LIGHT PRODUCTION IN CEPHALOPODS. 183 



cated in the same general region as these, and, by their appear- 

 ance, seeming to bear a closer relation to the intrapallial organs 

 than to the other systems outlined, yet scarcely to be regarded as 

 lying actually within the mantle cavity, are the conspicuous 

 paired photophores placed at the extreme posterior tip of the 

 body in Nematolampas. Lycoteuthis does not possess them. 

 They stand in a class quite by themselves at present, but if the 

 peculiar swellings to be noted in the same situation in certain 

 species of Abralia are susceptible of a photogenic interpretation, 

 or if Chun's identification of the posterior disk of Spirula as a 

 luminous organ be accepted, a further extension of this division 

 of the classification is afforded. 



7. STRUCTURE OF PHOTOGENIC ORGANS. 



Another most remarkable feature of the development of photo- 

 genic systems in Cephalopoda is, so far as I am aware, the quite 

 unparalleled variety of structural type manifested by their con- 

 stituent organs. It is entirely beyond the scope of this paper to 

 enter into any extended account of the histological detail, but 

 it will be useful to call attention to at least a few of the main 

 features. Suffice to say that since the first observations on the 

 finer morphology of cephalopod photogenic organs made by 

 Joubin in 1893, a most bewildering variety of structure within 

 the confines of this single, narrowly limited group of animals has 

 been brought to light, ranging all the way from the simple 

 discharging glands of the luminous myopsids, and the lump of 

 photogenic tissue which forms the proximal photophore in the 

 tentacle of Lycoteuthis, through almost innumerable intermediate 

 types, to the astonishingly complex bull's-eye lanterns of Abra- 

 liopsis and the mirrored searchlights of the Histioteuthidse. 

 Each species has in fact its own peculiar modifications and some- 

 times many of them. The histology of all affords a fruitful field 

 of investigation, which, with all due respect to the fine work of 

 Chun, Hoyle and Joubin, we can truly say has been hardly 

 skimmed. This is especially true of the embryology and he who 

 attempts to work out the origin and homologies of even the 

 simplest of these organs will have a virgin field. 



Cephalopod photophores appear only rarely to be made up 



