In the great class of the Articulata, we may start from an optic nerve simply coated with pigment, the latter sometimes forming a sort of pupil, but destitute of lens or other optical contrivance.
Artikulatuen klase zabalean, abiapuntu moduan nerbio optiko bat aurkitzen dugu pigmentuz estalia besterik gabe, honek batzuetan betsein antzeko bat, baina kristalino edo atal optikorik gabekoa, eratzen duelarik.