The phosphorescence or "burning of the sea," which is described by so many of the older explorers, is also due entirely to living organisms, both microscopic and macroscopic. The latter are mostly jelly-fish (_medusæ_) or comb jellies (_Ctenophores_) and give rise to the larger, more brilliant flashes of light often seen in the wake or about the sides of a steamer at night. The former are various species of dinoflagellates or cystoflagellates such as _Noctiluca_ (just visible to the naked eye) which collect at the surface of the sea and often increase in such numbers that the water is colored by day (usually pink or red) and shines like a sheet of fire when disturbed at night. Although _Noctiluca_ was recognized as a luminous animal in 1753 by Baker, the light of the sea was a mysterious phenomenon to the older observers. MacCartney, speaking before the Royal Society in 1810, outlines the various older theories as follows: "Many writers have ascribed the light of the sea to other causes than luminous animals. Martin supposed it to be occasioned by putrefaction; Silberschlag believed it to be phosphoric; Prof. J. Mayer conjectured that the surface of the sea imbibed light, which it afterwards discharged. Bajon and Gentil thought the light of the sea was electric, because it was excited by friction.... I shall not trespass on the time of the Society to refute the above speculations; their authors have left them unsupported by either arguments or experiments, and they are inconsistent with all ascertained facts upon the subject. The remarkable property of emitting light during life is only met amongst animals of the four last classes of modern naturalists, viz., mollusca, insects, worms, and zoöphytes." MacCartney recognized the true cause of the light, although he had little idea of the vast number of marine forms which are luminous and omits entirely any reference to the fishes, many of which produce a light of their own when living, apart from any bacterial infection. A survey of the animal kingdom discloses at least 36 orders containing one or more forms known to produce light and several more orders containing species whose luminosity is doubtful. In the plant kingdom there are two groups containing luminous forms. The distribution of luminous organisms is brought out in the accompanying classification of plants and animals. Those orders are printed in italics which contain species whose self-luminosity is fairly well established. It will be noted that further subdivisions into orders is not given in classes of animals which lack luminous forms. TABLE I _DISTRIBUTION OF LUMINOUS ORGANISMS IN PLANT AND ANIMAL KINGDOMS_ PLANT KINGDOM