PHOTOPHORES OF DECAPOD CRUSTACEA 313 



Burkenroad (1937, p. 317) describes the structures in freshly caught specimens of 

 S. richardi=\crassiis\ as " lenticular transparent structures invested on their inner sides 

 by a layer of vermilion pigment". Further, after receiving information from me on 

 their structure, this author has examined the photophores of S. regalis, and informs 

 me in a private communication that their colour in life is caused by the dense pig- 

 mentation of the rod-like bodies, the fibrous portion of the organ being translucent. 

 The organs of S. regalis and S. richardi are therefore different in structure, and a 

 detailed histological examination of all these lens-less organs in the genus Sergestes is 

 therefore very desirable. 



None of the organs studied in S. regalis showed any trace of innervation, but all were 

 in close proximity to the blood space of the limb. This relation of a photophore to a 

 vascular area has been noted several times in the course of the present work. 



It would appear that organs of the above type can have no evolutionary relationship 

 with the lensed organs occurring in S. challengeri and its allies (Hansen, 1903 ; Kemp, 

 1910^; Terao, 1917). Here in S. regalis there is no lens, and the source of luminescence 

 is not a regularly arranged cellular mass. The precise significance of the distribution 

 of these two types of superficial luminous organs within the Group I in S. regalis of 

 Hansen's scheme is of course a matter for the taxonomist working on a large collection. 



In its development the photophore presumably has an ectodermal origin, as its inner 

 sheath of connective tissue continues outwards underneath the general chitogenous 

 epithelium, of which the photophore is probably a specialized portion. 



The origin of the rod-like bodies in the photophores of S. regalis is not apparent. 

 They are not bounded by any definite limiting membrane which would suggest a 

 nuclear membrane and their staining reactions are not those usually exhibited by 

 nuclei. It is possible, however, that they may be derived from nuclei and that the 

 surrounding cylindrical spaces may represent the cytoplasm of the corresponding cell. 

 We are accustomed to the occurrence of regularly arranged photogenic cells, with definite 

 nuclei, in crustacean photophores (e.g. Euphausiacea, S. challejigeri, pleopod photo- 

 phores of Systellaspis debilis), but as I shall show (pp. 334, 358) there is strong evidence 

 that in the development of many of the photophores of members of the Hoplophoridae 

 the nuclei of the photogenic cells may undergo very profound changes. The rods and 

 spaces making up the photogenic "units" of the photophores of Sergestes regalis may 

 represent a parallel phenomenon. 



The mode of functioning of this type of photophore is not very clear. If my inter- 

 pretation is correct, and the structures I have described as "photogenic units" are 

 truly light producers, then it appears that the organ may be visible from the exterior in 

 two ways. First, the spaces in the fibre mass immediately distal to the rods, acting as 

 windows, would allow of the passage of light to the exterior through the chitin, so that, 

 if this were all, the organ would be visible merely as a group of minute points of light. 

 Second, light emitted laterally, that is, more or less in the plane of the fibres, might 

 undergo successive reflexion and scattering from their surfaces so that they themselves 

 would be illuminated and the organ then appear as an illuminated mass. 



