374 DISCOVERY REPORTS 



It will be remembered, however, that not all the fibres impinge on the granular cone, 

 but some, whether nervous or not, pass distally around the periphery of the organ, and 

 terminate in the neighbourhood of certain elongate nuclei in the plane of the photogenic 

 nuclei. The possibility exists that these fibres and nuclei are not concerned with the 

 reflector and screening pigments, but may be involved in the secretion of luciferin. 

 Both a detailed examination of the innervation of the organs, employing suitable special 

 methods, and an investigation into their physiological qualities are urgently necessary. 

 It will be apparent that the foregoing remarks apply equally to other photophores, such 

 as the carapace organs of Hoplophorus and those behind the bases of the last thoracic 

 limbs of Systellaspis affinis and S. debilis, which are composed of similar photogenic 

 cells with basal granular caps and related nerve fibres. 



The uropod photophores of S. affims are clearly related structurally to the pleopod 

 organs just discussed, but the absence of any object in their photogenic cells which 

 can be interpreted as a nucleus renders speculation concerning their mode of functioning 

 still more difficult. Further examination of the photophores of members of the Hoplo- 

 phoridae may possibly reveal a photophore intermediate between these and the pleopod 

 organs. The white opaque granular mass, situated basally in the organ, will be men- 

 tioned later after discussing the "rosette" type of photophore of S. affinis. 



This peculiar type of photophore, as instanced by that on the fifth thoracic limb 

 described on p. 356, is of considerable interest. The white opaque substance, laid down 

 in a very definite manner in the form of radially segmented spheres, was suspected 

 early in the course of this work to be either guanin, or a similar allied substance, since 

 the former is known to occur as a reflecting pigment in the eyes of Palaemonetes and 

 Macrobrochium (Welsh, 1932) and of Sergestes, Systellaspis, Hymenodora, and Hoplo- 

 phorus (Welsh and Chace, 1937, figs. 13-18 and 1938, figs. 10-15). ^^ is also present 

 in the carapace and limbs oi Hoplophorus (Coutiere, 1905, p. 3). In the photophores of 

 Systellaspis affinis the substance is finely granular and apparently amorphous. No 

 positive reaction was obtained to a test for xanthin, but owing to lack of material (only 

 one specimen being available for examination) no further tests were proceeded with. 

 The morphological evidence available from a study of the organs suggests that the 

 granules arise, in part at least, as a breakdown product of the large photogenic nuclei, 

 since only the last remnants of these nuclei are ever apparent when the granular 

 segments of the rosettes begin to appear, and usually the nucleus has completely 

 degenerated even before the first traces of granules are to be seen. On this evidence 

 alone it would seem to be highly probable that the granules might be expected to 

 consist of an accumulated product of nucleo-protein katabolism, and a precise identifi- 

 cation of their nature undertaken by a biochemist will be of interest. 



These organs of S. affinis therefore show a striking resemblance in their fundamental 

 features to those of Lampyrid beetles examined and described by Lund (1911) and 

 many other workers. In these beetles the luminous organs consist of a mass of photo- 

 genic cells, containing photogenic granules, situated ventrally in the abdomen and 

 penetrated by tracheae and nerves. Dorsal to and in contact with the photogenic cells 



