Some preliminary observations on luminescence in polynoid worms have been presented in a previous paper (Nicol, 1953). These animals produce light in scales (elytra) which cover the dorsal surface of the body. The source of the light lies in a layer of unicellular epithelium on the lower surface of the scale. Histologically, this tissue consists of columnar cells (photocytes), characterized by the presence of coarse eosinophilic granules in the cytoplasm. The nervous supply of the elytrum derives from a nerve trunk which ascends the stalk or elytrophore and proceeds to a ganglion in the centre of the scale. From this ganglion nerves radiate peripherally and proceed to the photocytes, and to sensory receptors on the dorsal surface and at the margin of the scale (Bonhomme, 1942).