52 COMPARATIVE PHYSIOLOGY 



tissues. According to Baglioni (1903) and Mines (1912), 

 addition of urea to the saline medium is essential to the mainte- 

 nance of activity in the artificially-perfused heart of Elasmo- 

 branchs. 



Bioluminescence. — Of all forms of secretion which are of 

 interest from a bionomic standpoint none have received so 

 much attention as the phenomena of light-production. 

 Bioluminescence is widely distributed throughout the animal 

 kingdom, and is also found among bacteria and fungi. It is 

 met with in many Protozoa {e.g. Noctiluca) ; in representatives 

 of all groups of Coelenterates, several Polyzoa {e.g. 

 Membranipora) ; Polychaetes {e.g. Chsetopterus) ; Ophiuroids, 

 Urochorda, many Crustacea {e.g. Cypridina), Myriapods and 

 Insects {e.g. Lampyris), many Cephalopods, a few other 

 molluscs {e.g. Pholas), and, among Vertebrates, in some genera 

 of fishes. 



The actual intensity of illumination produced by animals 

 is hardly ever such as to bear the scrutiny of the light-adapted 

 eye. In some forms the oxidative process involved proceeds 

 more or less continuously, independently of stimulation, as in 

 bacteria and a few fish. More generally it is an intermittent 

 form of activity, luminescence occurring only in response to 

 stimulation. In some cases, e.g. among Ctenophores, light 

 has an inhibiting influence. Previous exposure to illumination 

 is not essential to photogenic response in animals. The most 

 interesting physical aspect of the phenomenon is that the light 

 emitted is cold. Within the limits of experimental error a 

 hundred per cent, of the radiant energy emitted is light. Thus, 

 though feeble in quantity, light-production in animals is 

 prodigiously more efficient than any ordinary artificial source 

 of illumination. 



The organs concerned with light-production may be divided 

 into two categories, according as the production of luminous 

 material is extra-cellular or intracellular. In the former 

 category, exemplified by Pholas, Chaetopterus, Cypridina, and 

 Myriapods, are included those cases in which a slimy secretion 

 containing the phosphorescent substance is produced by 

 unicellular glands. These may be either scattered diflPusely 



