1910.] PHOTOPirORES OP DECAPOD CRUSTACEA. 049 



various suggestions wliicli have been made as to tlie use of 

 luminous organs to marine animals. He remarks that tliey 

 probably serve different functions in different groups of animals 

 a.nd c]a,sses them in four sections. 



i. Attraction of prey (chiefly important in sessile or slowly 

 moving animals). 



ii. Attraction of other individuals of the same species, either 

 (a) for the formation and mainteniuice of swarms or (b) to enable 

 the sexes to find and recognise one another. In this ccmnection 

 Doflein points out that animals with a complicated system of 

 photophores always possess highly developed eyes, and refers to 

 Brauer's theory that the varying arrangement of photophore.s 

 produces light patterns serving as recognition marks, like the 

 colour-piitterns of animals living in daylight. 



iii. Protection. The clouds of luminous secretion emitted by 

 some species may possibly serve the same purpose as the ink (if 

 the cnttlefish, and photophores may also by a sudden flash of light 

 scare a pursuer. In the fauna of land and shallow water a 

 brilliant colouring is often assumed as a signal that the species is 

 distasteful, and some deep-sea animals may, for the same purpose, 

 exhibit warning lights. 



iv. Illumination of o])jects viewed by the animal. On this 

 tlieory it is difficult to account for the ventral and lateral position 

 of the photophores in many marine animals*. In Crustacea 

 this is particularly well shown, for the large majority of the 

 organs illuminate regions which seem altogether out of range of 

 the eyesight. 



It is evident that these suggestions will not account foi- every 

 case which can be found ; the photophores in the roof of the 

 branchial chamber of Sergestes i-emain inexplicable. 



The vast majority of marine animals which possess photophores 

 live at the sui-face or at intermediate de2)ths and never occur on 

 the bottom. No exceptions to this I'ule have been noticed in the 

 deep-water fauna of the Irish Atlantic slope, but it seems that 

 the two Euphausians, Meganyctiphanes norvegica and yijctiphanes 

 couchii, are sometimes found on the bottom in shallow water. On 

 one or two occasions large numbers of these two species have 

 been caught off the Irish coast at depths of 40 to 60 fathoms, and 

 there are indications that the specimens which were obtained in 

 these hauls Avere actually living on the sea-floor. The same two 

 species are frequently obtained over depths of 400-800 fathoms 

 oft' the West coast of Ireland, and here they invariably occur in 

 rnidwater. 



It must be I'emembered that the ordinary open-mouthed nets, 

 which are generally employed for bottom work, frequently catch 



* Miss Massey informs me that when studying the development of the Cepliahiijod, 

 Histioteidhis bonelUana, which wlicn adult possesses pilot ojjhores all round its 

 hody. she noticed that the organs are developed first on the side whicli is vi-ntral 

 when the animal is swinimin''-. 



