422 PHYSIOLOGICAL LIGHT. 



Among the Tunicates, there has been noted the Appendicular ia of the 

 southern Atlantic, whose urocord emits a light of a variable color, being 

 red blue, green, and even white in the same individual. This varia- 

 bility in the color of the light has also been noted among the salpiform, 

 colonial Ascidians in Pyrosoma, whose form is like that of a pine cone 

 or an elongated thimble, and which is frequently found on the shore 

 at Mce. Besides artificial stimuli, which may lead to a sudden produc- 

 tion of light, spontaneous simultaneous movements of the colony may 

 produce the same result. Each colonist carries a pair of photogenic 

 organs at the base of its neck, near the upper border of the branclme. 

 They are part of the external layer and entirely composed of spherical 

 cells directly bathed by blood. A colony 0.08 centimeter in diameter, 

 containing 3,200 colonists, will therefore present 6,400 luminous points. 

 These little organs arise in the embryo from the ectoderm. 



Salpa and Doliola have also been reported as luminous. In the 

 Pacific they sometimes form streaks of light many leagues in length. 



Among vertebrates, aside from cases of phosphorescence, probably of 

 parasitic origin, found in man and some rare animals, the photogenic 

 function has been seen only in fishes and especially in those that live 

 at great depths. The photogenic organs may be situated in very dif- 

 ferent regions; along the body walls from the fins to the tail, near the 

 eyes, on the branchiostegal rays, the dentary bone, and the preopercu- 

 lum. By their position as well as by their organization and structure, 

 they recall the photogenic organs of the crustacean Euphaiisidce; like 

 these, they have sometimes been thought to be accessory eyes and some- 

 times photogenic apparatus. Perhaps they may combine the two func- 

 tions. They have, at any rate, ectodermic, muciparous glands connected 

 with nerves of general sensibility, an arrangement that in no way con- 

 flicts with this view when we consider the dermatoptic and photogenic 

 functions in Pholas dactylus. 



This rapid survey of the world of luminous animals and vegetables 

 shows that the photogenic function is more widely distributed than 

 had been generally supposed, and that this beautiful phenomenon 

 should not be considered as a mere biological curiosity. It is, like the 

 production of electricity and of heat, a great physiological function, 

 general in its distribution — that is to say, common to the two kingdoms 

 of living beings. 



The examples we have cited sufficiently show how independent this 

 general function is of the organs in which it arises, and yet how simple 

 is the cellular mechanism that produces it; this being always the same 

 whether we consider the Noctiluca, the Hippopodius, the glowworm or 

 its egg, etc. 



We shall see in the latter portion of this article that physiological 

 analysis may be pushed cpiite beyond the cell, and we shall endeavor 

 to ascertain whether this light, transmitted from generation to genera- 

 tion without extinction, having doubtless been burning for thousands 



