Light and dark adaptational changes in the accessory eye of the shrimp, Palaemonetes.

The fine structure of the accessory eye of the shrimp, Palaemonetes, was studied after light and dark adaptation. Adjacent to the principal compound eye, the accessory eye is a small compound eye composed of about 20 ommatidia which are smaller in structure to the ommatidia of the principal compound eye. During dark adaptation, rhabdomal microvilli increase in length; however, little pigment migration occurs. The implications of these changes in relation to the function of the accessory eye are discussed.