Morphology of the ommatidia of the compound eye of Limulus.

The sensory portion of the ommatidium of the compound eye of Limulus has been studied with the electron microscope. In axial longitudinal section the rhabdom appears to be made up of small polygons, and in transverse section the rhabdom appears as a banded structure of dark lines. Thus in three dimensions the rhabdom resembles a honeycomb composed of tubular units, the long axes of which lie in transverse planes and are oriented perpendicular to the retinula cell's contours. The tubular units, which are about 140 mmicro in diameter in Limulus (70 mmicro in diameter in the spider and Scutigera), are microvilli of the borders of the retinula cells. The walls of these microvilli are continuous with fine linear structures (membranes) in the cytoplasm of the retinula cells. In transverse sections of the ommatidium oval bodies interpreted as mitochondria are observed in an annular zone at the tips of the rhabdom's rays. These mitochondria, which are 2 to 10 micro in diameter, are crowded with irregular closed outlines about 100 mmicro in diameter. Possible functions of components of the ommatidium are discussed.