Chapter XXXI: Section IV: , 1. The experiments of Table IX, C, repeat those of A with (11)

_b. Reactions to Vertical Colored Light._ The same box was used as in the previous experiments, but the end was closed with a black cloth, and an electric light marked 32 c. but measuring only 22 c. was hung exactly over the middle of the box, 40 cm. from the bottom. By means of wires it was arranged that a plate of colored glass could be swung in such a manner that all of one half the box (the whole of one end) was illuminated with the desired color, while the other half was either left white or illuminated with another color. In this way there was a fairly sharp dividing-line between the two colors. The animals were observed at intervals of one minute for 40 minutes, and reset at the middle on the dividing-line every ten minutes as before. Table VII gives the results of the observations.

In this set of experiments it was possible to keep the groups intact except that in group 1 no. 13 had to be replaced by no. 46. If now we conceive the colors arranged in the order of the spectrum with black at one end and white at the other, and consider the black a lower stimulus than the white, we have the ascending series black, red, yellow, green, blue, white. Now since the animals have already been shown to be somewhat negatively phototactic, we should expect them to prefer a color of lower stimulus to one of higher. Turning to the sum totals in the table we find that the first color of each pair (which is always the lower stimulus) has the larger number of reactions in every case but one, the first pair of red-blue. As was stated above, it was impossible to secure colored light of the same intensity by means of the glass at our disposal, and in the present case the red was considerably brighter than the blue. Owing to the fact already mentioned that different intensities of white light seem to have no effect on the reactions it was thought that these differences in the intensities of the colored lights might be overlooked. Since the only thing that could be thought of to account for the anomalous behavior to the red-blue was this difference in intensity, another experiment was undertaken with the same animals under slightly different conditions. A glass aquarium about 40 cm. long by 20 cm. wide was covered with black cardboard and black cloth in such a manner that light could enter only through a space 5 cm. wide at the bottom of each end. Each of these ends was covered, the one with blue, the other with red glass, and 15 cm. from each end was placed an electric light marked 32 c. Later, however, it was found that one of these lamps measured 30 c. and the other 22 c. The red light was found to be much more intense to the eye than the blue, so the former was damped down with tissue paper until the two appeared to have the same intensity. The second pair of red-blue in Table VII gives the results of the observations under these conditions, and these are found to be in harmony with the rest of the table, _i. e._,[236] the color giving the lower stimulus has the higher number of reactions.

TABLE VII. REACTIONS TO VERTICAL COLORED LIGHT

Group 1 Group 2 Group 3 Sum
Animal
no. 13 37 41 43 44 Sum 21 27 36 38 52 Sum 54 56 58 60 62 Sum Total

Blue 17 22 14 25 29 107 11 21 26 27 26 111 26 32 26 28 10 122 340
White 23 18 26 15 11 93 29 19 14 13 14 89 14 8 14 12 30 78 260

Green 17 26 11 21 29 104 21 27 30 7 26 111 29 31 26 10 9 105 320
White 23 14 29 19 11 96 19 13 10 33 14 89 11 9 14 30 31 95 280

Yellow 25 21 12 20 35 103 12 20 34 25 29 120 19 20 9 28 22 98 321
White 15 19 28 20 15 97 28 20 6 15 11 80 21 20 31 12 18 102 279

Red 37 33 23 22 28 143 21 30 30 33 22 136 32 37 26 33 28 156 435
White 3 7 17 18 12 57 19 10 10 7 18 64 8 3 14 7 12 44 165

Black 23 20 1 28 34 106 9 12 25 40 32 112 25 32 18 34 16 125 343
White 17 20 39 12 6 94 31 28 15 14 88 15 8 22 6 24 75 257

Black 20 15 13 27 37 112 16 11 16 27 24 94 36 40 18 29 29 152 358
Red 20 25 27 13 3 88 24 29 24 13 16 106 4 22 11 11 48 242

Black 20 25 26 20 32 123 22 15 39 40 32 148 33 22 34 34 11 134 405
[236]
Blue 20 15 14 20 8 77 18 25 1 8 52 7 18 6 6 29 66 195

Red 25 17 28 18 8 96 3 15 21 28 20 87 21 13 15 14 28 91 274
Blue 15 23 12 22 32 104 37 25 19 12 20 113 19 27 25 26 12 109 316

Red 21 27 22 10 36 116 13 40 24 21 28 126 22 25 18 21 16 102 344
[237]
Blue 19 13 18 30 4 84 27 16 19 12 74 18 15 22 19 24 98 256

The most striking feature of the table is the marked predominance of the red over the white. Here the red reaches 73% of the total number of reactions, and inspection shows that this predominance is uniform not only through the groups but even for the individuals. The constancy of this reaction and the fact that it is so much more frequent than the one to the black as compared with the white, would lead one to expect that the red would have the higher percentage in the combination black-red. Such, however, is not found to be the case, although it does happen with one group. If the arrangement of our color-scale in accordance with increasing intensity of stimulus were correct, we should expect a gradually increasing predominance in reactions to colored light over those to white in the first five pairs. Instead of this we find that green and yellow stand nearest to the white, blue and black come next and are almost equal, while red is very much higher than any. In the pairs black-red and black-blue the red holds its predominance over the blue at about the same rate as in the second pair of the direct comparison, red-blue. The wide individual variations, however, in all these reactions to colored light, except perhaps in the case of red-white, indicate that there is nothing very regular, stereotyped, or mechanical about them. The most that can be said is that in a general way the red end of the spectrum furnishes a less intense stimulus to negative reaction than the blue.

