PHOTOPHORES OF DECAPOD CRUSTACEA 37i 



are truly luminous they, with the organs of the Pandahds, stand alone among Crustacean 

 photophores in having an endodermal origin. 



As already mentioned (p. 318) the appearance of these organs of Pesta indicates great 

 secretory activity, and the morphological evidence suggests that in different zones of the 

 tubules two different substances may be elaborated. That these substances may be 

 luciferin and luciferase, together responsible for the production of light, is possible, 

 but it must not be overlooked that the function of the organs may be merely one of 

 excretion alone, or of excretion accompanied by incidental luminescence. If the 

 difficulties of keeping these animals alive after capture can be overcome (and the success 

 of Welsh and Chace with Systellaspis debilis (1937) is encouraging), then the physio- 

 logical investigations so clearly necessary may be possible. But whatever the function 

 of the organs of Pesta their presence in some species of Sergestes and absence from 

 others is a very curious and at present inexplicable feature. The proximity of the 

 anterior pair at least of the organs of Pesta in Sergestes, and of the anterior pair of liver 

 organs in some of the Pandalids, to the roof of the branchial chamber may be a purely 

 inevitable result of the situation of the liver, but if the organs should prove to be 

 definitely luminous their position may perhaps have the same significance as that of 

 other organs illuminating the branchial chamber. 



It must be admitted, however, that the meaning of photophores, whether complex 

 and lensed organs like those of S. challengeri, or a simple glandular streak like that of 

 S. corniculiim, so placed as to throw their light downwards on to the gills, is entirely 

 obscure. It is difficult to imagine any way in which their presence might influence the 

 respiratory efficiency of the branchiae, and as far as I am aware there are no structural 

 peculiarities in the gills of those species possessing photophores in this situation. That a 

 supply of oxygen is necessary for luminescence is shown both by physiological in- 

 vestigations and by such morphological findings as the occurrence of tracheoles pene- 

 trating deep into the photogenic tissue of luminous beetles, and the existence of a 

 special photophore blood vessel in Hoplop/iorus. The possibility exists that the situation 

 of photophores in the branchial chamber roof is influenced by the passage of an oxygen- 

 laden stream of water through the branchial chamber. The thin integument Uning the 

 branchial chamber no doubt allows of ready gaseous interchange, so that the photo- 

 phores are bathed by body fluid rich in oxygen. If the complex photophores of the 

 Decapoda are luminous by virtue of the oxidation of luciferin induced by the enzyme 

 luciferase, as occurs in the luminous secretion of the ostracod Cypridina hilgendorfii 

 (Harvey, 1919) and of a species of Systellaspis (Harvey, 193 1), and there seems no 

 reason to doubt it, then the situation of photophores in the branchial chamber roof 

 might be regarded merely as a response to a rich oxygen supply. 



There are several serious objections, however, to such a view. In the first place, the 

 luminescence of Cypridina secretion still continues even in the presence of only very 

 small amounts of oxygen, and great increases in oxygen tension, while increasing the 

 light emitted, do not do so proportionately (Harvey, 1919). It appears then that 

 variations in oxygen tension of the order that might be expected to occur within 



