Two new sensory receptors in Gyrodactylus sp. (Platyhelminthes, Monogenea, Monopisthocotylea).

Two pairs of sub-surface ciliary aggregations, not previously described in gyrodactylid monogeneans, have been found in the antero-dorsal region of adult Gyrodactylus sp. One pair lies more anteriorly and has several basal bodies without rootlets as well as modified cilia with single microtubules projecting into an extra-cellular cavity. A neuronal process extends posteriad. An anterior pocket connecting to this cavity is surrounded by cytoplasm that adjoins the ciliated cell by a septate junction. This type may have a pressure/contact or photoreceptoral function. Each of the second pair of receptors contains approximately 12 basal bodies without rootlets in the periphery of a cytoplasmic cavity. Flattened ciliary membranes extend from the basal bodies into the interior, forming a number of whirls. A neuronal projection extends posteriad. This structure closely resembles presumed photoreceptors found in several other parasitic and free-living platyhelminths. The first type does not appear to have been described for any other species.