PRODUCTION OF LIGHT 425 



dently acts as a reflector and resembles a glass mirror, being 

 backed by a silvery reflecting layer like the mercury at the 

 back of the glass, and this layer being covered by opaque pig- 

 ment like the opaque substance over the mercury. 



Of the various theories which have been suggested concern- 

 ing the process by which light is produced in the luminous 

 organs of fishes and other animals, the most definite and pro- 

 bable is that the essential part of each organ consists of glan- 

 dular cells, which secrete a substance containing phosphorus, 

 that this substance is oxidised by oxygen supplied by the blood, 

 and that the light is due to this oxidation. Our knowledge of 

 the skin in general indicates that its glandular cells are usually 

 derived from the epidermis, and that the cells of the con- 

 nective tissue of the skin do not become glandular. In the 

 organs of the Myctophidae it seems difficult to apply this theory, 

 firstly because the special organ does not appear to be com- 

 posed of glandular cells, and secondly because it has been stated 

 by Emery that this organ is developed in the derma and is not 

 derived from the epidermis. Accordingly other investigators 

 have maintained that the light is produced in these organs 

 electrically ; but this statement has little definite meaning, as 

 no attempt is made to connect it with what we know of electric- 

 ity in general ; it seems to be based merely on the laminated 

 structure of the special body which suggests the plates of an 

 electric battery. On the other hand, the investigators last 

 mentioned state that the special body is derived in development 

 from the epidermis and not from the derma, and this if correct 

 would be in favour of the glandular theory. 



2. The light-organs of Sternoptychidce, Stomiatidce, etc. — 

 These organs, although they differ greatly in degree of com- 

 plexity, agree in the glandular character of the essential part of 

 the organ by which the light seems to be produced. Modified 

 scales take no part in the structure of these organs ; they are 

 provided on the internal surface with a reflector formed of the 

 reflecting substance of the skin, i.e. of iridocytes, and this is 

 covered by pigment cells ; there is no definite single structure 

 acting as lens, but the outer part of the organ consists of 

 transparent refracting cells, whose function is believed to be 

 optical. As examples of this type, we may take the organs of 

 Sternoptyx (Fig. 31). In this fish the lateral organs consist 



