Surface ultrastructure of the redia ofParorchis acanthus Nicoll (Digenea: Philophthalmidae)

Rediae ofParorchis acanthus were examined by scanning electron microscopy and the ultrastructure of the surface related to migration through the digestive gland ofNucella lapillus. The median ventral birth papilla, ventro-lateral processes, and posterior papilliform process, of young active rediae, apparently serve to anchor the body during migration. The honeycomb-like apex of the birth papilla and ventro-lateral processes may be a strengthening device. Uniciliate sensory receptors around the mouth, on the birth papilla, and posterior terminal papilla are probably touch receptors or chemoreceptors concerned with feeding and orientation during migration. After the formation of the birth pore, and with increase in size and in the number of contained cercariae, the redia becomes less active, loses its characteristic shape, and is eventually immobile.