4 i8 FISHES 



Luminous organs, on the other hand, must be supplied by 

 efferent nerve-fibres, along which impulses pass from the 

 brain or spinal cord to the organs. Sense-organs, therefore, 

 could never become luminous organs. It would be necessary 

 to show that light in Halosauridae is produced by glandular 

 cells surrounding, but distinct from the sensory cells, and no 

 attempt has been made to prove this, nor have the organs in 

 question been seen to emit light. Specimens of Halosauridae 

 have been obtained in all the great oceans. Halosauropsis also 

 possesses a white glandular patch on the skin behind the gills 

 covered in the natural state by the operculum ; it seems prob- 

 able that this is really a luminous organ. 



Among the Spiny-finned Fishes (Acanthopterygii) organs 

 which can with certainty be regarded as luminous occur more 

 rarely than in the more primitive forms above considered, 

 although in various cases the function of producing light has 

 been attributed on insufficient evidence to various structures in 

 the skin. In many of the deep-sea forms, for example in the 

 families Zoarcidae or Brotulidae, the dermal organs of the lateral 

 line and head are much enlarged as in the Halosauridae, and 

 like the latter have been supposed, especially by American 

 ichthyologists, to have a light-producing function, of which there 

 is no direct evidence. Several oceanic or abyssal forms of the 

 angler family (Lophiidae) possess organs, of whose light-emitting 

 function there can be little doubt, at the extremities of the 

 dorsal filaments which are characteristic of the family. In these 

 cases the light seems to serve as a lure to attract other fishes 

 within reach of the angler's jaws. One of these forms is 

 Ceratias uranoscoptis, of which a specimen 9 cm. or nearly 4 in. in 

 length was taken by the Challenger at a depth of 2400 fathoms 

 between the Canary and Cape Verde Islands. The body is 

 compressed and deep and the first spine of the anterior dorsal fin 

 forms a long filament arising from the top of the head and 

 nearly as long as the body : the filament terminates in a pear- 

 shaped bulb at the outer end of which is a semi-transparent 

 whitish spot, probably a luminous organ. Cryptopsaras couesii 

 is a somewhat similar fish in which the bulb of the filament is 

 larger and continued into a whitish thread. In Melanocetus 

 murrayi, taken in the Atlantic at 1850 and 2450 fathoms, the 

 dorsal filament is dilated at its extremity, but it is not certain that 



