SEAS OF FLAME. 81 



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 aggre- 

 gation of individuals, forming a hollow cylinder closed at 

 one end, and from two inches to four feet in length. 41 



The Pyrosomce 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, distin- 

 guishing by their light the forms of fishes, that, bathed by 

 their gleams, stood out in bold relief far below the surface. 



The light is extremely beautiful. That of the Atlantic 

 forms is said to be polychroic, or an intense green ; while in 

 the very large species it is azure. So brilliant and striking 

 is the light, that the impression is gained that it proceeds 

 from the entire surface of the animal ; but this is not the 

 case, according to Panceri. 42 When the Pyrosoma is moving 

 along in its curious fashion, which calls to mind the old 

 stern-wheel steamers, and is undisturbed, the light is inter- 

 mittent, now flashing from one cell, and now from another ; 

 the vast number of gleams giving it the appearance at times 

 of constant light over the entire surface. 



Panceri found that the luminous bodies produced an albu- 



