A Unique Photoreceptive Structure in the Arrowworms Sagitta Crassa and Spadella Schizoptera (Chaetognatha)

The ultrastructure of photoreceptors in two species of chaetognaths, Sagitta crassa and Spadella schizoptera, was studied by electron microscopy using a goniometer specimen stage as well as by freeze-fracture techniques. In contrast to earlier descriptions, the photoreceptor is made up of a stack of lamellae with pores. The lamellae, each 30-45 nm in thickness, are piled one on top one another at intervals of 10-20 nm. The lamellar surface is often perpendicular to the incident light. The pores are 35-55 nm in diameter and arranged in an orderly square lattice with a center-to-center distance of 80-95 nm. These perforated lamellar structures, resembling annulate lamellae, are entirely new as photoreceptive structures.