Choroid Choroid The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissues, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye (at 0.2 mm), while in the outlying areas it narrows to 0.1 mm. The choroid provides oxygen and nourishment to the outer layers of the retina. Along with the ciliary body and iris, the choroid forms the uveal tract. The structure of the choroid is generally divided into four layers (classified in order of furthest away from the
Ocelloid hypothesized to represent the eye of a multicellular organism coincidentally engulfed by a single-celled organism. The possibility that it had an evolutionary relationship to plastids had been considered at least since the 1970s, although until the 2010s direct evidence was scarce. Ocelloids contain subcomponents analogous to eye structures including the lens, cornea, iris, and retina. It can be divided into two substructures, the translucent, roundish "hyalosome" and the heavily pigmented "melanosome", also known as the retinal body or pigment cup. The hyalosome serves as the refractive lens of the ocelloid; it is surrounded by a layer of mitochondria serving as