The ciliary epithelium has a pigmented and an unpigmented
portion. It's really a bilayered covering of the ciliary processes. The pigmented cells
(Ep2) rest on a basal lamina and are close to an underlying blood vessel (BV).
The unpigmented cells (Ep2) are exposed to the posterior chamber.
The two layers are continuous with the retinal pigment epithelium and derived from the optic cup. Aqueous humor produced by the ciliary body has a composition with exactly the right refractive index, and other properties which serve the needs of the internal structures. The aqueous humor is also the route for nutrient transport to the avascular lens. Its production has to take into account the special requirements of the fragile and physiologically delicate internal structures.
The eye, like the brain, has to have a barrier between it and the rest of the real world, to prevent passage of deleterious materials from the blood directly into the aqueous humor. The composition of aqueous humor with respect to electrolytes and so forth is very different from that of the blood. In order to maintain this difference, free passage of ions and so forth can't be allowed. These two layers of cells are part of that barrier: they're sealed together by occluding junctions, so that blood-borne material has to go through both layers of cells before reaching the posterior chamber. In other words, the occluding junctions between these two layers physically seal off the eye and give these cells total control over what does and doesn't get in.
Monkey eye; H&E stain, paraffin section, 400x