Physiology.

The animal is strongly negatively heliotropic. Sudden light and sudden heat causes a quick cessation of movement, which is followed by motion away from the stimulus. Experiments seem to suggest that there is no perception of red, green or blue light, and that the eyes are infra-red sensitive. Strong sunlight is rapidly fatal.

The optimum temperature for movement is about 37°, the most favourable temperature being 18° to 25° according to conditions, such as humidity. At this range of temperature the animal is most active. Temperatures above 45° and below 0° are rapidly fatal. At a temperature of below 11° the animals can be induced to move only with strong stimulation, and then with difficulty. Hibernation takes place by the worm burrowing into the earth when the temperature falls below 15°, and will follow severe cold, even in summer.