A tendency to habit formation was noticed during the course of these experiments, and it is possible that this may have influenced the results somewhat. Many individuals apparently formed a habit of going to a certain corner as soon as they were reset at the centre. The positions in which they were set were varied and they were headed in different directions, but within a minute after they were released in the middle of the box they would be found in their favorite corner. This was especially the case with no. 38 in Table VI, and I think accounts in some measure for the persistent avoidance of the white. In no case did this continue throughout the whole series, but would sometimes be noted for two or three days at a time in the case of an individual. What were the controlling factors in this habit formation, the means by which orientation and recognition were effected, I was unable to determine.

3. _Reactions to Objects_

In no case did an animal give any sign of perceiving stationary objects in its path or of avoiding them in any way that could be referred to a visual stimulus. When the animal approached an obstruction there was no hesitation in the movement until the object was touched. Usually even when the antenna had touched the object the animal did not stop, but continued until the contact of the chelæ or even of the rostrum made further movement in that direction impossible.

With moving objects the case was quite different. Here the condition and disposition of the individual animal seemed to be the deciding factors. Often when the animals were trying to climb out of a shallow pan in which they were kept in the experimenting-room, raising a finger or holding out a pencil would be sufficient to make them stop or even start back into the pan. Nor was this response occasioned by any change in the intensity of light, such as that caused by a shadow falling on the animal, for they would react to a movement made on the opposite side of them from the window. In fact, no. 56, the most active in response to moving objects, seemed to react more vigorously to a motion made on the opposite side than when it was made between him and the light. Whenever a person came near the aquarium he and one or two others would take an attitude of defence, and would "face about" to correspond to any movement the person made toward one side or the other. When in the pan mentioned above, any movement of a person within two or three yards of him usually called forth a reaction on his part, and if the pan were placed on the table and the person moved slowly round it, the animal turned with the person, making a complete circuit of the pan.

Reaction to a smaller moving object, however, was not so marked. A black object, 20×8×8 cm., was suspended above the middle of the pan so that if set swinging it would just pass over the top. When it was pulled to one side the animal responded slightly, but after the first swing he seemed to pay no more attention to it. When the operator stepped out from behind the screen, the animal was as keen in its response as before. The experiment was now tried of allowing the object to approach from one direction while the operator moved to a position at right angles to its line of movement. Without hesitation the animal moved so as to keep fronting the operator, without paying any attention to the movement of the smaller object, although this was much nearer.

These observations on the reactions of the crayfish to stationary and moving objects are in line with the conclusions of Plateau[238] and Exner[239] drawn from observations on other Arthropods. It is Exner's belief that the compound eye is a visual apparatus which is almost worthless for detecting the _forms_ of objects, especially if these objects are stationary, but that it may furnish a very keen perception of _moving_ objects.

II. EXPERIMENTS WITH SOUNDS

Hensen[240] stated that Palæmon and Mysis reacted to sounds made by striking a thin, resonant board floating on the surface of the water, or by tapping the walls of the aquarium or of the room. Beer[241] repeated Hensen's experiments, but denied that the Crustacea reacted to sounds, and claimed that their movements were due to visual and tactual stimuli. Prentiss[242] confirmed Beer's results on Palæmonetes, and noted that the reactions were only slightly diminished by the removal of the otocysts, but that removal of the antennæ and antennules caused their almost complete cessation. More extended experiments were made on the fiddler crab, _Gelasimus pugilator_, which is on land a good deal of the time, and Prentiss's conclusions are: "(1) The reactions formerly attributed to sound-stimuli are nothing more than tactile reflexes. (2) The otocyst has little or no part in calling forth these reactions. (3) There is no direct evidence to prove that decapod Crustacea hear, and until such evidence has been obtained, we are not warranted in ascribing to the otocyst a true auditory function."

The experiments performed on the crayfish in this connection all resulted negatively and go to confirm Beer's and Prentiss's conclusions. Rapping upon a board floating in the water, and tapping the sides of the aquarium did not cause the slightest apparent reaction in the animals under observation, even though the vibration of the water could be plainly perceived by the sense of touch in the hand. When a rather large electric bell was sounded just over the surface of the water some reactions were observed which were evidently due to the movements of the hammer, but there was nothing which could be referred to the sound-stimuli. If the bell was held against the sides of the aquarium, or in the water near the animals, the vibration could be plainly felt by the fingers, yet no reactions on the part of the crayfish were observed. A metal snapper making a crack like a small pistol-shot was tried both in and out of the water but with no success in producing a reaction. A large hand tuning-fork, when held with its base pressed firmly against the glass walls of the aquarium, gave a deep rich tone of great volume, or when lightly touched to the glass produced a shrill, piercing, penetrating sound which was extremely sharp and disagreeable. Here again the vibrations of the water were quite perceptible to the hand at a distance of 10 cm., yet in neither case was there a sign of a reaction. Finally two electric tuning-forks, one of 256, the other of 512 vibrations per second, were tried on the animals taken one by one, and especial attention was given to the regular movement of the little thread-like appendages which keep up the current of water to the gills, with the idea that perhaps their rate of movement might be affected. In no case was there the slightest movement that could be referred to vibration, although here again the tactile stimulus was very perceptible to the finger. None of these experiments, then, give any indication that the crayfish reacts to vibratory stimuli which to the human ear produce sound.

