LIGHT PRODUCTION IN CEPHALOPODS. 147 



include around 225 species (over one third of the total). The 

 Octopoda, with only 8 families (one quarter) and 25 genera 

 (slightly over one fifth), yet develop nearly 200 species (one 

 third of the total). In comparison to this the single family and 

 genus of Tetrabranchiata (3 generally recognized species) hold 

 a minor place in the fauna. 



The extraordinary development of the CEgopsida in families 

 and genera is indicative, as the reader may anticipate, of the 

 more than usual modification to which the different branches of 

 this group have been subject, and we are to see that this is 

 particularly true of the photogenic organs. The number of 

 species on the other hand seems to have been held down by the 

 circumstance that a very considerable proportion of the genera 

 are pelagic types of widespread distribution, and, like so many 

 other animals showing somewhat similar ecologic relations, do 

 not break up well into species with our present degree of refine- 

 ment in perception. 



This brief survey of the classification may seem a digression, 

 but without it as a guide, no consideration of the distribution 

 of the photogenic function within the group as a whole or within 

 its components could be entirely intelligible. 



3. DISTRIBUTION OF THE PHOTOGENIC FUNCTION AMONG 



CEPHALOPODS. 



By no means all cephalopods are luminous. Among the entire 

 major division of octopods but two species, Melanoteuthis 

 lucens Joubin and Eledonella alberti Joubin, have been described 

 as possessing photogenic organs, while even here the fact that 

 the structures so described are actually designed for the produc- 

 tion of light still remains to be demonstrated. The various 

 instances where octopi have been observed to emit light are 

 almost always poorly authenticated, though it is not impossible 

 that in some cases, such as the observation by Darwin during 

 the Voyage of the "Beagle," which will be noted later, are ex- 

 plicable on the assumption of infection by photogenic bacteria 

 or protozoa. However that may be, and perhaps the point is 

 not yet definitely settled, in the morphological evidence offered 

 the two species mentioned stand quite alone. 



