112 BIOLOGICAL LECTURES. 



Appendicularia showed in one individual first red, then bhie, 

 and finally green} This remarkable property seems to be pos- 

 sessed by other tunicates, for Huxley ^ states in his well-known 

 paper on Pyrosoma, quoting Peron, that Pyrosoma exhibited 

 movements of alternate contraction and dilatation at regular 

 intervals ; and that each contraction was accompanied by the 

 development of a luminosity, which, when at its brightest, was 

 red, but in dying away passed through shades of orange, green, 

 and blue. Newport ^ also noticed a change in the color of light 

 emitted by the glow-worm at the end of the season from that 

 at the beginning. I have learned in the study of several species 

 of fire-flies, in the neighborhood of Woods Holl laboratory, to 

 distinguish them at a distance by the color of their light, 

 which, though slight, is still quite characteristic of the species. 

 The difference in color, when exhibited by different organ- 

 isms, is probably due to some slight chemical differences in the 

 light-giving substance. It may be supposed that under the 

 influence of oxygen, the molecules of the given photogenic sub- 

 stance are set in vibration, the rate of vibration depending on 

 and being characteristic of the particular species. And in those 

 cases where a series of colors are displayed in succession by the 

 one and the same organism, it may be supposed to be due to 

 either of two causes : (i) the same photogenic substance is agi- 

 tated with different degrees of frequencies at different periods 

 in the life of the organism, or (2) a series of photogenic sub- 

 stances are produced, each one of the series representing a stage 

 in the chemical metamorphosis of the substance. Without 

 some definite chemical knowledge on the nature of such photo- 

 genic substances, however, it is useless to make any conjecture 

 at present. 



Nor is it easy to offer any. plausible explanation as to the use 

 of such light to the organism, which will apply with equal force 

 to all cases. We may say this much, however, that if heat inci- 



1 The above is taken from W. E. Hoyle's article Phospiiorescence, in the 

 EncyclopcEdia Brittanica, Vol. XVIII, 1S85. 



2 Huxley : On the Anatomy and Development of Pyrosoma. Phil. Trans., 



1859- 



3 Newport : On the Natural History of Glow-worms. Proc. Lin. Soc, Vol. I, 



1857- 



