The response of the planarian Dendrocoelum lacteum to an increase in light intensity

Abstract

Dendrocoelum is said to respond to an increase in light intensity by an increase in its rate of change of direction (r.c.d.). An infrared viewing technique was used to observe and record, on 16-mm film, the response of dark adapted Dendrocoelum to sudden increase in light intensity of 1 lux. The planarians moved faster, turned through bigger angles, and turned more frequently: the response has both ortho-and klino-kinetic components. The increase in the speed of gliding was maintained after the r.c.d. had fallen back to its original level. Dendrocoelum reacted to intensities as low as 10 −5 lux, and at these intensities they moved faster and turned more frequently but the mean angle of turn decreased: there was a fall in r.c.d. The results are discussed in relation to Ullyott's earlier work and shock reactions.