72 THE NATURE OF ANIMAL LIGHT 



glands. A large number of forms possess luminous glands 

 or gland cells, including some of the medusce, the hydroids 

 (probably), the pennatulids ( ?), the molluscs {PJiolas and 

 PhyllirJioe (probably), some cephalopods {Heteroteuthis 

 and Sepietta), most annelids, ostracods, copepods, some 

 schizopods (GnatJiopJiausia) and decapod (Heterocarpus 

 and Aristeus) crustaceans, all myriapods, and the balano- 

 giossids. The remaining organisms burn their material 

 within the cell. These include the bacteria, fungi, pro- 

 tozoa, some medusae (?), ctenophores (probably), most 

 cephalopods, a few annelids (Tomopterns (?)), ophiu- 

 roids( ?), some schizopod (Nyctiphanes, Eiiphasia, Nema- 

 tocelis, Stylochiron) and decapod {Sergestes) Crustacea, 

 allC?) insects, Pyrosoma, and fishes {selachians and tele- 

 osts). It is among this latter type that the most compli- 

 cated luminous organs have been developed. While a 

 description of all the types of luminous organs and lumi- 

 nous structures cannot be attempted here (excellent de- 

 scriptions have been given by Dahlgren and Mangold) it 

 is necessary to understand the structural conditions 

 in a few of the forms whose physiology has attracted 

 most attention. 



Luminous bacteria are so small that the light from a 

 single individual cannot be seen. It is almost impossible 

 to make out structural diii'erences within the cell and we 

 cannot definitely state in just what special region, if any, 

 the luminescence is produced. We do know that the light 

 is intracellular and that filtration of the bacteria from 

 their culture medium gives a dark sterile filtrate abso- 

 lutely free from any luminous secretion. 



Among protozoa, in certain forms at least, it is easy to 

 observe that luminescence is connected with globules or 



