PHOTOPHORES OF DECAPOD CRUSTACEA 377 



for the purposes of swarming or breeding. This, however, cannot account for the exist- 

 ence of photophores in such curious situations as the branchial chamber and interior 

 of the cephalothorax. Furthermore, Dr M. D. Burkenroad has pointed out to me that 

 in most of the small-eyed species of the genus Sergestes organs of Pesta which are 

 probably luminous are present, and that in certain undescribed species of Sergestes 

 allied to S. challengeri, with photophores of the general type which, alone in the genus, 

 have been observed actually to luminesce (see Terao, 1917), the eyes may be relatively 

 quite as small as in such a small-eyed species as S. atlanticiis. Burkenroad himself 

 suggested (1936, p. 118) that a large eye might perhaps be correlated with the presence 

 of photophores in the species of section 2 of group IV of the genus Hymenopenaeus , 

 although I am informed by the author that further investigation has not confirmed this 

 possibility. The presence in some members of the Hoplophoridae of movably articulated 

 photophores whose musculature is under the control of the nervous system and whose 

 whole organization is fitted for concentrating emitted light and directing it in a particular 

 manner strongly suggests, however, that in some cases at least photophores may have a 

 definite function involving vision on the part of the animal itself. 



The luminous secretion which is poured out by a number of members of the Crus- 

 tacea Decapoda has not been touched upon in the course of the present work, although 

 it is hoped to make it the subject of further investigation in the future. It appears to 

 be a not uncommon phenomenon. Alcock (1902, p. 134) remarks that specimens of 

 Heterocarpus alphonsi emitted a luminous secretion, glowing with a blue light, from the 

 antennal glands, and this was also observed, though it was less abundant, in Plesio- 

 penaeus (as Aristaeus) coniscans. Harvey (1931) has given the results of his observations 

 on the luminous secretion of the "deep sea shrimp or prawn, Systellaspis'\ and Schmitt 

 (1931) gives a luminous frontispiece depicting the clouds of luminous secretion emitted 

 by S. debilis. In his ' Discovery ' notes Dr Kemp records that several specimens of 

 Hoplophorus novae-zealatidiae emitted a luminous secretion from the neighbourhood 

 of the mouth. Dahlgren (1916, pp. 829 32) states that the origin of the luminous 

 secretion is to be found in some of the tegumental glands. That such clouds of luminous 

 secretion, poured out into the exhalant respiratory current, may serve for protection 

 during the backward "escape reaction" of the animal seems highly probable. 



Structures, usually marked by well-defined pigment spots and which may be luminous, 

 have been mentioned in many pelagic decapods, and detailed examination of these 

 structures is clearly necessary in order to determine what likelihood there is of these 

 organs proving to be luminous. In addition, from the experience gained in the course 

 of the present work it seems desirable, although tedious, to examine many more of the 

 photophores found in any one species of Hoplophorus or Systellaspis, as it is possible 

 that there may be an even greater variety of photophores in some of these species than 

 has so far been recorded. The detailed innervation of known photophores, using suitable 

 methods on fresh material will also prove to be necessary. In the physiological and 

 biochemical fields investigations into the functioning of the various types of organs 

 are urgently necessary when opportunity for such work occurs. I hope in the future 



