Fine structure of the optic cusion in the asteroid Nepanthia belcheri

The optic cushion of Nepanthia belcheri (Perrier) is a prominent pigmented sense organ situated on the oral surface below the terminal tentacle. The distal region contains up to 170 optic cups, whilst proximally are numerous pyriform glandular cells traversed by supporting fibres. The outer margin of the optic cup is formed by alternating pigmented and photoreceptor cells. The pigmented cells contain numerous densely staining granules of scarlet pigment. The distal ends of the photoreceptors are elaborated into many long microvilli regularly arranged about a modified cilium. There is a clear circumciliary space delimiting the cilium from the microvilli.