574 History of Luminescence 



present genus, Physalia, the Portuguese man-of-war is not luminous. 

 Only the delicate transparent forms produce light. They have been 

 known for a long time but apparently the first recognition of lumi- 

 nosity came from F. J. F. Meyen (1804-1840) in 1834. C. W. Peach 



(1850) also found that siphonophores were responsible for some 

 of the light along the English coast and E. H. Giglioni (1870) 

 named four species as luminous on the high seas, while Panceri 



(1871) studied luminescence of the varieties at Naples in some 

 detail. The light is relatively weak and has not attracted the atten- 

 tion lavished on the more striking organisms. 



Hydroids 



The light of sea growths described by Aelian under the name of 

 " Aglaophotis marina " was most probably due to hydroids. Since 

 that time many naturalists must have noted the sparkling when the 

 hand is rubbed over hydroids growing on rocks or piles in the sea 

 at night. It is difficult to name the actual time of identification of 

 the animals, but one of the first to recognize luminous hydroids 

 appears to have been Charles Stewart. In 1802 he published Ele- 

 ments of the Natural History of the Animal Kingdom. Speaking of 

 Serularia pumila growing on Fucus, he wrote: 



This species and probably many others, in some particular states of the 

 atmosphere, give out a phosphoric light in the dark. If a leaf of the 

 above Fucus, with the Sertularia upon it, receive a smart stroke with a 

 stick in the dark, the whole Coralline is most beautifully illuminated, 

 every denticle seeming to be on fire. 



Later Francois Peron (1804) described luminous Sertularia, and 

 Charles Darwin, during the voyage of the "Beagle" in 1831-1836, 

 noticed a luminescent Clytia-like growth while collecting at Tierra 

 del Fuego. By 1841, the year that M. Sars (and in 1842 J. J. Steen- 

 strup) worked otit the alternation of generations for hydroid and 

 medusa, A. H. Hassal was able to state: " All the most transparent 

 Zoophytes possess highly luminous properties. This fact I discovered 

 in a specimen of Laomedia gelatinosa. . . ." The statement is not 

 entirely true, as many transparent hydroids are now known to be 

 non-luminous. 



Further observations were made by D. Landsborough (1842) , by 

 E, Forbes (reported by George Johnson in 1847) , and particularly 

 by P. Panceri (1878) , who identified the luminous cells and studied 

 stimulation of the hydroids. 



