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. 1012141618Sum 79151725Sum
I III
90°– 85681037 2475523
90° 4672221 31 4
90°+ 89710842 5535523
II IV
90°– 212 27 931215
90° 1 1 24 3533116
90°+ 79710639 1219111144
Nos. 4143464864Sum Sum
V Totals
90°– 9111214248 130
90° 212139 54
90°+ 98651543 191

Table XI. No. II, first row, 90deg: the minus was added.

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.