Like so many coloured Protista, they frequently possess a pigmented " eye-spot " in which may be sunk a spheroidal refractive body (" lens ").
It is marked by a number of rings of papillae placed transversely to its long axis, the dorsal of which are pigmented like the dorsal surface of the body, and the ventral like the ventral surface.
Dorsal surface arched and more darkly pigmented than the flat ventral surface.
In the next stage of complication, seen in the supernumerary (seventh) ocellus of Charybdaea, the patch of pigmented and sensory epithelium is pushed in to form a little pit, in the T Aurelia aurita.
8); the primitively open cup has now closed over to form a vesicle lying beneath the ectoderm; the outer wall of the vesicle becomes thickened to form a cellular lens (1), while the proximal wall consists of sensory and pigmented cells and forms a retina.