Sensory receptors in the head of Stenostomum leucops. I. Presumptive photoreceptors.

General body sensitivity to light is present in Stenostomum leucops, expressed as increased activity and locomotion of resting animals by exposure to light. The sensory structures involved have not been identified so far. The head of S. leucops bears sensory cells in the ciliary pits, along the epithelium and in connection to the brain lobes. Two types of presumptive photoreceptor, quite distinct from eyespots, are found. The first type, a pair of ciliary lamellate bodies, is found on both sides of the worm between the ciliary pits and the anterior brain lobes. Both cells contain an internally ciliated vacuole. The sparse lamellae are derived from membranes of more than 20 milia, most of them consisting of a typical 9 + 2 axoneme pattern. The cells are connected to brain ganglia by nerve fibres, but there are no connections to the surface. No pigment cups are observed, either. The second type, two cells with light-refracting bodies, is observed more posteriorly, near the dorsal surface and in close nerve connection to the posterior brain lobes. Both cells contain numerous (20-30) refractile, round or ovoid granules of various sizes. They form a cup-shaped structure to one side of the cell, the concave surface directing forward. The granules resemble lipid spheres under the electron microscope. In addition to comparable similarities with presumed photoreceptors in other platyhelminths, the photoreceptive function of the ciliary lamellate bodies and light-refracting bodies was tested using tritiated thymidine ([3H]T) vitamin-A autoradiography. No specific labelling, however, was observed in these cells nor in other sensory cells in S. leucops.