Type III

This organization is present only in some dinoflagellates of the order Warnowiales, such as Nematopsides sp. and Erytropsidinium sp. The photoreceptor system is very specialized and it is termed ocellus. It is situated towards the left side of the ventral surface of the cell. It consists of a refractile structure termed hyalosome, thought to act as a lens, subtended by a domed pigmented part, divided into two sections, a retinoid and a pigmented cup. Between the lens and the retinoid is a chamber representing an invagination of the cell covering, which is lined by the cell membrane, and allows the contact of the ocellus with the external medium. The pigmented cup wrapping the retinoid represents the eyespot and is made up of pigment containing droplets enclosed in a vesicular layer. Small droplets contain carotenoid pigments, large droplets contain melanoid pigments. The retinoid is an extremely complex membranous construction made up of numerous regularly arranged layers giving an almost paracrystalline appearance.

No data are available on the structure and localization of the photoreceptive system in the divisions and classes of algae other than those listed above. It seems unlikely that Type II and Type III systems have not been identified so far in other algal groups, while it is more reasonable to assume that this lack of information is mainly due to the difficulty to reveal photoreceptor systems belonging to Type I. We can conclude that those algae should possess photoreceptor systems that can be taken back to Type I.