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

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

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

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

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.