Luminous animals are all either marine or terrestrial forms. No examples of fresh water luminous organisms are known. Of marine forms, the great majority are deep sea animals, and it is among these that the development of true luminous organs of a complicated nature is most pronounced. Many of the luminous marine animals are to be found in the plankton, while the littoral luminous forms are in the minority. Some members of all the above groups are found at one or another of our marine laboratories with the possible exception of _Pholas_, _Phyllirrhoë_ and squid. Although earthworms and myriapods which produce light are found in the United States, they are rather rare and seldom observed forms. Not only adult forms but the embryos and even the eggs of some animals are luminous. The egg of _Lampyris_ emits light within the ovary and freshly laid eggs are quite luminous. The light does not come from luminous material of the luminous organ adhering to the egg when it is laid but from within the egg itself. _Pyrophorus_ eggs are also luminous. The segmentation stages of _Ctenophores_ are luminous on stimulation, as noted by Allman (1862), Agassiz (1874) and Peters (1905), but the eggs themselves do not luminesce. _Schizopod larvæ_ (Trojan, 1907), _Copepod nauplii_ (Giesbrecht, 1895), _Chætopterus larvæ_ (Enders, 1909), and brittle star _plutei_ (Mangold, 1907) also produce light. Apparently there is no rhyme or reason in the distribution of luminescence throughout the plant or animal kingdom. It is as if the various groups had been written on a blackboard and a handful of sand cast over the names. Where each grain of sand strikes, a luminous species appears. The _C[oe]lenterates_ have received most sand. Luminescence is more widespread in this phylum and more characteristic of the group as a whole than any other. Among the arthropods luminous forms crop up here and there in widely unrelated groups. In the mollusks, excluding the cephalopods, only two luminous species are known. Several _phyla_ contain no luminous forms whatever. It is an extraordinary fact that one species in a genus may be luminous and another closely allied species contain no trace of luminosity. There seems to have been no development of luminosity along direct evolutionary lines, although a more or less definite series of gradations with increasing structural complexity may be traced out among the forms with highly developed luminous organs.