M. Giglioli agrees with Huxley in stating that “the phenomenon of phosphorescence in these animals does not reside in the protoplasmic branches, which, as is known, are sometimes wanting; but in the cortical substance it is not uniform, but manifests itself in distinct and very minute luminous points, which sparkle, go out, and light up again.” [2] PAGE 9. Kiel observed this phenomenon in _Peridinium_. The following species of luminous forms existing in the Baltic Sea have been described by Ehrenberg: _Prorocentrum micans_, _Peridinium michælis_, _Peridinium micans_, _Peridinium fuscus_, _Peridinium furca_, _Peridinium acuminatum_, _Lynchata baltica_, and a species of _Stentor_. [3] PAGE 9. Giglioli and his assistant, De Fillipi, observed luminosity in the gelatinous mass described by Hækel as _Citophora_. The genera of those low forms most remarkable for luminosity are _Thalassicolla_, _Collozoum_, _Sphærozoum_, and _Collosphæra_. Giglioli states that the forms of this group which are found in the Indian Ocean and China seas are not luminous. [4] PAGE 11. _Dymophora fulgurans._ [5] PAGE 12. Other light-givers of this group are _Willsea prolifera_, _Bourganivillia_, and _Lizzia_. [6] PAGE 12. _Mueniopsis leidyii._ [7] PAGE 13. The _Lucernaria_ is a very rare form of _medusa_ on our northern shores, and particularly characteristic in color and form. It is more like a polyp in texture, and its rich beryl color distinguishes it from all other forms. It is related to the _Discophores_, animals belonging to one of the groups of jelly-fishes, or _medusæ_. [8] PAGE 14. Schafer has observed radiating fibres on the under side, but there is no evidence to show that the luminosity originates here. In fact, the outer surface, where the cells of the delicate epithelium, or skin, contain minute points of fatty material, is equally phosphorescent. The tentacles become luminous, and it is supposed that they contain no nerves except at the margin of the disk. In some instances the light seems well defined at the so-called eye-spots at the edge of the disk, but its sudden fluctuations render any attempt at locating a photogenic structure difficult. While numerous theories are advanced, investigators are entirely at fault as regards any satisfactory explanation of the phenomenon. There are certain conditions which are not favorable to the emission of light; and observers have seen _medusæ_, vividly luminous at one time, and not so at another. It has been suggested that the light is subject to the so-called will of the creature. A better theory, perhaps, would attribute the luminosity to certain peculiar conditions, or to certain stages of existence.