(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.