title: A unique photoreceptive structure in the arrowworms Sagitta crassa and Spadella schizoptera (Chaetognatha).
abstract: 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.
mesh_terms: D000818:Animals; D007448:Invertebrates / Q000648:ultrastructure*; D010786:Photoreceptor Cells / Q000648:ultrastructure*