Ultrastructure and Development of the Eyes of Larval Kronborgia isopodicola (Platyhelminthes, Fecampiidae)

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Abstract
The structure and development of the paired eyes of the larva of Kronborgia isopodicola were studied by electron microscopy. Each ocellus, located anterolateral to the brain, is of the inverse rhabdomeric type. A supportive cell contains 10–12 rows of concentrically arranged crystalline plates forming a cup‐shaped reflective structure (mirror), the opening of which faces laterally. Three large dendritic processes penentrate the opening and each terminates in a rhabdomere. The cell body of each dendrite lies slightly behind the margin of the mirror cell. Cytoplasmic extensions of the supportive/mirror cell project across the opening, interposing between the dendritic processes. A secretory process (possibly neurosecretory) passes in front of the eyecup in contact with dendritic processes and the extensions of the supportive cell. Reflective layers consisting of rows of crystalline platelets are widely distributed in the animal kingdom, but among the Platyhelminthes this type of reflective ocellus has previously been reported only from Polystomatidae (Monogenea).