animal at first advances in the general direction shown by the arrow x, following thus the course a to e. The heat is supposed to be advancing from the direction opposite the arrow x. When the rotifer reaches the point e the heat becomes effective as a stimulus. The animal reacts by turning toward the dorsal side, and continues this so as to describe a complete circle,/", ^, A, /,/", etc. ; often it describes such a circle several times. Finally, at some point in the circular course, as _^, it resumes the usual spiral course, following thus the path ^, /, /. Its original course, shown by the arrow x, has thus been exchanged for a course having the general direction shown by the arrow v. $2 THE BEHAVIOR OF LOWER ORGANISMS. continue this till death intervenes. In many cases they repeat the dart forward and some escape in this way, while others do not. The reaction of Anuraea to heat is, therefore, not very precise, and many individuals swim into the heated region and are killed. Those which escape do so through a reaction which is similar to that of those which do not ; in the one case the forward movement carries the animal out of the heated region ; in the other it does not. The essential point to the reaction is that the animals when stimulated by heat change their course (through a " motor reflex"). This changed course nat- urally is an advantage, and in accordance with the laws of probability carries some of the organisms away from the source of danger. Others, likewise in accordance with the laws of probability, are carried even by the changed course toward the heated region, where they may be killed unless a repetition of the '* motor reflex" with its change of course carries them finally away. The reaction is by the method of" trial and error," and is not always successful. Altogether, the reaction of the rotifer Anuraea to heat is of a charac- ter similar in principle to that of Oxytricha (Fig. 7, p. 16). The direction of turning depends on an internal factor ; the reaction takes the form of " a motor reflex," and is by no means compatible with the typical tropism schema. REACTION TO LIGHT. In light, as I have already set forth in the account of the reactions of Stentor, we have a stimulating agent of a different character from that found in chemicals or in heat, since the distribution of the stimulating agent is not afl^ected by the currents of water produced by the motor organs of the animal. There is thus no reason in the distribution of the stimulating agent to favor a turning toward one side rather than the other. I have been able to study accurately the light reaction in but one rotifer, Anurcsa cochlearis Gosse. The conditions necessary for precise observation of the nature of the reaction are very difficult to fulfill, and the usual movements of the animals are such that the nature of the reaction is obscured. As will be recalled, the organism is normally swimming rapidly in a spiral, continually swerving toward its dorsal side. This in itself is very confusing when one attempts to observe just how the organism turns when stimulated. When light is thrown upon it, or when the direction of light falling on it is changed, the response is usually not given at once, and when it does occur, as we shall see, it may be in the form of an accentuation of certain features of the normal movement. From these conditions it results that it is exceedingly difficult to tell, after a reaction to light has clearly occurred, REACTIONS TO STIiVlUI.I IN CERTAIN HOTIFERA. S3 just how the reaction took place. Of course, only sharply defined posi- tive observations are of value in deciding between two opposing possi- bilities ; hence, although I have studied a number of other rotifers in this connection, I give the results only where absolutely sure of them. But in the two or three other rotifers I have examined in this connection the reaction is apparently the same as that in Anurcea cochlearis^ to be described at once. The specimens of AnurcBa cochlearis studied had been in a small aquarium in the laboratory some months, and were distinctly negative to light, gathering at the side of vessel farthest from the window. The freshly collected animals are, I believe, usually positive to light. These negative individuals were placed in a small flat-bottomed rectangular glass vessel, on a dark background, in a dark room. At opposite sides of the vessel and somewhat above were clamped two incandescent electric lights, A and B^ at a distance of about lo inches* in the manner described for Stentor (p. 41 and Fig. 15). One of these lights could be extinguished while the other was simultaneously turned on. In this way the direction of the light falling on the rotifers could be reversed. When only one of the lights, as A^ was turned on, the Anuraeas all collected at the opposite side of the vessel, next to B. When A was extinguished and B turned on, they turned and swam in the opposite direction, toward A. By reversing the direction of the light while the animals were crossing the vessel their course could be reversed while in full career. 