I9O S. STILLMAN BERRY. 



its components, is now admitted practically without debate as 

 ample ground for taxonomic discrimination. Where such differ- 

 ences are shown to occur, further differences in the remaining 

 organization seem practically predestined for eventual discovery. 

 Good characters for specific discrimination are to be found, not 

 only in the presence or absence of photogenic organs, but also in 

 their distribution on or within the body, in their number, in 

 their size, and in the veriest details of their intrinsic structure. 

 The taxonomist has in fact few more convenient points of attack 

 in the pursuit of his primary objects of classification and relation- 

 ship than that afforded by the light organs. And this is exactly 

 what we find, if to somewhat less degree, among the fishes and 

 the few other groups where the photogenic organs have attained 

 some considerable complexity. One can construct a fairly 

 workable taxonomic key based on the photogenic organs alone, 

 for such species as possess them. 



10. PROBABLE POLYPHYLETIC ORIGIN OF PHOTOGENIC ORGANS. 



Before concluding this paper a somewhat general answer may 

 be attempted to a question which has no doubt occurred more 

 than once in the mind of the reader, and which indeed has been 

 touched upon very nearly on more than one occasion Is photo- 

 genesis a primitive function among cephalopods? In other 

 words, are our present day species descended from an ancestral 

 photogenic stem, some branches of which have now yielded up 

 the function? Or has photogenesis arisen several times in this 

 class of animals, possibly to meet altogether diverse conditions or 

 associations in the environment, so that its presence therefore 

 becomes of secondary ratl er than primary significance? 



At first glance the widespread distribution of the function in 

 the great and, comparatively, primitive oegopsid group of cephalo- 

 pods favors an affirmative answer to our first query. But in 

 reply to this it may be said that the varied pelagic environment 

 of these forms would almost per se favor the development of the 

 light-producing function after a manner which would be hardly 

 likely to hold true among the more littoral Myopsida and Octo- 

 poda, the former of which are mainly frequenters of much 

 shallower water than the (Egopsida, the latter hardly ever 



