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The four eyes of the prolecithophoran Allostoma sp. are disposed in two pairs in a dorsolateral position at the periphery of the brain and beneath its capsule. They are rhabdomeric pigment-cup ocelli. Each eye in the anterior pair consists of one pigment cell and one receptor cell; each in the posterior pair is made up of a larger, single pigment cell and two photoreceptor cells. A lens in front of the pigment cell’s aperture is formed by electron-dense, refractive, finger-like protrusions which arise from unpigmented cytoplasmic extensions of the pigment-cup margin. Degenerative signs are sometimes visible in the lens.

The appearance and development of the embryonic and larval eyes of the polyclad turbellarian Stylochus mediterraneus were studied. In the embryo, the left epidermal eye appears first. Subsequently, the right epidermal eye appears, and within hours it sinks into the parenchyma and turns into a cerebral eye. Newly hatched Gotte’s larvae possess both the left epidermal and the right cerebral eye. Three days after hatching, an incomplete eye appears adjacent to the left epidermal eye. The left cerebral eye then originates from this incomplete eye as it sinks into the parenchyma. This third eye is believed to originate through a process of induction.

Examination of the larvae of Thysanozoon brocchii and Stylochus mediterraneus shows that they have both epidermal and cerebral eyes, while the young worms of Notoplana alcinoi have only cerebral eyes. A description is given of the ultrastructure of both kinds of eyes. The epidermal eye consists of one cup-shaped pigmented cell, whose cavity is filled with lamellae of ciliary origin. A small covering cell is located over the cup-opening. The cerebral eye is made up of three cells: one pigmented cell with ciliary projections and two rhabdomeric-type photoreceptor cells. The cerebral eye in the adult is formed of a pigmented cup without cilia and at least three rhabdomeric-type photoreceptors. A number of remarks of a morpho-onthological nature are presented.