LIGHT PRODUCTION IN CEPHALOPODS. 145 



specially modified "tentacles" as well; have the suckers pedun- 

 culate, reinforced with a chitinous ring, and often very curiously 

 modified; and have a so-called buccal membrane surrounding 

 the mouth. 



The Octopoda are principally shore or bottom-loving forms; 

 short bodied; lack fins, or have them only secondarily developed; 

 have eight arms only, with their suckers sessile and lacking 

 chitinous rings; and lack the buccal membrane. There are also 

 important internal characters which need not concern us here, 

 but it may be said that few living groups are more sharply de- 

 limited. As a whole the group of the Decapoda has seemed to 

 most students more archaic than the Octopoda, at any rate is 

 less uniformly divergent from what must have been the ancestral 

 stock, but it includes many highly specialized types, and prob- 

 ably neither group as we now know it can be taken a<s especially 

 "primitive." 



Of the two groups the Decapoda are much the less uniform, 

 and are therefore still further to be divided. The classical 

 bifurcation, which has found general acceptance until quite 

 recently, is that of d'Orbigny, into CEgopsida, or those forms in 

 which there is a free eyelid, and the Myopsida, in which there is a 

 simple fold-like pseudo-lid, or the skin passes uninterruptedly 

 over the eyeball. There seems to be no doubt but that the 

 CEgopsida, at least, form a monophyletic, natural group, for on 

 this point there is reasonable agreement. This cannot be said 

 of the Myopsida. 



The principal alternative system is that of Naef (1916). l 

 He sets apart the Sepias and Sepiolids together as one of the two 

 main subgroups, the Sepioidea. The other group, which not alto- 

 gether satisfactorily he denominates Teuthoidea, is subdivided 

 into Myopsida (here restricted by the elimination of the Sepioidea 

 and hence comprising only the single family Loliginida?) and 

 CEgopsida, the latter as outlined above. Naef's classification 

 avoids several very serious difficulties involved in the standard 

 arrangement, but encounters, perhaps, certain others, which 

 need not be dwelt upon here. 



For convenience the two systems are summarized in the 



1 Naef, A. J., Pnbblicazione Slazione Zoologica Napoli , V. i, pp. 14-17, 1916. 



