LUMINOSITY IN ANIMALS. 891 



marine Crustacea appear like little luminous specks in glasses of sea-water, 

 especially when this is agitated ; and they are even discernible in the stomachs 

 of larger Molluscoid animals, which feed upon them. Some of the marine 

 Annelida are distinguished by being able to emit sudden scintillations of light 

 along the body, which may be repeatedly excited by mechanical irritation. It 

 has been suggested that, in these cases, the light may be excited through ner- 

 vous agency, which may possibly undergo conversion into light. (Carpenter. ) 

 Amongst the Annuloid animals, certain Star-fishes are said to be luminous. 

 But the Coelenterata yield the largest number of luminous marine creatures, 

 especially the Acalephee and the Hydroid Polyps, such as Pennatula and 

 others. Lastly, a minute jelly-like creature, formerly assigned to the group 

 Acalephse, amongst the Coslenterata, but now classed amongst the Protozoa, 

 by some as a Rhizopod, but by others as a peculiar and gigantic Infusorial ani- 

 malculse (Huxley), the Noctiluca miliaris, is the most common of all light- 

 giving creatures m the temperate oceans, and is the chief cause of the lumin- 

 ous nocturnal appearance in our ^Northern Seas. In the Tropics, the phe- 

 nomena is much more striking and brilliant, and depends upon a greater 

 variety of animals, especially upon the Medusae and the Hydroid Polyps. 



The luminosity of these various marine animals is said to depend upon a 

 mucous secretion from their integument, which will even impart luminosity 

 to water or milk with which it is mixed. The so-called phosphorescence is 

 always more marked in warm than in cold climates ; it is increased by moder- 

 ate elevation of the temperature of the water, and, most remarkably, by brisk 

 agitation of the fluid, either because the secretion is detached from the ani- 

 mals, or simply owing to their excitement. The light is extinguished by ex- 

 tremes of either heat or cold ; it disappears in tacwo, and is restored on renewed 

 exposure to air ; it is rendered more vivid by various stimulating substances, 

 if moderately used, and also by electricity ; but it is extinguished by the ex- 

 cessive employment of these, and especially by such vapors as those of ether 

 and chloroform, which would interfere with oxygenation. Oxygen increases 

 and maintains the phosphorescence ; carbonic acid first excites, and then de- 

 stroys the luminous property ; sulphuretted hydrogen almost instantly arrests 

 it. The luminosity may continue for a time after death, unless this has been 

 produced by some specially poisonous substance. It entirely ceases as soon 

 as putrefaction begins. 



From the preceding facts, it is obvious that the luminosity of animals is 

 owing to some living action, and not to decomposition. 



Its use is by no means understood. The supposition that it serves occa- 

 sionally to guide one sex to the other, affords a very partial explanation of the 

 facts ; for it does not apply to the cases of the multitudes of hermaphrodite 

 marine luminous animals. It may conduce to their destruction, by assisting 

 other animals in seeking them as food ; or it may serve to illuminate deep 

 waters. But this curious phenomenon affords a good example of the imper- 

 fection of our knowledge of final causes. 



In the glow-worm it appears not to be phosphorus, but some carbonic com- 

 pound which produces the light. Even in the case of the numerous marine 

 luminous animals, it is not proved that the light is owing to the slow oxida- 

 tion of a phosphuretted substance. This, however, seems more probable in the 

 case of animals living in water, in which the luminous oxidation of a phosphu- 

 retted body is more conceivable than that of a hydro-carbonaceous substance. 

 It is possible that some, at least, of the feeble light exhibited in these phenom- 

 ena, or its intensification, is due to fluorescence developed in a high degree ; 

 fluorescent substances certainly exist in living animals. The term phospho- 

 rescence must be regarded as descriptive and provisional only, for the light 

 may not depend in any case upon the oxidation of a phosphuretted compound. 



The evolution of light from these animals as a normal phenomenon, and that 

 from the human body as an occasional or morbid occurrence, must be accom- 

 panied by chemical change, in. which the chemical energy passes into the form 

 of light. The photic work of the animal body must therefore depend on the 

 chemical energy evolved by it. But the quantity of matter subjected to change 

 in its production is very small. 



