The cornea of the sand lance, Limnichthyes fasciatus (Creeiidae).

The sand lance, Limnichthyes fasciatus, is a small fish, 15-30 mm in length, found on the Great Barrier Reef. The eyes, which are not covered by a spectacle, are externally placed, move independently, and measure about 1 mm in diameter. The cornea, unusual even in the teleost world, is about 0.14 mm thick, which represents one-seventh of the length of the eye. The layers of the cornea include an epithelium with a complex pattern of surface microplicae, a grossly enlarged basal cell layer, and a thick basement membrane. Structures (iridophores), which may represent the vestigial remnants of a secondary spectacle, are present in the peripheral corneal epithelium. The stroma, which has no Bowman's layer, is composed of lamellae of collagen fibrils but contains no keratocytes. Posterior to the stroma is a thick (0.1 mm) cellular layer that may represent the autochthonous layer seen in some teleost species. An iridescent layer consisting of approximately 70 parallel cytoplasmic plates oriented at right angles to the visual axis lies anterior to a thick Descemet's membrane. There is a single layer of endothelial cells on the posterior surface.