Structure of the eye of an adult bony fish, Gambusia (Haplochilus) schoelleri.

The study showed that the eyeball of Gambusia is spherical and that it has two thin transparent lids--a small upper lid and a larger lower lid. The cornea is composed of four layers, and the lens, which is relatively large, is covered externally by a lens capsule. The lens fibres are arranged in concentric lamellae. The ciliary body is present only on the ventral aspect of the eye. The iris is triangular and is densely pigmented with guanin crystals. As in other vertebrates, the retina consists of ten layers. The hyaloid or retinal artery lies among the optic nerve fibres. The photoreceptor cell layer is generally thick and contains rods and cones. The internal retinal envelope, the choroid coat, appears on the medial aspect of the eyeball as a thickened vascular part, referred to as the suprachoroidal layer. This layer contains a horseshoe-shaped gland, the choroid gland, the outer portion of which is surrounded by a layer of silvery guanin crystals generally termed the argentea.