Among animals the best known forms are the dinoflagellates; Noctiluca; hydroids; jelly-fish; ctenophores; sea pens; Chætopterus and other marine worms; earthworms; brittle stars; various crustaceans; myriapods; fireflies and glowworms, the larvæ of fireflies; Pholas dactylus and Phyllirrhoë bucephala, both molluscs; squid; Pyrosoma, a colonial ascidian; and fishes.
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 labora
tories 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 Cœlenterates 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.
While the accompanying list of luminous genera aims to be fairly complete, there are no doubt omissions and some inaccuracies in it. Anyone who has ever tried to determine what animal is responsible for the occasional flashes of light observed on agitating almost any sample of sea water will realize how difficult it is to discover the luminous form among a host of non-luminous ones, especially if the animal is microscopic in size. It is not surprising, then, to find many false reports of luminous animals in the literature of the subject and we cannot be too careful in accepting as luminous a reported case. The difficulty lies chiefly in the fact that all luminous organisms with the exception of bacteria, fungi, and a few fish, flash only on stimulation, and, while it is easy enough to see the flash, the animal is lost between the flashes. The only safe way to detect luminous organisms is to add a little ammonia to the sea water. This slowly kills the organisms and causes any luminous forms to glow with a steady, continuous light for some time, a condition accompanying the death of the animal. Not all observers, however, have followed this method. One must always be on guard against confusing the light from a supposed luminous form with the light from truly luminous organisms living upon it. The reported cases of luminosity among marine algæ are now known to be due to hydroids or unicellular organisms living on the alga.
We know also that many non-luminous forms may become infected with luminous bacteria, not only after
death, but also while living, so that their luminescence is purely secondary. Giard and Billet (1889-90) succeeded in inoculating many different kinds of amphipod crustacea (Talitrus, Orchestia, Ligia) and isopod crustacea (Porcellio, Philoscia) with luminous bacteria, in some cases passing the infection from one to the next through nine individuals. Curiously enough the bacterium did not produce light on artificial culture media but did when growing in the body of the crustacea, which were killed in about seven days by the infection. The species of Talitrus and Orchestia might easily have been taken for truly luminous animals if not carefully investigated.
Tarchanoff (1901) has injected luminous bacteria into the dorsal lymph sac of frogs with the result that the animals continued to glow for three to four days, especially about the tongue. I remember once while collecting luminous beetles in Cuba, I was astounded to find a frog which was luminous. Expecting this animal to be of great interest, I examined it further only to find that the frog had just finished a hearty meal of fireflies, whose light was shining through the belly with considerable intensity.