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.