PRODUCTION OF LIGHT 417 



ventral fin. As the pelvic fins are abdominal in position they 

 are doubtless allied to the more primitive soft-finned malacop- 

 terygian and open-bladdered fishes, but the air-bladder is 

 stated to be closed. Some members of this family, all of 

 which belong to the deep-sea fauna, are generally stated to 

 possess luminous organs, but the organs supposed to produce 

 light are very different in nature and structure from the un- 

 doubtedly luminous organs of other fishes, and there is no satis- 

 factory evidence that these organs have anything to do with 

 the production of light. In all other cases the light-organs are 

 situated in various parts of the skin, are innervated by branches 

 of the ordinary dermal nerves, and are not connected with the 

 lateral line. 



In Halosauridse, the organs supposed to be luminous are on 

 the lateral line and seem to be merely the proper lateral line 

 organs enlarged. The organs of the lateral line are sense-organs 

 supplied by branches of the lateral-line nerve, and it seems to 

 be held by some zoologists that the sense-organs in this case 

 have developed a light-producing function. Only three genera 

 of Halosauridae are recognised, and in two of these (Halosanrus 

 and H alosauricJitJiys) the scales of the lateral line are not much 

 enlarged and are stated to be destitute of luminous organs ; in 

 the third genus, Halosaaropsis, of which there are several species, 

 the scales of the lateral line are much enlarged and expanded, 

 and are said to bear light-organs. The only reason for this 

 statement seems to be that each scale of the lateral line in this 

 genus is somewhat transparent and shows a conspicuous opaque 

 white spot in preserved specimens, which is not visible in the 

 other genera ; but in several deep-sea fishes the tubes of the 

 lateral line and sensory canals on the head are much enlarged 

 and this is apparently due to the greater development of the 

 dermal sense-organs under abyssal conditions. Halosauropsis 

 lives on the whole at greater depths than the other two genera, 

 and it seems most probable that the supposed light-organs are 

 nothing but the enlarged sense-organs of the lateral line. The 

 lateral line sense-organs, like other sense-organs, are con- 

 nected with sensory, i.e. afferent nerves, along which nerve- 

 impulses travel towards the brain. It is well known that 

 nervous impulses pass along nerves only in one direction, 

 an afferent or sensory nerve cannot become an efferent nerve. 

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