III. ROTATION EXPERIMENTS

It has been found that the higher vertebrates, on being rotated on a turn-table, exhibit all the symptoms which accompany the sensation of dizziness in man. The question arises, to what degree and in what manner do invertebrates respond to rotation? Schaefer,[243] the first to take up this question, denied on rather meagre observations that Crustacea respond in any way to rotation on the turn-table. Kreidl[244] showed that this statement was altogether too sweeping, that Palæmon reacts very definitely to rotation by running in the opposite direction. Bunting[245] tried the crayfish, but all the rotation experiments resulted negatively, so she was led to confirm Schaefer's statement so far as the crayfish is concerned. Bethe[246] found that Carcinus behaved in a very definite manner on being rotated, that during the rotation the animals ran in the opposite direction to that in which they were turned, and as soon as the motion ceased they began running in the other direction. Finally Lyon,[247] while agreeing with Bunting that adult crayfish do not react to rotation, discovered that young animals two or three centimetres long react very prettily to the movement, going in a direction opposite to the turn. To confirm and if possible extend these observations on the crayfish was the purpose of the following experiments.

It was soon found that a great deal depended on the method of experimentation. None of the experimenters mentioned above gives any detailed description of the manner in which the experiments were carried out. One is left uncertain whether the animals were placed on the periphery of the turn-table or over the centre, whether in the former case they were set with their heads toward the centre or away from it, or placed at right angles to a radius, or whether they were merely set down in any chance fashion and whirled about. The same indefiniteness exists in most of the accounts as to how fast they were turned, and whether the experiments were performed in the air or in the water. Finally it is not stated whether the rotation was always in the same direction, or whether its direction was alternated.

The turn-table used in the following experiments was one that had to be turned by hand, so that it was impossible to regulate the speed accurately. The crank, however, was not attached directly to the rotating board, but was connected with it by means of a gearing so that one turn of the crank produced about ten turns of the table. This gearing gave a steadying effect to the motion so that the speed could be kept tolerably constant. A circular pan, about 15 cm. in diameter at the bottom with the sides slightly sloping outward, was set so that its centre coincided with the axis of the rotating table. It was in this pan that all the experiments were tried. Through various preliminary experiments to determine the most favorable speed, it was found that a rotation rate of over one turn of the table per second produced such a strong centrifugal force that unless the animals were set exactly over the centre they were swept off against the side of the pan in such a manner that it was difficult to decide whether the rotation as such had any effect upon their movements. It was finally decided that the best results were obtained from a rate of approximately one rotation in two seconds.

It soon became evident that when the larger and more sluggish crayfish were merely dropped in the pan and rotated there was no particular reaction. This was true whether the animals rotated were in the air or in the water. The smaller and more active crayfish, however, showed a decided tendency to run either with or against the direction of the rotation, especially when the experiments were carried on in the water. In no case was there any tendency to go in the opposite direction when the rotation ceased, except in so far as the animals were carried along by the water. To get a quantitative expression for these tendencies a more delicate method of experimentation was resorted to. If there was a tendency on the part of the active animal to move either with or against the rotation, such a tendency might also be supposed to exist in the sluggish animal, only in the latter the inertia was sufficiently strong to prevent its appearance. If, however, the animals should be set radially to the periphery of the pan, the tendency to go with or against the rotation would be exhibited in the direction in which they turned out of the radial position. For it was found that no animal would remain in that position for any great length of time. Two groups of animals were used for these experiments, five animals in each group, and the first group was selected from the smallest and most active animals, the second from the largest and most sluggish. Each animal was set in two positions, position I, with the head toward the centre, position II, with the head away from the centre. Each animal was given ten trials in each position, and the number of times it turned in a direction _with_ the rotation is set down in the + column, the number of times it turned _against_ the rotation is indicated in the - column. In general from 5 to 15 turns were necessary for the orientation of the animal, though sometimes the number ran up to 30 or 40. Each trial was made in the opposite direction to the preceding one, in order to avoid the formation of any habit in turning. All these experiments were carried out in water, the depth of which in the pan was about 4 cm. In order that there should be no difference between the velocity of the water and that of the pan, the table was rotated a few times before the animal was put in. As a check a series of experiments of 5 in each position was performed in the air on the more active group.

