1910.] PIIOTOPIIORES or DECAPOD CRUSTACEA. G41 



any direct effect on the production of light, it is none the less the 

 case that phosphorescence lias actually been observed only in a 

 single species, Sergestes challanyerl. Prof. Gardiner informs me 

 that the examples of this form which he obtained in the Indian 

 Ocean were brilliantly phosphorescent on the occasion of their 

 capture. In the other species the function of the organs has 

 been deduced from their structure only. 



The Photophores of Sergestes. 



The only two species of S'eryestes which are known to possess 

 photophores are aV. chccUenc/eri, which was described by Hansen * 

 from a single specimen obtained by the ' Challenger ' Expedition 

 near the Fiji Islands, and a closely allied form, *S'. gloriosiis 

 Stebbing f, which was found in South African waters. Hansen 

 discovered 117 photophoi'es in his mutilated example of ;S'. chal- 

 lengeri, and estimated that at least 150 would be found in a 

 perfect specimen. According to 8tebl)ing's account an even 

 greater number exist in S. gloriosus. 



It is not necessary to describe the distribution of the photo- 

 phores in detail. They occur on the lower sides of the e>'estalks, 

 antennules and antennal scales, on the oral appendages, on the 

 thoracic and abdominal sterna, on the ventral surface of the outer 

 uropods, and on many of the leg joints. All are so situated thnt 

 the light which they jjroduce is thrown directly or obliquely 

 downwards. In both the species photophoi'es have been described 

 on the lateral face of the carapace. These, however, at any rate 

 in S. chcdlengeri, are not external, but are placed in the roof of 

 the branchial chamber and illuminate the gills from above 

 (PI. LIV. fig. 4). To find photophores in such a, position as 

 this is most astonishing, and it is not easy to suggest any theory 

 which will account for their curious situation. 



The photophores are all practically identical in strnctin-e and 

 all are quite immovable, though a few are supported on veiy 

 short, thick stalks in order to make them bear more directly 

 downwards. They vary considerably in size, but even the largest 

 are much smaller than the organs on the pleopods of Acaiit/i- 

 ephyra dehilis. In a single individual the diameter of the lens 

 Avas found to range from -06 to "14 mm. 



The structure of the organs is illustrated in PI. LIII. figs. 2-4 

 and in PI. LIV. figs. 2, 3, & 5. 



Externally there is a double convex lens. Tliis is made up of 

 two distinct portions, which are formed from the two layers com- 

 posing the cuticle. The outer part is dou])le convex, while the 

 inner, which is closely a,p[)lied to it, is concavo-convex. In sei-tions 

 treated with picro-carmine the inner lens often stains to a, rather 

 deeper red tone than the outer, but the complete alisence of any 

 yellow colour indicates that the cuticle of which both ai-e formed 



* ITiiiison, P. Z. S. 100;j, p. 72. 



t !?tel;l)iiig, MariiK' lii\t>t. in S, Atfica, vol. iv. 19Uo. !>. 84. 



