There are probably few subjects of natural investigation which have excited so many and such long-continued contentions as the phosphorescence of sea-water. All that is known with certainty regarding this much disputed question may be reduced to the following simple facts. There are many luminous mollusca which possess the property when alive of emitting at will a faint phosphoric light; which is of a bluish tinge in _Nereis noctiluca_, _Medusa pelagica var._ β,[KE] and in the pipe-like _Monophora noctiluca_, discovered in Baudin’s expedition.[KF] The luminosity of sea-water is in part owing to living light-bearing animals, and in part to the organic fibres and membranes of the same, when in a state of decomposition. The first-named of these causes of the phosphorescence of the ocean is undoubtedly the most common and the most widely diffused. The more actively and the more efficiently that travellers engaged in the study of nature have learnt to employ powerful microscopes, the more our zoological systems have been enriched by new groups of mollusca and infusoria, whose property of emitting light either at will or from external stimulus has been recognised. The luminosity of the sea, as far as it depends on living organisms, is principally owing, among zoophytes, to the Acalephæ (the families of Medusæ and Cyaneæ), to some Mollusca, and to an innumerable host of Infusoria. Among the small Acalephæ (Sea-nettles), the _Mammaria scintillans_ presents us, as it were, with the glorious image of the starry firmament reflected in the surface of the sea. When full-grown this little creature scarcely equals in size the head of a pin. The existence of siliceous-shelled luminous infusoria was first shown by Michaelis at Kiel. He observed the coruscation of the Peridinium. (a ciliated animalcule,) of the Cuirass-monad (_Prorocentrum micans_), and of a rotifer, which he named Synchata baltica,[KG] the same that Focke subsequently found in the lagoons of Venice. My distinguished friend and fellow traveller in Siberia, Ehrenberg, succeeded in keeping two luminous Infusoria of the Baltic alive for nearly two months at Berlin. I examined them with him in 1832; and saw them coruscate in a drop of sea-water on the darkened field of the microscope. When these luminous Infusoria (the largest of which was only ⅛ and the smallest from ¹⁄₄₈ to ¹⁄₉₆ of a of a Parisian line in length) were exhausted, and ceased to emit sparks, they would renew their flashing on being stimulated by the addition of acids or by the application of a little alcohol to the sea-water.