The following table shows the results of these experiments in rotation:

TABLE VIII. REACTIONS TO ROTATION

In Water In Air
Sum
I II Sum I II Sum Total
Group 1 + - + - + - + - + - + - + -
44 5 5 10 5 15 1 4 5 1 9 6 24
49 2 8 4 6 6 14 5 1 4 1 9 7 23
56 3 7 8 2 11 9 5 3 2 8 2 19 11
62 4 6 10 14 6 2 3 2 3 4 6 18 12
64 2 8 9 1 11 9 1 4 1 4 2 8 13 17
Sum 16 34 31 19 47 53 9 16 7 18 16 34 63 87
Group 2
21 5 5 3 7 8 12
27 2 8 5 5 7 13
36 7 3 8 2 15 5
37 2 8 3 7 5 15
54 5 5 6 4 11 9
Sum 21 29 25 25 46 54 46 54
Sum Total 37 63 56 44 93 107 9 16 7 18 16 34 109 141

Examination of the table reveals great individual variation. Some animals, as nos. 44, 49, and 37, turn rather constantly against the direction of the rotation, while others, as nos. 56 and 36, are almost as constant in their movement with the rotation. On the whole we observe that for each group, and for Group 1 in both water and air, there is a slightly greater tendency to go against the rotation than with it. This tendency, strange to say, comes out much more clearly in the air than in the water. It is evident, however, from the variation exhibited that there is nothing very stereotyped or mechanical about the reaction. Mention should be made of the fact that usually (though not always) the animals not only oriented themselves with reference to the rotation, but moved forward in that direction as long as the rotation continued.

IV. GEOTAXIS, BAROTAXIS, AND TURNING

(1) _Geotaxis._ So far as my knowledge extends, no experimental work has been done to determine the geotaxis of decapod Crustacea. Most of the vertebrates are positively geotactic, while a great many of the invertebrates, particularly unicellular organisms, larvæ of moths and butterflies, slugs, etc., are negatively geotactic. Parker[248] found that in the case of the Copepod, _Labidocera æstiva_, the females exhibited strong, the males weak, negative geotaxis. In the investigation of the geotaxis of the crayfish, two sets of experiments were undertaken. In the first the method of procedure was as follows:

On a level table before a window a board was so arranged that it could be set at an inclination of 5°, 10°, 15°, 20°, and 25° either toward or away from the window. Starting, let us say, with the inclination toward the window, each one of a group of five animals was placed on the board with the right side to the window five times. The board was then inclined the same amount away from the window and the process was repeated. The same procedure was carried out with the animals set with the left side to the window. The following table gives the results of this set of experiments.

TABLE IX. GEOTAXIS IN FRONT OF WINDOW

10 12 14 16 18 Totals
+ - ± + - ± + - ± + - ± + - ± + - ±
5° 14 5 1 11 7 2 11 9 13 7 8 10 2 57 38 5
10° 13 7 12 7 1 13 6 1 16 4 14 6 68 30 2
15° 17 3 14 6 16 3 1 17 3 9 11 73 26 1
20° 12 8 17 3 18 1 1 19 1 14 6 80 19 1
25° 18 2 19 1 19 1 17 3 16 4 89 11

From this table it appears that the crayfish is positively geotactic, and that the positive geotaxis increases regularly with the increase in inclination. As a check on these results another set of experiments was undertaken with different animals under different conditions. The board was placed on a level table in the centre of a darkened room, and the operator stood behind a screen so as to be quite hidden from the animals. In order to observe the orientation a 2 c. incandescent electric light was suspended directly above the spot where the animals were set, at a distance of 60 cm. above the board. Each animal of a group of five was set five times in each of four positions, viz., head down the incline, head up the incline, and at right angles to it with first the right and then the left side down the slope. The results were as follows:

TABLE X. GEOTAXIS IN DARKENED ROOM

41 46 48 51 64 Totals
+ - ± + - ± + - ± + - ± + - ± + - ±
5° 12 4 4 13 4 3 8 10 2 14 5 1 11 7 2 58 30 12
10° 14 5 1 13 6 1 11 8 1 13 6 1 14 3 3 65 28 7
15° 16 4 15 5 13 7 11 6 3 14 2 4 69 24 7
20° 16 4 19 1 16 4 20 17 2 1 88 11 1

It will be observed that Tables IX and X agree quite well in the main, and we may conclude that the crayfish is positively geotactic and that the positive reactions vary from 58% at 5° to 89% at 25°.

(2) _Barotaxis._ Verworn[249] uses the term barotaxis in an inclusive sense to cover all pressure phenomena that can be classed under the sub-heads of thigmotaxis, rheotaxis, and geotaxis. It seems preferable to me to employ the term in a more restricted sense of reaction to pressure other than the pull of gravity, the flow of a current, or the contact with bodies. The following experiment with the crayfish furnishes us, I think, with a case in point.

