1. The rate of movement in Pl. alpina is increased during a period of about 10 days after feeding.

2. Sexual individuals move at a faster and more irregular rate than do immature individuals.

3. Immature individuals do not respond to a weak current but are positively rheotactic to strong currents. Various weak non-orientating stimuli facilitate the positively rheotactic response of these animals.

4. Sensory appreciation of temperature differences is slight.

5. After feeding, immature individuals respond positively to a weak current.

6. Sexual individuals respond positively to a weak current.

7. Above 12° C. Pl. alpina responds positively to weak currents, this fact explains their distribution under natural conditions.

8. Strong stimuli or combined weak stimuli cause negative rheotaxy.

9. After negative rheotaxy has been induced the positive response only occurs after a period of rest or after a period of time long enough for the excitatory state of the central nervous system to have become reduced.

10. The nature of the rheotactic response depends on the excitatory state of the central nervous system, and under different circumstances the same individuals will respond differently to the same stimulus.

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