376 DISCOVERY REPORTS 



limbs (p. 337) and on the limbs themselves (p. 334) are clearly of the same type as those of 

 Systellaspis affinis, although their granular substance is of a different nature and exhibits 

 staining reactions not shared by the material in the latter organs. The telson photophores 

 of S. debilis (p. 367) also are clearly similar. 



With regard to the curious inversion seen in the photophores of the base of the last 

 thoracic limb and the limb itself both in Hoplophorus and in Systellaspis debilis, I am 

 entirely unable to offer any explanation. Its precise significance is obscure, but is probably 

 that the structures seen in the photogenic cells of the type with a distal nucleus, character- 

 istic clear area, and basal granular mass or cap, while being of importance in actual light 

 production, are not concerned with the way in which this light is emitted to the exterior, 

 a conclusion at variance with the constant and regular manner in which the photogenic 

 cells of the pleopod organs are found to be arranged. 



The method of control of the photophores of the bathypelagic Decapoda is com- 

 pletely obscure. In the first place, very few observations on the intensity, colour and 

 periodicity of the flashes emitted are available. Even where such observations have 

 been made it must not be forgotten that the photophores are functioning under com- 

 pletely abnormal conditions. Terao (1917) has recorded some observations on the light 

 emitted by the photophores of S. lucens (as S. prehensilis) and states that they give an 

 intermittent dim greenish yellow light. Sometimes a serial anterior-posterior succession 

 of flashes takes place, often only one photophore glowing at a time. The photophores in 

 the neighbourhood of the eyes often give a steady light. 



It might be expected that photophores with a well-defined nervous supply would 

 function in a similar manner to the above rather than produce a steady glow, but the 

 precise nature of the nervous control cannot be stated. The absence of any morpho- 

 logical evidence of control of non-innervated photophores, such as those of S. regalis 

 and those on the limbs of Systellaspis affinis, suggests that they produce a luminescence 

 of constant intensity. No structures capable of varying the rate of oxygen supply, or 

 the rate of secretion of any of the cells appear to be present, and it is difficult to see 

 any way in which the amount of light given by such organs might be controlled. It is 

 striking, however, that whereas in the thoracic limb organs, for instance, of S. affinis, 

 no indications of innervation have been discovered, organs of a similar nature in Hoplo- 

 phorus possess a well-developed nerve supply. 



Many suggestions have been made with regard to the function of the photophores of 

 bathypelagic animals, and reference may be made to such papers as those by Nutting 

 (1899), Doflein (1907), and Kemp (19106). During the course of this purely morpho- 

 logical enquiry no concrete information has been gained on this point. Recently, Welsh 

 and Chace (1937, 1938) have attempted to correlate the size of the eyes of a number of 

 species of Sergestids and iVcanthephyrids with the presence or absence of the photo- 

 phores and the depth of occurrence of the animal, and have come to the conclusion that 

 those species possessing photophores have larger eyes than those lacking these organs. 

 This may indicate that photophores play some part in the life and habits of the animals 

 affecting them directly through the eye, such as facilitating food capture or recognition 



