While the Botryllus is fixed and adherent, the Pyrosoma, on the contrary, is perfectly free. The animal colony which constitutes it floats and balances itself upon the waters, like the sea-pen or the physalia, of which we have spoken in treating of the zoophytes. The name Pyrosoma has been given to these animals in consequence of their brilliant phosphorescent properties. According to the observations of Péron and Lesueur, nothing can exceed the brilliant and dazzling light emitted in the bosom of the ocean by these animals. From the manner in which the colonists dispose themselves, they form occasionally long trains of fire; but it is a singular fact that this phosphorescence presents the same curious characteristics that distinguish the cilia of the Beroë; namely, that the colours vary instantaneously, passing with wonderful rapidity from the most intense red to yellow, from golden colour to orange, to green, or to azure blue. Von Humboldt saw a flock of these brilliant living colonies floating by the side of his ship, and projecting circles of light having a radius of not less than twenty inches in diameter. He could see by this light the fishes which followed the ship's track, during many days, at the depth of from two to three fathoms. Bibra, a Brazilian navigator, having caught six Pyrosoma, employed them to light up his cabin. The light produced by these little creatures was so bright, that he could read to one of his friends the description he had written of these his living torches. Three species of Pyrosoma are known; namely, _P. elegans_, two or three inches in length, which inhabits the Mediterranean; _P. giganteum_, which is found in the same sea. It is a long bluish cylinder, bristling with tubercles, each of which is the abode of an animal, a citizen of this moving republic, and is attached to its colleagues by means of its gelatinous envelope: an alliance imposed by inexorable Nature--a forced species of socialism. The third species, _P. atlanticum_, was discovered by Péron and Lesueur in the Equatorial seas.