A glass aquarium, 54 cm. long and 28 cm. wide, was so inclined that the water was 20 cm. deep in one end and 8 cm. deep in the other. A board was so anchored that one end rested on the bottom at the shallow end of the aquarium while the other end projected slightly out of and above the deepest water. The board was about 45 cm. long, so that its slope was very gradual. Nine animals were placed in this aquarium and observed for three successive days. If we denote the bottom of the deep end of the aquarium by A, the shallow end under the board by B, the shallow end on top of the board by C, and the end of the board at the surface of the water by D, the results of the observations were as follows: On the first day 1 animal was found at D, 6 at C, and 2 at B. On the second day 5 were at C, 3 at B, and 1 at A. On the third day 1 was at D, 4 at C, and 4 at B. Totals, 2 at D, 15 at C, 9 at B, and 1 at A.

While these observations were too few to base very positive statements on, the striking fact that only one animal was found at A, the deep end of the aquarium, whereas 15 were noted on top of the board at C, indicates strongly that the animals avoid the deeper water. That the animals were found _on top_ of the board, not under it, indicates that the observation is not to be referred to thigmotaxis, although the latter is doubtless very strong, as we shall see later. It should be observed that the negative barotaxis works against and overcomes the marked positive geotaxis which, as we have just seen, the animals exhibit in the air. Under the influence of the positive geotaxis, we should expect to find the greater number of the animals at A,--a condition which is speedily realized if we let the water run out of the aquarium. We conclude, therefore, that at certain pressures (specifically at the pressure exerted by water at a depth of 20 cm.) the crayfish is negatively barotactic.

(3) _Turning._ In the experiments with light it was observed that very seldom do the animals, when placed upon a surface, move off at once in a straight line, but usually they first turn through an angle of 90° or more and then start off straight. This came out strongly in the work on geotaxis, where oftentimes, when the animal was set with the head up the incline, the reactions would be preponderantly positive, whereas when set with the head down the incline the reactions were on the whole negative. In other words, when headed up the incline the animal would go down, and when headed down he would more often go up. Some experiments were tried under various conditions to determine how general this tendency is. The table presents the results in condensed form.

TABLE XI. EXPERIMENTS IN TURNING

Nos. 10 12 14 16 18 Sum 7 9 15 17 25 Sum
I III
90°- 8 5 6 8 10 37 2 4 7 5 5 23
90° 4 6 7 2 2 21 3 1 4
90°+ 8 9 7 10 8 42 5 5 3 5 5 23
II IV
90°- 2 1 2 2 7 9 3 1 2 15
90° 1 1 2 4 3 5 3 3 1 16
90°+ 7 9 7 10 6 39 12 1 9 11 11 44

Nos. 41 43 46 48 64 Sum Sum
V Totals
90°- 9 11 12 14 2 48 130
90° 2 1 2 1 3 9 54
90°+ 9 8 6 5 15 43 191

In this table the first line indicates the number of times each animal turned less than 90° when starting off from the position in which it was set, the second line the number of times the amount of turn was practically 90°, and the third more than 90°. The five parts of the table mark the different conditions; in Part I twenty observations were made on each animal placed on a level board before the window, and set now with the right now with the left side toward the window. In Parts II and III the animals were set with the head turned now toward now away from the window. In Parts IV and V the animals were placed on a level board in the middle of a darkened room with a 2 c. light about three feet above them. This was to exclude any possible directive influence of light. In all cases the operator was concealed by a screen. In Parts I and V twenty observations were made on each animal, in II and III ten, and in IV fifteen.

Rarely the animal would turn completely round and start off in the direction originally set, but usually the turn was between 90° and 180°. When once the animal began to move off, it would ordinarily keep to an approximately straight line. How seldom this was observed when the animals were first set down may be judged from the fact that out of a total of 375 observations in only 18 did the animals move straight ahead from their original position. From the table we observe that in over 65% of the cases (a proportion of almost two to one) the animals turned through 90° or more before starting off. At present the writer has no explanation to offer for this phenomenon.

V. THIGMOTAXIS AND TOUCH REACTIONS

(1) _Thigmotaxis._ Experiment has shown that there are some animals which tend to avoid contact with objects as much as possible, and on the other hand there are animals that seek to get as much of the surface of their bodies as possible in contact with objects. The former are spoken of as negatively, the latter as positively thigmotactic. Does the crayfish show any tendency in the one way or the other, and if so is it positively or negatively thigmotactic? In a large glass aquarium, 80 cm. long and 40 cm. wide, was a thin wooden box, 22 cm. long and 16 cm. wide, set in one corner 4 cm. from the glass walls. At the bottom of the box was an opening where the crayfish could enter. The following table shows the disposition of the animals for 27 different days, on which one examination was made each day. Line I indicates the number of times each animal was found against the walls of the aquarium, line II in the 4 cm. space between the box and the walls of the aquarium, line III inside the box against its sides, and line IV resting freely in the middle of the aquarium or in the middle of the box.

