In his memoir on the Pyrosoma, M. Péron describes with lively colors the circumstances under which he first made its discovery, during a dark and stormy night, in the tropical Atlantic. “The sky,” says this distinguished naturalist, “was on all sides loaded with heavy clouds; all around the obscurity was profound; the wind blew violently; and the ship cut her way with rapidity. Suddenly we discovered at some distance a great phosphorescent band stretched across the waves, and occupying an immense tract in advance of the ship. Heightened by the surrounding circumstances, the effect of this spectacle was romantic, imposing, sublime, riveting the attention of all on board. Soon we reached the illuminated tract, and perceived that the prodigious brightness was certainly and only attributable to the presence of an innumerable multitude of largish animals floating with the waves. From their swimming at different depths they took apparently different forms--those at the greatest depths were very indefinite, presenting much the appearance of great masses of fire, or rather enormous, red-hot cannon-balls; while those more distinctly seen near the surface perfectly resembled incandescent cylinders of iron. “Taken from the water, these animals entirely resembled each other in form, color, substance, and the property of phosphorescence, differing only in their sizes, which varied from three to seven inches. The large, longish tubercles with which the exterior of the Pyrosomas was bristled were of a firmer substance, and more transparent than the rest of the body, and were brilliant and polished like diamonds. These were the principal scene of phosphorescence. Between these large tubercles, smaller ones, shorter and more obtuse, could be distinguished; these also were phosphorescent. Lastly, in the interior of the substance of the animal, could be seen, by the aid of the transparency, a number of little, elongated, narrow bodies (viscera), which also participated in a high degree in the possession of phosphoric light.” In the Pholades or Lithodomes, that bore their dwellings in hard stone, as other shell-fish do in the loose sands, the whole mass of the body is permeated with light. Pliny gives us a short but animated description of the phenomenon in the edible date-shell of the Mediterranean (_Pholas dactylus_): “It is in the nature of the pholades to shine in the darkness with their own light, which is the more intense as the animal is more juicy. While eating them, they shine in the mouth and on the hands, nay, even the drops falling from them upon the ground continue to emit light, a sure proof that the luminosity we admire in them is associated with their juice.”