REACTIONS OF CERCARIA HAMATA TO LIGHT AND TO MECHANICAL STIMULI

¹ ZOÖLOGICAL LABORATORY, WASHINGTON UNIVERSITY, ST. LOUIS

1. Cercaria hamata swims intermittently. A short, rapid swim alternates with a relatively long period of quiet sinking.

2. The duration of the period of sinking decreases with rise in temperature. Below 15° C. the cercariæ exhibit jerking and lashing movements in place of swimming.

3. Shadowing a dense group of cercariæ results instantly in the almost simultaneous swimming of most of them. The response is partly due to the shadow stimulus and partly to the mechanical stimulus occasioned by the colliding of active individuals with quiet ones.

4. Repeated shadowing at short intervals results in inhibition of swimming.

5. Mechanical stimulation by touch (or a stream of water) initiates swimming instantaneously. Only a very short interval between stimuli is necessary to keep a cercaria in almost continuous locomotion. This indicates that a dual mechanism is involved in response to shadowing and to touch.

6. The duration of swim in response to shadow, touch, and stimulation by a stream of water is significantly greater than that of a spontaneous swim.