Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye.
The natural fracture lines (sutures) of the head run along the top edges of the compound eye.
The resulting eye is a mixture of a simple eye within a compound eye.
The retinula (literally, small retina) cells of the ommatidium of the compound eye contain areas from which membranous organelles of conceivable size (rhabdomeres) extend.
Most copepods have a single median compound eye, usually bright red and in the centre of the transparent head; subterranean species may be eyeless.
Each compound eye is made up of up to ten thousand ommatidia of the apposition type.
Chitons have a dispersed network of tiny eyes over the surface of their shells which may act together as a compound eye.
There is a single large compound eye which takes up much of the animal's head.
