“When one examines in a dark chamber the abdomen detached from a Luciola which has been plunged in a solution of osmic acid, it is seen that a part of the segments occupied by the luminous organs shine with a feeble and variable light; whilst another part (ordinarily in the neighborhood of the median line) is obscure, or as it were veiled by a light phosphorescent cloud. When the preparation is placed under the microscope, the luminous parts exhibit towards the top the appearance which we have already noted in examining normal Luciolas; that is to say, the existence of obscure round spots surrounded by brilliant field. In observing more attentively, one perceives around the spots other little spots, less obscure, and sometimes hardly visible, disposed with a certain degree of regularity. “Now, if we compare these images with those which are presented under the microscope by the luminous organs when hardened in alcohol, and cleared up by caustic potash, or else a tangetized section made of the organ of an animal killed by osmic acid, and colored by carmine, it becomes evident that the large, obscure round spots correspond to the central part of the digitiform lobes of _Targioni Tozzetti_; that is to say, to the cylinders constituted by the matrix of trachea (_Tracheenendzellen_ of M. Schultze), whilst the luminous part is represented by the parenchymentous cells, and the little obscure spots are due to nuclei of these same cells. Still towards the limit of the brilliant and obscure regions of the luminous organ a very varied spectacle is observed.... “From all the facts which we have just described, one may conclude with full certainty that the light of the Luciola has its seat in the parenchymentous cells of the luminous organ.” “It remains to be seen if the luminous combustion does not also take place, though, with luminosity in other parts. In my previous work I had it that the surface of the cylindrical lobes formed by the matrix of the tracheæ was the principal focus or seat of the combustion. The facts which result from later observation oblige me to abandon this opinion.... “In the moments of mean luminous activity, one may say that the combustion is situated exclusively in the parenchymentous cells of the superficial layer of the luminous organ.” [39] PAGE 73. _Gammarus caudisetus_, _Gammarus longicornis_, _Gammarus truncatus_, _Gammarus heteroclitus_, _Gammarus crassimanus_. _Cyclops exiliens_ is also luminous. [40] PAGE 77. Another species in which this change had taken place is _Galathodes antonii_, an allied form which is shown in the central figure of the frontispiece. Many more, as _Willemœsea_, _Pentacheles_, _Polycheles_, and others, have organs of vision, which have undergone more or less change. It has been suggested that certain deep-sea crabs, as _Geryon tridens_, _Gonoplax_, _Donychus_, and _Munida_, have phosphorescent eyes.