ON LUMINOUS ORGANS OF FISHES. 21 



They are definite in number and arrangement according to species, 

 complicated in structure and are in some cases provided with a 

 system of intercommunication between the internal parts of the 

 neighbouring organs. The photogenic body is formed by nu- 

 merous polyhedral gland-cells filling the interior of the organ 

 in either regular or irregular arrangement. The lens is multi- 

 cellular, and lenticular or flat in form. The blood capillaries 

 form a network between the photogenic cells. The presence of 

 nerves has been proved in a few cases. The luminescence is 

 not momentary but continuous, the light being not very intense. 

 The photogenic body emits light in the air even when taken 

 out of the organ. 



(3) The organs of the Myctophidae. These are the most highly 

 differentiated forms of luminous organs, some species presenting 

 sexual differentiation as regards their position on body. The 

 photogenic body consists of a number of small lamellar gland-cells. 

 The lens, if present, is formed by a local thickening of the super- 

 posed scale. The photogenic body is richly supplied with nerves, 

 and the blood vessels form a plexus around it. The spark-like 

 light is rather bright and is undoubtedly controlled by nerves. 



The different forms of the organ have several common 

 features, namely : 



(a) They are abundantly supplied with blood. 



(b) Within one and the same species, the photogenic bodies 

 are of the same structure, irrespective of their belonging to 

 different %'ms of the organ situated in different parts of body. 



(c) The photogenic cells are always of a glandular nature. 



(d) The secretion product in photogenic cells is always 

 stainable with plasma dyes. It is never set free. 