TABLE XII. THIGMOTAXIS REACTIONS

Animal 4 5 9 13 21 27 29 31 33 35 36 37 38 39 41 42 43 44 45 46 47 48

I 4 9 2 10 4 17 16 6 6 10 3 2 11 9 3 2 1 9 1 15
II 5 2 13 18 9 5 2 4 5 22 1 14 11 3 6 11 1 9 4 7
III 2 1 2 5 1 9 3 2 3 11 2 24 7 1 4 14 9 1 5
IV 1 11 2 1 2 4 4 1 2

Animal 49 51 52 54 56 58 60 62 64 Sum
Totals
I 6 11 3 1 2 4 7 10 12 195
II 8 3 7 5 8 1 3 2 1 190
III 1 16 9 5 10 3 2 2 154
IV 1 1 2 1 33
572

In order to appreciate the significance of the figures in this table it is necessary to consider the amount of lateral surface with which it was possible to come in contact in each case. In IV of course it was zero, in III it was 76 cm. with four corners in close proximity, in II it was only 38 cm. and one corner, but the space was so narrow that there was practically a contact-surface on both sides, and in I there was 212 cm. of lateral surface with three corners. I mention corners in this connection because they were almost invariably occupied. If we examine the table with these facts in mind, we find, (1) that the number of animals resting freely without contact with any lateral surface is very small, only about 6% of the whole; (2) that the number of animals found in the narrow space between the box and the walls of the aquarium is very large in proportion to the length of the space: indeed the animals were frequently found wedged into this space three or four deep; (3) that the number of animals found in the box was probably due largely to the fact that they found in it a greater lateral contact-surface, particularly in the corners, than was possible outside.

Two or three minor considerations are of interest. The animals were frequently observed "on edge" about half out of the water, that is, with the ventral surface of the body pressed against the vertical surface against which they were resting. This was also observed where the water was so deep that none of the members could touch the bottom. It was perhaps on account of the quality of the surface affording a rougher contact that so large a number of the animals were found in contact with the wooden box rather than the smooth, slippery surface of the glass. In the centre of the aquarium a wooden stopper 2 cm. in diameter projected about 15 cm. above the surface of the water. Very often a crayfish would be found almost at the top of this stopper, completely out of the water. This tendency to climb was frequently observed in the light-reaction experiments, where the animals would climb up on any piece of wood that chanced to be left in the box. It reminds one of the tree-climbing crabs of the West and East Indies. Along the creeks of Ohio I have frequently seen crayfish that had climbed up on logs or sticks that projected some feet out of the water.

In the table we see decided evidences of "habit" in the sense of an animal returning to the same place which it had occupied. No. 5 has almost half the observations in the open, nos. 21, 37, and 39 showed a decided preference for the space between the box and the aquarium wall, while nos. 38, 44, and 52 were more frequently found on the inside of the box. This recurrence to a particular position also came out in the light-reaction work, where an individual would return to the same spot in the box for days at a time as soon as released.

From the above considerations we conclude that the crayfish is strongly positively thigmotactic and that this thigmotaxis probably plays a most important part in the life of the animal.

(2) _Touch Reactions._ Lemoine[250] investigated the reactions of crayfish to touch-stimuli and found that the plates of the telson, the sternal portions of the thorax, the abdominal pleopods, the chelæ, and particularly the antennæ toward their points are especially sensitive, but that nowhere, even on the back of the carapace, is a touch-stimulus altogether devoid of reaction. Gulland[251] found that a needle could be inserted between the tufts of setæ on the chelæ without causing any reaction, but as soon as one of the hairs was touched, the chelæ closed with a snap. Considering the setæ as the organs of touch, he claimed to have found that the eyes, eye-stalks, and carapace (which he says have no setæ) are impervious to tactile impressions. This claim of Gulland's is strangely at variance with the facts. In no case have I been able to bring about retraction of the eye-stalk by visual stimulation, but a very light touch-stimulus on the eye itself or on the eye-stalk or a stronger stimulus on some portion of the head will cause the eye to be drawn in. It is true that after repeated stimulation the eye is retracted no longer, and with a heavy bristle one can make a perceptible indentation in the corneal surface without the eye being withdrawn.

The antennæ, from their anatomical structure, their position, and the manner in which they are carried, are generally considered the special organs of touch. Nevertheless, as far as the reactions of the animal are concerned, a stimulation of the antennæ by touch produces a less decided response than almost any other portion of the body. If the stimulus is very light no reaction at all is observed in most cases, and if stronger the antennæ are moved away, but that is all. A stimulation of the edge of the telson produces a more decided reaction. Either the animal folds it under the abdomen at once or faces about like a flash in an attitude of defence; frequently both reactions occur. While the response to stimulation of the chelæ was decided, that to touch on the first chelipedes was quicker and more accurate. The mouth-parts are also very sensitive to touch. I cannot agree with Gulland's assertions as to the insensitiveness of the carapace, for I have been able to find no place upon it where a light-stimulation would not produce a reaction. In this connection a curious phenomenon is characteristic of the animal. If the carapace or the front portion of the abdomen be lightly stroked with a solid object such as a pencil, the animal will slowly turn toward the stimulus on its antero-posterior axis. If, now, a like stimulus be applied on the other side, the animal will roll back through the normal position to a like inclination toward the stimulus on the other side. If the alternation be kept up and the change made quickly, a continuous and curious rolling movement is maintained, the animal growing more and more excited until it scampers off with a kind of cramp-like motion. With some animals this rolling reflex is more marked than with others, but in no case is it altogether lacking. Some animals have been known to roll so far over that they topple over on their backs. Dr. Yerkes informs me that he has observed the same phenomenon in a less degree with turtles when the edge of their shell is stimulated by scratching. The movement seems to be caused by the reflex stimulation of the extensor muscles on the opposite side of the body from the part stimulated. The thrust of the legs thereby brought about raises that side of the body and thus causes a rotation to some extent about the antero-posterior axis. But how was this connection between the stimulation of one side of the body and the contraction of the extensor muscles of the other side established? I have no doubt that it is intimately connected with the positive thigmotaxis described above. These animals live under loose stones for the most part, and thus the carapace gets a great deal of stimulation. If the animal is stimulated on one side, a contraction of the extensor muscles of the opposite side tends to roll the animal toward the source of the stimulus, and hence to increase the contact. In the race-history of the animal this has doubtless been advantageous in enabling it to escape the dangers of its habitat.

