372 DISCOVERY REPORTS 



the body of the animal can have no influence on the efliciency of the photophores, and 

 that organs in regions of relatively high oxygen concentration are actually in a no more 

 favoured position than those elsewhere, and all photophores, wherever placed, will 

 glow with much the same intensity. 



Secondly, although in Sergestes challetigeri the photophores on the carapace (Hansen, 

 1903, pi. xii, fig. 2a), shown by Kemp (igiob, pi. liv, fig. 4) to be internal and near the 

 roof of the branchial chamber, appear to be situated above the middle of the series of 

 branchiae, in S. corniculum, S. sargassi, S. diapontiiis, and S. edioardsi the suspected 

 luminous streak is placed a little in advance of the foremost gills, that is, in the least 

 favourable position in the branchial chamber from the point of view of oxygen supply, 

 since the respiratory current flows anteriorly. Furthermore, the occurrence of photo- 

 phores in this situation is, as far as is known, by no means universal even in the luminous 

 species of Sergestes, and it would be difficult to account for this discrepancy even if it 

 could be shown that the situation under consideration is a peculiarly favourable one for 

 photogenesis. 



It may be fairly stated that the degree of structural complexity attained by some of 

 the photophores of the Hoplophoridae is equalled nowhere else in the Crustacea, 

 although the complex Euphausid organs closely rival them. The pleopod photophores of 

 the species oi Hoplophortis and Systellaspis examined, movable in virtue of a surrounding 

 arthrodial membrane and well-developed musculature, and showing complexities of 

 innervation such as those seen in Hoplophonis novae-zeolandiae, might be expected to 

 be remarkably efficient. However, the single available observation of active light pro- 

 duction, in H. grimaldii, does not confirm this. Dr Kemp records in his notes made at 

 the time that, on placing the specimen in fresh water all the photophores emitted light, 

 which was "much feebler than that of Eiiphaiisia longirostris, caught in abundance in 

 the same haul". It is possible, of course, that the animal was more enfeebled than the 

 Euphausids, and clearly additional observations are desirable, but it should not be 

 assumed that structural complexity of photophores necessarily indicates a high degree 

 of physiological efficiency. The organs of the Lampyrid beetles are relatively simple in 

 structure and yet are remarkably efficient light producers. 



Whether the distribution of photophores in the members of the Hoplophoridae will 

 prove to have any profound taxonomic significance cannot be answered from the 

 information at present available. But from the observations so far made it would perhaps 

 appear unlikely. The apparently sporadic occurrence of photophores of diff'erent 

 structure in corresponding positions in different species and in difl^erent positions in the 

 same species, and of photophores of corresponding structure in difl'ering positions in 

 different species (see table, p. 369) is at present merely confusing. But if a form were 

 to be discovered bearing photophores of only one type these difficulties might be 

 resolved, at least in part. The close similarity of the pleopod organs, and their early 

 appearance in development (Kemp, i9ioZ>, p. 646) suggests that they are of phylo- 

 genetic significance, while those organs containing photogenic cells of entirely diflferent 

 form, producing the granular "rosettes", are supernumerary and secondary. 



