The fact that light was emitted in the dark by a Schizopodous shrimp was first noticed by J. Vaughan Thomson, who, in his Zoological Researches, published about 1820, describes a species which he observed to be luminous under the name Noctiluca. He mentions the presence of scattered spots of red pigment in the animal, but was quite unaware that the production of light was confined to certain definite organs enveloped by this pigment—was indeed unaware of the existence of the organs which form the subject of this paper.