LIGHT PRODUCTION IN CEPHALOPODS. 



pelagic at all, and then generally surface forms or confined to 

 the shallower water like so many of the myopsids. 



There are many other arguments which may militate against 

 any theory of monophyletism and as strongly support the con- 

 trary view as brought out by the last query above. These, 

 having already been largely elaborated elsewhere or to be dealt 

 with in another connection later on, need be merely summarized 

 here. Such considerations are: 



1. The uneven distribution of photogenic organs throughout 

 the entire group, and, as a corollary of this, their appearance in 

 distantly related groups more or less sporadically. 



2. The variety and sporadic character of the development 

 of photogenic organs in different regions of the body. 



3. The large number of strongly diverse structural types. 



4. The evidence from ecological considerations, the distribution 

 upon the body, and similar facts that these organs have arisen 

 in response to very diverse environmental requirements. 



How then may one bespeak a photogenic sy 'stem? Exactly as 

 one speaks of a muscular system, or a skeletal system, or a 

 receptor system in almost any animal body. The term is used 

 in the sense not necessarily indicating an aggregation of homo- 

 logous structures, but an assemblage of organs within a single 

 organic body exhibiting more o'r less similar or coordinate 

 physiological reactions, if at times neither in fact phylogenetically 

 nor ontogenetically related. 



ii. CONCLUDING NOTE. 



This paper is mainly a compilation from the scattered work of 

 other authors. No doubt there are omissions, but the aim has 

 been to present simply a concise summary of the knowledge of 

 this subject which has been gained to the present time. It cannot 

 be too strongly emphasized that not only are many more species 

 of luminous cephalopods likely to be discovered in the future, 

 but some of those now known but not yet recognized as possessing 

 photogenic properties are likely to be revealed as having them. 

 Of the known luminous forms some will no doubt prove to 

 possess luminous organs or properties additional to those de- 

 scribed. Bearing all this in mind, if this little paper but fur- 



