THE ROLE OF THE EYE AND CNS COMPONENTS IN PHOTOTAXIS OF THE ARROW WORM,SAGITTA CRASSATOKIOKA

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Abstract
The arrow worm Sagitta crassa swims by a slow tactic behavior toward a light of moderate intensities and by a quick target-aiming upon sudden reduction in light intensity. The swimming stroke is achieved by a dolphin-type thrust. Microcautery of eyes, brain and ventral ganglion has shown that one-eyed worms can show the two kinds of photic responses and that specimens with both eyes or the brain damaged can survive, but fail to respond to light. Swimming itself, however, depends on the ventral ganglion being intact. These results indicate that the responses are mediated telophototactically through the eyes, and that no extraocular photoreceptors can be substituted for them. The eye structure which possesses one central pigment cell with seven depressions on four sides is fitted for the telophototaxis, enabling photoreceptive endings in the depressions to be stimulated to different degrees by light coming from one particular direction.