LIGHT PRODUCTION IN CEPHALOPODS. l8l 



generally easy to distinguish, many of them being of unusual 

 size and often of conspicuous coloration, while the situations 

 which they occupy are peculiarly limited and, within a given 

 species, constant. By reason of this last fact the intrapallial 

 organs may readily be subclassified into four series, (i) anal, 

 (2) branchial, (3) gastric, and (4) axial. Such a classification, 

 too, in spite of its obviously superficial foundation, is a con- 

 venient one. That it is at the same time in all respects a natural 

 or phylogenetic arrangement is probably not true, and it will no 

 doubt be greatly improved upon by the first worker who takes 

 up the relationships of these organs in any sort of adequate 

 detail. 



The term anal organs is misleading, but has become so well 

 established in the literature that I use it pending the invention 

 of a more appropriate term. The photophores so classified 

 appear usually as a pair of quite large, often very brightly 

 colored organs of rounded or ovoid outline, lying on the ink sac 

 on either side of the rectum, with which they would otherwise 

 appear to have no particular connection. Being often situated 

 just back of the funnel, or sometimes almost within it, they are 

 therefore sometimes termed the siphonal photophores, a . name 

 which in its turn is open to objection as inappropriate to the 

 actual morphological relationships involved. Anal organs occur 

 in a considerable number of little related genera, and the dis- 

 charging photophores of the luminous Sepiolidae are noteworthy 

 for occupying an analogous situation. 



The branchial organs are always paired, being situated one 

 near the base of each gill. They are confined, so far as known, 

 to the Lycoteuthidse, Lampadioteuthidae, and the pterygiomorph 

 section of the Enoploteuthidse. 



The gastric and axial organs are classed together by most 

 writers under the general term abdominal, but I prefer to separate 

 the mesially situated, unpaired organs, which are often extended 

 into a considerable series in the hinder portion of the mantle 

 cavity, from the paired organs which sometimes occur near the 

 middle of the body on either side of and often in close association 

 with the anteriormost of the axial organs. There is evidence 

 that in at least some genera the division here postulated into the 



