PHOTOPHORES OF DECAPOD CRUSTACEA 



347 



the actual specimen I examined, made on the Murray Expedition by Lt.-Col. R. B. 

 Seymour-Sewell, no such absence of pigmentation is indicated. It is possible that such 

 a transparent area of small size might perhaps be overlooked in the preparation of 

 such a figure intended to show the general pigmentation.) The pleopod photophores 

 are readily observed, even in the preserved specimen, on account of the distinct nature 

 of the lens, which appears as a sharply circumscribed shining elevation. 



phot pi 5. 



rs. 



phot, pi 



z_ 



■phot pi. 3. 



Fig. 20. a, the last four abdominal somites and telson of Systellaspis affinis in lateral view. /;, the telson, 

 uropods, and posterior portion of the last abdominal somite in dorsal view, to show the position of the 

 uropod and pleopod boss-shaped photophores and of certain pigment spots. x3-5. r.s. pigment spots; 

 phot.pl. T,, phot.pl. 4, phot.pl. 5, photophores borne on the protopodites of the third, fourth and fifth pleopods ; 

 phot.ur. photophore just anterior to the base of the uropod. 



An essential similarity of structure is readily apparent between these photophores 

 and those similarly situated in Hoplophorus and Systellaspis debilis (Kemp, igioZ*, 

 p. 643), but at the same time there exist noteworthy differences which will be 

 mentioned in passing. Apart from these differences the photophores require little 

 description. 



The lens of these organs has the same structure as has been seen in Hoplophorus 

 (p. 329) and differs but little from that of Systellaspis debilis. It shows inner and outer 

 layers, and a middle layer with again a less dense central layer (Fig. 21, l.o., Li., l.m.), 

 whereas in S. debilis the middle layer appears to be uniform. The peripheral depression 

 around the lens, noted in Hoplophorus (p. 330) and in Systellaspis debilis (Kemp, 19106, 

 p. 643), is not very well marked, but this may be due to the condition of preservation 

 of the specimen. The longitudinal photophore muscle (Fig. 21, phot.l.m.; cf. Hoplo- 

 phorus, p. 333) has been distinguished, but no circumferential muscle loop like that of 

 Hoplophorus could be discovered. A study of carefully fixed material would in all 

 probability reveal this muscle loop, as it is also present in the pleopod photophores of 

 Systellaspis debilis. 



