Animal Luminescence 569 



doubt similar to that used for his previous nerve and muscle studies, 

 which were made in 1797, soon after Galvani's famous frog nerve 

 discoveries were published in 1791. 



During the nineteenth century there were innumerable records 

 of luminous jellyfish, both the large more or less opaque scypho- 

 medusae and the smaller transparent hydromedusae, whose asexual 

 polypoid forms are hydroids. Further studies of stimulation— mecha- 

 nical, electrical, thermal and chemical— were made by J. Macartney 

 in 1 8 10 on a small species, Medusa hemispherica. His experiment 1 1 

 reads as follows: 



Some hemispherical medusae were placed in contact with the two ends 

 of an interrupted chain, and slight electric shocks passed through them. 

 During the very moment of their receiving the shock no light was visible, 

 but immediately afterwards the medusae shone like illuminated wheels, 

 which appearance remained for some seconds. Upon the closest inspec- 

 tion with a magnifying glass, no contractile motion could be perceived to 

 accompany the exhibition of the light. The application of electricity in 

 this instance seems to have acted merely as a strong mechanic shock. 



Macartney also found that when heated or when plunged into 

 " spirits," the luminescent spots again appeared " like illuminated 

 wheels." 



He placed some medusae in a vacuum and could not discover that 

 the light was any less brilliant, in fact it was more easily excited by 

 shaking and continued longer in a vacuum. This experiment is of 

 considerable interest, for the author discovered in 1926 that medusae, 

 (Pelagia) at Naples, will luminesce under conditions of complete 

 anoxia, in this respect differing from most luminous animals in 

 which the light production only occurs if dissolved oxygen is present. 



Macartney continued: 



It seems proved by the foregoing experiments, that so far from the lumi- 

 nous substance being of a phosphorescent nature, it sometimes shews the 

 strongest and most constant light, when excluded from oxygene gas; that 

 it in no circumstances undergoes any process like combustion, but is 

 actually incapable of being inflamed; that the increase of heat, during 

 the shining of glow-worms, is an accompaniment, and not an effect of 

 the phenomenon, and depends upon the excited state of the insect; and 

 lastly, that heat and electricity increase the exhibition of light, merely 

 by operating like other stimuli upon the vital properties of the animal. 



Nevertheless, Macartney did believe that " the power of shewing 

 light resides in a peculiar substance or fluid, which is sometimes 

 situated in a particular organ, and at others diffused throughout the 



