XXX THE VOYAGE OF H.M.S. CHALLENGER. 



a considerable amount of this luminosity of the abyssal depths ; but the various degrees 

 of differentiation of the luminous organs, as well as their location on very ilifferent paits 

 of the fish, prove that the production of light is dependent on a variety of circumstances 

 and subserves difl'crcnt purposes. 



1. Light may be produced by the luminous organ to enable its possessor to see. In fishes 

 which secrete merely a large quantity of luminous mucus without specially developed 

 organs (Macruridse), or in which innumerable minute organs are disseminated over the 

 greater piirt of the body, the luminosity proceeds from the general surface of the fish 

 whenever the animal is active, and probably ceases whilst it is asleep or at rest. But in 

 those fishes in which the organs are highly developed and specialised, the production of 

 light is evidently subject to the will of the fish. Only thus can the luminous apparatus 

 be of advantage to the fish ; if tlie production of light were constant, or could not be 

 suppressed instantaneously, the fish would be a most conspicuous object and fall a ready 

 prey to its enemies. The liigh degree of development of the lumiuous organs on the 

 side of the head, in close proximity to the eye, as it is found in Anomalops and the 

 Stomiatida3, can be accounted for only by assuming that these fishes are able at will 

 to shoot rays of light in the direction which they want to explore for the jaurpose of 

 discovering their prey, or for some other object. In fact these organs are used by them 

 as we would use a " bulls-eye." The circumstance that some of the organs are lodged 

 below membranes or even in the cavities of the gills or within the mouth, cannot be 

 regarded as an objection to this explanation of their function, as the memljranes as well 

 as the bones are semi-transparent, and would not much interfere with the effect. No 

 doubt the intensity of light produced by the various organs is not the same, and it is 

 probably least in those least specialised : perhaps no more than the glimmer produced 

 by a number of minute particles of phosphorus ; but the light which issues from the large 

 peai'ly organs of the Scopelida), the infraorbital organs of the Stomiatidaj, and from the 

 lenticular organs of the Halosauridse, must Ijc intense and jjenetrate to a considerable 

 distance. 



2. The luminous organs which are placed on barbels, filamentous fin-rays, or tentacles 

 have evidently the function of attracting other animals and of serving as lures. It is a 

 matter of common obsen'^ation that aquatic animals are in the dark attracted by a light ; 

 and therefore these appendages will prove most eflicient lures in the abyssal darkness, 

 when, with one or several bright phosphorescent spots at the end of the tentacle, they 

 are played about by the fish. Thus, whilst the appendages retain the original function 

 which they Imve to perform in surface fishes like Lopldua, Antemuirms, etc., in which 

 they simulate u worm or other similar creature, the means by which the final end is 

 attained is changed in accordance with abyssal conditions. Their luminous property 

 could not be of any other use to these fishes, many of which, a.s, for instance, the 

 deep-sea Pediculates, have their eye in a most rudimentary condition. 



