370 DISCOVERY REPORTS 



Without entering into any discussion on the possible diagnostic significance of the 

 varying number and positions of superficial photophores in the genus Sergestes, the 

 suggestion may be entertained that a detailed examination and comparison of all the 

 structures suspected to be luminous, including the organs of Pesta, which are found in 

 the genus, might lead to a clearer elucidation of the relationships existing between the 

 various species. Burkenroad (1937, p. 317) has already suggested that the absence of 

 the organs of Pesta from S. chaUengeri and its close allies is compensated for by the 

 presence of complex lensed photophores, and in S. robtistus and its relatives by simple 

 subcuticular bodies. The assumption, however, that all the simple bodies are identical 

 in structure would be most unwarranted. In any such survey the occurrence in 

 S. corniculum, S. sargassi, S. diapoiithis, and S. edwardsi, and probably also other species 

 possessing organs of Pesta, of the glandular and probably photogenic streak in the roof 

 of the branchial chamber (p. 320) must be taken carefully into account, as Burkenroad 

 (1937, p. 317) has stated that superficial photophores are lacking in all of the species 

 possessing organs of Pesta. 



The photophores described by Ramadan (1938) in Hymetiopetiaeus debilis are also 

 very different from those which I have described in Sergestes regalis. While the difl^er- 

 ences between these organs of Hymenopettaeiis and those of Sergestes chaUengeri and 

 S. lucens mentioned by the author are noteworthy, it would nevertheless appear from the 

 descriptions available that in these three species the luminous organs follow the same 

 basic plan. Extensive proliferation inwards of the photogenic cells of an organ such as 

 that of S. lucens, for example, might well lead to a condition very similar to that seen 

 in Hymenopenaeiis debilis. 



The lack of innervation of the simple structures of Sergestes regalis is paralleled by 

 the condition in some of the photophores of the Hoplophoridae, and will be again 

 mentioned in discussing the possible means of control of Decapod photophores. The 

 appearance of the organs in S. regalis suggests that the fibrous mass may possibly consist 

 of modified muscle strands, and thus the organs may be of mesodermal origin. (In the 

 Lampyrid insects the luminous organs are also of mesodermal origin, being apparently 

 derived from the fat-body.) In other phyla muscle cells are said to be luminous, as for 

 instance in a species of Ophiura (Dahlgren and Kepner, 1908, p. 123) and it may perhaps 

 be that these simple photophores of Sergestes regalis have resulted from the increasing 

 specialization of portions of luminous muscle. In this connexion, however, the lack of 

 a continuous chitogenous epithelium between the integument and the photogenic mass 

 may perhaps be taken to indicate that the structures are of ectodermal origin. Only 

 observations on the development of the organs can determine the truth concerning 

 their origin, and the difficulties in the way of carrying out such work are only too clear. 



The occurrence of dense clusters of modified liver tubules in a number of species of 

 Sergestes was first recorded by Pesta (1918, p. 56), who suggested that they might 

 perhaps be luminous organs. (Organs of a similar gross appearance are also found in 

 certain Pandalids (Kemp, 1925, p. 277 and this paper, p. 322).) Their pigmentation 

 supports this view (Burkenroad, 1937 p. 316 and this paper, p. 319), and if the organs 



