In the United States, there are ten thousand enrolled young naturalists, comprising the Agassiz Association. As one of a committee solicited to answer questions propounded by the young people, members of this association and of the Chautauqua Circle, I have often been surprised at the nature of the queries, which shows that this army of young observers includes many who are not merely collectors of curiosities, but are naturalists in the best sense. They are systematic inquirers, and working in the right direction to become scientists, should they continue. It is to these young scientists, their unscientific elders, and the boys and girls in general who have not yet had their interest aroused in Nature’s works, that this volume is addressed; and if some information is conveyed, while appearing merely to entertain, one object of the author will have been accomplished. The subject of phosphorescence is one which affords the widest field for investigators; as, while the most careful descriptions of the light-emitting organs have been made, the actual cause of animal phosphorescence is unknown. Material for study is ever at hand; the fire-fly courts attention at every summer door-yard, and the pools of beach and cove are illumined by ocean forms. Even the simplest experiments are of the greatest interest. I have read by the light of a luminous beetle, and have determined the time of night while holding my watch in the glare of ocean animals. Von Bibra wrote his description of the _Pyrosoma_ by its own light; the shark of Bennett illuminated his cabin like a chandelier; photographs have been taken by the light of luminous beetles and by phosphorescent plates; and probably the day is not distant when more important uses will be found for this wonderful light, which, in default of a better name, we term phosphorescence. It is found in the animal, vegetable, and the mineral kingdoms; in life and in death; in growth and in decay. It illumines, but does not appear to consume, and without perceptible heat exists where ordinary combustion is impossible. From the nature of the subject, it is evident that illustrations of the phosphorescence of marine animals must be more or less conjectural; and those given, representing over fifty luminous forms, show as nearly as possible the probable effect produced. As this work is scientific only so far as to secure accuracy, some technical details have been omitted. To compensate in a measure, I have appended a fairly complete bibliography of most important monographs and papers on the subject, which may be of value to those who wish to pursue the subject in its technical relations. To render the work as popular as possible, certain systematic portions necessary to the student are placed in an appendix, and referred to by number. The whole work is also thoroughly indexed.