* * * * *

In a succeeding paper the writer hopes to discuss the reactions of the crayfish to chemical stimuli. In conclusion he desires to make acknowledgment to Dr. Robert M. Yerkes of the Harvard Psychological Laboratory for kindly suggestions and helpful criticism throughout the course of the investigation.

SUMMARY

(1) Crayfish are somewhat negatively phototactic, going away from rather than toward the source of light in the ratio of 62% to 38%. The different intensities employed in this investigation produced very little difference in the reactions. The average reaction-time was much less for the group of animals which showed the highest percentage of negative reactions, indicating a greater general sensitiveness. Variations of the position in which the animals were set affected the results very slightly.

(2) Previous confinement in the dark tended to increase slightly the number of negative reactions, and previous exposure to strong light tended to decrease the number, but the results were not constant. An increase in temperature tended to decrease the number of negative reactions to light, but here again the results were somewhat conflicting.

(3) Reactions to horizontal colored light showed a tendency to go to the colored light rather than to the white in the following order: Blue 47%, green 50.5%, black (or the absence of light) 51%, red 54%, yellow 59%. In the case of vertical colored light the comparison with the white resulted somewhat differently, as follows: Green 53%, yellow 53.5%, blue 57%, black 57%, red 72.5%. In the latter experiments the animals showed a marked and constant preference for the red.

(4) The animals showed no signs of reaction to static objects from visual stimulation, i. e., there is no evidence of visual perception of form in the case of stationary objects. Moving objects, especially large ones, are plainly perceived and definitely reacted to.

(5) There were no reactions whatever caused by those vibrations which to the human ear produce sound. So far as these experiments go, the animals cannot be said to hear.

(6) In rotation experiments individual animals were rather constant in moving either with or against the direction of the rotation, but no definite tendency for all animals was observed.

(7) The pull of gravity was followed with constantly increasing frequency from 58% at 5° to 89% at 25°. Therefore we conclude that the animals are positively geotactic. They are negatively barotactic, avoiding the pressure of water at the depth of 20 cm., and this is sufficient to overcome their positive geotaxis. When placed upon a level surface the animals show a peculiar tendency to turn through a greater or less angle before starting out in a straight line. In only 18 out of 375 observations, or 5%, did the animals start straight, in 30% they turned through an angle of less than 90°, and in 65% they turned through an angle of 90° or more.

(8) The crayfish is positively thigmotactic in a marked degree, as is indicated by the fact that in only 33 out of 572 observations, or less than 6%, were the animals found resting in the open, while in 190, or 33%, they were found in a narrow opening between two vertical surfaces.

(9) The animal is sensitive to touch over the whole surface of the body, but especially on the chelæ and chelipedes, the mouth-parts, the ventral surface of the abdomen, and the edge of the telson. If one side of the carapace or of the dorsal surface of the abdomen be stimulated, the extensors of the legs on the opposite side are contracted, and the animal turns on its antero-posterior axis toward the source of the stimulus. If opposite sides be stimulated alternately, a peculiar rolling motion is set up.

FOOTNOTES:

[Footnote 218: An Introduction to the Study of Zoölogy, illustrated by the Crayfish, Internat. Sci. Ser., 1880.]

[Footnote 219: How the Burrowing Crayfish works, Inland Monthly, Columbus, Ohio, vol. 1, pp. 31, 32, 1885.]

[Footnote 220: Cave Animals from Southwestern Missouri, Bull. Mus. Comp. Zoöl. Harv. Univ., vol. 17, pp. 225-239, 1889.]

[Footnote 221: Notes on the Individual Psychophysiology of the Crayfish, Amer. Jour. of Physiol., vol. 3, pp. 404-433, 1900.]

[Footnote 222: Reactions of Entomostraca to Stimulation by Light, II, Reactions of Daphnia and Cypris, Amer. Jour. of Physiol. vol. 4, pp. 405-422, 1900; Reactions of Daphnia pulex to Light and Heat, Mark Anniversary Volume, pp. 359-377, 1903.]

[Footnote 223: Heliotropism of Cypridopsis, Amer. Jour. of Physiol., vol. 3, pp. 345-365, 1900.]

