302 THE VOYAGE OF H.M.S. CHALLENGER. 



The parcaboloid cups are in a very slanting position. And as they meet the 

 surface very obliquely, the outline which is the intersection of the cup, and the surface 

 appears as a very elongate ellipse. The ventral organs which lie at the extremity of 

 the body in the median line have a circular outline, because they are situated close 

 to the ventral margin of the laterally compressed body. 



These ellipses are very conspicuous ; they apjjear surrounded 1)y a dark line of 

 jiigment, whilst they shine with a brilliant silvery lustre in the centre. They measure 

 1 to 2 by 3 to 6 mm. in diameter. A dark mass — the pigment coat of the sac-shaped 

 portion — is attached to one end and looms through the semitranspareut skin of the fish. 



The two rows of ventral organs of Argyropelecus and also of Sternoptyx approach eacli 

 other so closely in the median line of the body that they are nearly in contact (PI. LXX. 

 fig. 17). Their orifices (PI. LXX. fig. 19), therefore, appear nearly square, the cups 

 becoming by mutual compression prismatically four-sided in their outer portions. 



In external appearance there is a great difference in these organs, although the 

 internal structure is the same in all. Four varieties may be distinguished. 



(1) The organs of Gonostoma. 



These have been studied by Leydig.^ I have not seen any specimens of this genus ; 

 but they do not coalesce like the other varieties and resemble more the composite 

 phosphorescent organs without reflectors. They are smaller than the other varieties 

 and not so oblique. Leydig gives no detailed description of their shape. 



(2) The dorsal organs or "stern-chasers" of Scopelus. 



The sac-shaped portions do not coalesce, and the constriction dividing the sac- 

 shaped and cup-shaped portion is very conspicuous, it is circular and has a diameter 

 of 0"3 mm. The axis is nearly parallel to the surface. The cup-shaped portion is a 

 rotation-paraboloid mth very smaU focal length, about O'l mm. The cup terminates on 

 the surface with a very elongate, elliptical contour measuring 1 by 4 mm., and the posterior 

 margin approaches the constriction to within 0'2 mm. The sac-shaped proximal part (if 

 the organ is elongate, conical, tapering towards the rounded end, and widest just l^elow 

 the constriction, where it has a width of nearly 1 mm. It is 2*5 mm. long. 



The sizes vary according to the species ; the more numerous these stern-chasers are, 

 the smaller they appear to be. The largest are possessed by those species which have 

 only a single one. 



The propoi'tions are always as indicated by the above numbers, and the shape of 

 the organ is constant. 



' F. Leydig, he. cit., ji. 12. 



