REPORT ON THE DEEP-SEA FISHES. . 303 



(3) The groups of lateral organs in Argyropelecus and Sternopt;/x. 



These groups consist of from three to five organs, the sacs of which coalesce. 

 Generally there are three (PI. LXX. figs. 22, 23, 24). The constriction is circular and 

 has a diameter of O'l mm., and the plane in which it lies is perpendicular to the axis. 

 The axis is nearly parallel to the surface. The cup-shaped portions are rotation- 

 paraboloids, and the line with which they terminate on the surface of the fish is a very 

 elongate eUipse measuring 1 "5 by 5 mm. This circumference touches the constriction, 

 that is to say, only one side of the cup, namely the lower, is developed (PI. LXX. 

 fig. 23). The focal length of the paraboloid is O'l mm. 



The sac-shaped portions of the organs coalesce to form an irregular flattened sac, 

 with from three to five circular apertures on one side, the original strictures of the 

 organs. 



Seen from the surface this combined sac is in the triplex organ of Sternoptyx nearly 

 semicircular (PI. LXX. fig. 22). In sections its flattened shape becomes apparent (PL 

 LXX. fig. 23). It measures 0'3 mm. in height and 1'2 mm. in length. The width of 

 course depends on the number of cups attached to it. In the triplex organ it is about 

 1"2 mm., in the quinquei^lex 2 mm. In the interior of it crests or ridges can be discerned 

 which are the remnants of the partitions between the adjacent phosphorescent organs, 

 the internal portions of which have coalesced. 



(4) The ventral rows of Argyropelecus and Sternoptyx. 



Wherever composite ocellar phosphorescent organs are observed, they invariably form, 

 as stated above, two lateral lines or rows on each side of the body. The groups of 

 triplex and quinqueplex organs of Sternoptyx and Argyropelecus mentioned above, 

 belong to the upper of the two rows which is split up to form groups along the side of 

 the fish. 



The lower rows, however, remain unbroken and continuous. They approach each other 

 very closely (PL LXX. fig. 17). The lower portions of the organs on both sides coalesce 

 to form one large ventral canal lying in the median line of the body, from the lower side 

 of which two rows of cups project (PL LXX. fig. 20). The optical axes of all the cups are 

 parallel, and so the rays of light from all these cups are concentrated in one direction. 



/8 Histology. 



The histological structure of the diff"erent organs belonging to this grouj) is ].retty 

 much the same, so that it can be dealt with in a summary manner. 



