In the summer months in tropical and semi-tropical waters, often during several days in succession, the ocean presents a surface almost unruffled. The fin of some roving shark, the splash of the flying-fish, or, if near shore, the plunge of the pelican or gull, are the only objects that disturb the sea of glass. At such times, after the sun had gone down, we have lain in our boat, with faces as near the surface as possible, and watched the wondrous panorama of the submarine world. Here great globes of light seemed to shoot through the watery space: every fish left a train of light; while the dolphin, or other great forms, gliding by, appeared converted into fiery monsters; and, as they rose to the surface, fountains of phosphorescence burst from the sea. The forms which tend to produce this remarkable appearance in the ocean depths are many; but, in the warm waters of the tropics, the most noticeable are those belonging to the class known scientifically as _Tunicata_. Aside from their luminous properties, the Tunicates are extremely interesting, from the fact that they are now supposed to represent, with perhaps one exception, the lowest form of backboned life,--being what are called degenerate forms. In the larval stage of some species a noto-cord is present, which is supposed to represent the backbone of higher vertebrates. In some, when the animals assume the adult form, the little spinal cord is absorbed; but in others, as the _Appendicularia_ (Plate XVI., Fig. 3), the noto-cord and neural cord persist throughout the entire life of the animal. The life-history of these forms is of extreme interest; but, as it can be found in any text-book, we will pass to the feature that has rendered some of the class most conspicuous. In exploring the depths of southern seas, among others we shall see a columnar form, the _Pyrosoma_, or “fire-body” (Plate XVII.), the giant of the Tunicates. It is an aggregation of individuals, forming a hollow cylinder closed at one end, and from two inches to four feet in length.[41] The _Pyrosomæ_ are richly tinted during the day; but at night, as their name implies, they resemble incandescent bodies. Humboldt refers to the spectacle he enjoyed when passing through a zone of them in the Gulf Stream, distinguishing by their light the forms of fishes, that, bathed by their gleams, stood out in bold relief far below the surface.