[Footnote 224: Das Nervensystem von Carcinus mænas, I, Arch. f. mikros. Anat., vol. 50, pp. 460-547, 589-640, 1897.]

[Footnote 225: The Color Physiology of Higher Crustacea, Phil. Trans., London, Series B, vol. 196, pp. 295-388, 1904.]

[Footnote 226: An Establishment of Association in Hermit Crabs (_Eupagurus longicarpus_), Jour. Comp. Neur. and Psych., vol. 14, pp. 49-61, 1904.]

[Footnote 227: The American Lobster; A Study of its Habits and Development, Bull. U. S. Fish Comm., vol. 15, pp. 1-252, 1895.]

[Footnote 228: Notes on the Senses and Habits of some Crustacea, Jour. Marine Biol. Assoc'n., Plymouth, N. S., vol. 1, pp. 211-214, 1889.]

[Footnote 229: The Reactions of Copepods to Various Stimuli, Bull. U. S. Fish Comm., vol. 21, pp. 103-123, 1902.]

[Footnote 230: Recherches pour servir à l'histoire des systèmes nerveux, musculaire, et glandulaire de l'écrevisse, Ann. des Sci. Nat., Series 5, vol. 9, pp. 99-280; vol. 10, pp. 5-54, 1868.]

[Footnote 231: The Sense of Touch in Astacus, Proc. Roy. Physiol. Soc., Edinburgh, vol. 9, pp. 151-179, 1886.]

[Footnote 232: _Loc. cit._]

[Footnote 233: Contribution to the Comparative Physiology of Compensatory Movements, Amer. Jour. of Physiol., vol. 3, pp. 86-114, 1899.]

[Footnote 234: The Retina and Optic Ganglia in Decapods, especially in Astacus fluviatilis Mitth. Zool. Stat. Neapel., vol. 12, pp. 1-73, 1895.]

[Footnote 235: Photomechanical Changes in the Retinal Pigment Cells of Palæmonetes, etc., Bull. Mus. Comp. Zoöl. Harv. Univ., vol. 30, pp. 273-300, 1897.]

[Footnote 236: No. 46 substituted.]

[Footnote 237: The second pair of red-blue gives the results of an experiment under somewhat different conditions as described above.]

[Footnote 238: Recherches expérimentales sur la vision chez les Arthropodes, Mém. Corronnés de l'Acad. Roy. des Sci. etc. de Belgique, vol. 43, pp. 1-91, 1889.]

[Footnote 239: Die Physiologie der facettirten Augen von Krebsen und Insekten, 1891.]

[Footnote 240: Studien über das Gehörorgan der Dekapoden, Zeitsch. f. wiss. Zool., vol. 13, pp. 319-412, 1863.]

[Footnote 241: Vergleichend-physiologische Studien zur Statocysten-function, I. Ueber den angeblichen Gehörsinn und das angebliche Gehörorgan der Crustaceen, Arch. f. d. ges. Physiol., vol. 73, pp. 1-49, 1898; Idem. II. Versuche an Crustaceen, Arch. f. d. ges. Physiol., vol. 74, pp. 364-382, 1899.]

[Footnote 242: The Otocyst of Decapod Crustacea, Bull. Mus. Comp. Zoöl. Harv. Univ., vol. 36, pp. 165-251, 1901. (Contributions, no. 123.)]

[Footnote 243: Das Verhalten wirbelloser Thiere auf der Drehscheibe, Zeitsch. f. Psych. und Physiol. d. Sinnesorgane, vol. 3, pp. 185-192, 1892.]

[Footnote 244: Weitere Beiträge zur Physiologie des Ohrenlabyrinthes, II. Mittheilung, Versuche an Krebsen, Sitzungsb. Kais. Akad. Wiss., Wien., vol. 102 (Part. 3), pp. 149-174, 1893.]

[Footnote 245: Ueber die Bedeutung der Otolithenorgane für die geotropischen Functionen von Astacus fluviatilis, Arch. f. d. ges. Physiol., vol. 54, pp. 531-537, 1893.]

[Footnote 246: Das Nervensystem von Carcinus mænas, I. Arch. f. mikros. Anat., vol. 50, pp. 460-547, 589-640, 1897.]

[Footnote 247: Contribution to the Comparative Physiology of Compensatory Movements, Amer. Jour. of Physiol., vol. 3, pp. 86-114, 1899.]

[Footnote 248: The Reactions of Copepods to Various Stimuli, Bull. U. S. Fish Comm., vol. 21, pp. 103-123, 1902.]

[Footnote 249: General Physiology, English translation by Frederic S. Lee, 1899.]

[Footnote 250: Recherches pour servir à l'histoire des systèmes nerveux, musculaire, et glandulaire de l'écrevisse, Ann. des Sci. Nat., series 5, vol. 9, pp. 99-280; vol. 10, pp. 5-54, 1868.]

[Footnote 251: The Sense of Touch in Astacus, Proc. Roy. Physiol. Soc. Edinburgh, vol. 9, pp. 151-179, 1886.]

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