In some of the flatworms (platyhelminthes) the lens is formed from mitochondria, and it is intriguing to speculate whether a mitochondrial enzyme has been co-opted to provide a crystallin.

Mitochondria are, of course, very well known as organelles central to the respiratory processes in eukaryotic cells, and are clearly derived by endosymbiosis from once free-living bacteria, closely related to those that cause typhus.

Mitochondrial eyes

Less well known, perhaps, is that in some of the flatworms (platyhelminthes) the eye lens is formed from mitochondria. This involves, of course, the evacuation of most of the contents but the diagnostic cristae around the margins betray the mitochondrial origin. The contained material is refractile, and it is intriguing to speculate whether a mitochondrial enzyme has been co-opted to provide a crystallin. The arrangement of the mitochondria and the lens they produce varies quite widely, but sometimes involves giant organelles. This arrangement is typically found in the rhabdocoels, but its occurrence in another group (neodermatans) is suggestive that it is convergent within the platyhelminthes.

Chloroplast ‘eyes’

This is not the only example of an organelle being employed in a visual system, because in the warnowiid dinoflagellates the eye of this protistan is partly derived from a chloroplast.