Cephalic structures in the nemertine Parborlasia corrugatus-Are they really eyes?

Abstract

The opinion as to whether tiny, approximately 0.1 mm large spots around the innermost margin of the cephalic slits in the Antarctic nemertine worm Parborlasia corrugatus represent photoreceptors or not has fluctuated over the years. This first electron microscope study of the enigmatic spots fails to detect any screening pigment granules, rhabdomeres, or lamellae, but reveals that the structure in question is principally made up of two types of cell, characterized by vesicular and vacuolar material of approximately 80 nm and 0.3 mum in diameter, respectively. Filamentous connective tissue strands with gaps for axons surrounds the 'eye-spot' and it is suggested that either exposure to the bright Antarctic summer light has led to a total disintegration of all visual membranes or these structures do not represent eyes at all.