PHOSPHORESCENCE. Physicists apply the word phos phorescence to the emission of light at low temperatures by cer tain substances (chiefly sulphides), after previous exposure to light or radiation of other kinds. However, common usage has made it applicable to the glow of phosphorus, or of the sea, or of dead fish or meat, or of wood, the "fox-fire" of forests. Phosphorescence of the sea or of flesh, wood, etc., is due to living organisms of various kinds and should more properly be called bioluminescence. Such light is the result of a slow oxidation of material manufactured by the organism.
At least 4o different orders of animals contain luminous species, and also two groups of plants, the bacteria, responsible for the luminescence of flesh, and the fungi, responsible for the lumines cence of wood. Either the fine strands of fungus mycelium, which penetrate the wood, or the fruiting body, the mushroom, or both, may be luminous.
The luminous bacteria are so small that individuals cannot be seen by their own light, whereas natural colonies are easily visi ble. They may be cultured on artificial media and form excellent material for the experimental study of bioluminescence. They are not pathogenic to man but are known to infect living animals, giving rise to a natural luminescent disease of sand-fleas, shrimps and possibly midges, which is eventually fatal. Luminous bac teria may also live symbiotically in definite organs of fish, notably Photoblepharon and Anornalops of the Banda islands. Some be lieve that the luminescence of all luminous animals, even the fire fly, is due to symbiotic luminous bacteria, but this is far from proven. The characteristic of the light of bacteria and fungi, which distinguishes them from all other luminous organisms, is its uniformity, shining night and day, independently of stimulation. Other animals light only when disturbed or stimulated.
Phosphorescence of the sea is largely due to Protozoa, of which the Radiolaria, Dinoflagellata and Cystoflagellata (including Noc tiluca) are luminous. The latter two groups may develop in such . enormous numbers that the sea is a pink or red by day and a vivid sheet of flame by night. Larger patches of light in the
ocean are mainly due to coelenterates, jelly-fish, Siphonophora or Ctenophora (comb-jellies). The latter are often very abundant and show the interesting phenomenon of loss of luminescence in sunlight or on bright illumination by electric light. The lumines cence again appears on stimulation after about one-half hour in the dark. The coelenterates contain more luminous species than any other group, the sea pens or pennatulids showing especially brilliant luminescence, which travels along nerves over the colony of polyps in the form of a wave.
Among the Crustacea, Copepoda and Ostracoda (Cypridina) contain many luminous forms while the schizopod and decapod shrimps possess members with true luminous organs or photo phores, often consisting of lens, reflector and pigment screens, veritable lanterns on a microscopic scale, earlier mistaken for eyes.
Among the worms, many marine forms and some earthworms are luminous. Among the echinoderms, only the brittle-stars (Ophiuroids) possess luminous members.
Luminous molluscs are represented only by Pholas dactylus (a bivalve) and Phyllirrhoe bucephala (a nudibranch), together with the cephalopods, of which many members produce light. The cephalopods and deep-sea fish possess the most complex lantern like luminous organs.
Some centipedes are luminous, as well as the spring-tails, lar vae of the fly, Bolitophila, which live in New Zealand caves, and beetles (fire-fly, glow-worm and Pyrophorus of the West Indies). All stages, including the egg, of some fire-flies are luminous.
Balanoglossus and Pyrosoma, primitive chordates, are also lumi nous. The latter forms large colonies, floating near the surface of the sea, each individual possessing two luminous spots.
Finally, mention should be made of the fish, both elasmobranchs and teleosts, containing luminous members. The deep-sea forms are often bizarre in appearance and sometimes contain the lumi nous organ on the end of a long process dangled before the animal as a lure for prey.