Layer of the retina composed of the outer segments of the photoreceptor cells. The photoreceptor outer segment is a specialized primary cilium consisting of a microtubule-based axoneme and regular stacks of membrane folds or disks containing the photopigments (opsins) required for the phototransduction cascade. [ GOC:mh http://zfin.org/ZDB-PUB-110811-12 ORCiD:0000-0001-9114-8737 ]