410 FISHES 



mackerel, Cavanx trackurus, in the sun-fish, Ovthagoriscus mola, 

 and in a species of Balistes or trigger-fish : in the last case 

 the sound is described by Moseley in the " Naturalists on the 

 Challenger," as heard by himself when the living fish was held 

 in the hand. I noticed the same thing myself at Ascension in 

 a specimen of Balistes vetula, a large, brilliantly coloured species, 

 common in many parts of the world. This specimen had a 

 drum behind the pectoral fin and produced a sound from it like 

 Balistes buniva, but it also produced a quite distinct sound from 

 inside its mouth, and I have no doubt this second sound arose 

 from friction between the pharyngeal teeth. 



The luminous or light-producing organs in fishes which are 

 best known and which have been known for the longest time 

 are those of the Scopelida^, or as the family is now called 

 Myctophida;. In these fishes the organs usually have the form 

 of small pearl-like beads arranged in rows and groups on the sides 

 of the lower half of the body, reminding one of the pearlies of a 

 coster dandy. (Plate XXXI., C.) Several species of these fish 

 are not uncommon in the Mediterranean and specimens taken 

 there were described and named in 1810. A great number of 

 species are now known both from the Mediterranean and from 

 the Atlantic and also from the Pacific. The family belongs to 

 the sub-order Haplomi, typified by the common pike. All the 

 members of the family in the restricted sense of the name as here 

 used have luminous organs or photophores. Many of the 

 commonest species are taken on the surface of the ocean, but 

 only at night ; in the day time and in rough weather they 

 descend to unknown depths. The range in depth of these fishes 

 is one of the puzzles of Ichthyology. As they have an air- 

 bladder, although it is small and has an open duct, it is difficult 

 to understand how they can accommodate themselves at one 

 time to the surface and at another to a depth of some hundreds 

 of fathoms ; yet the records of oceanic expeditions often state 

 that specimens of the same species have been taken at differ- 

 ent times under these different conditions. The explanation 

 in some cases at least is doubtless that a dredge or trawl worked 

 on the bottom at a depth of a thousand fathoms or more may 

 catch small fishes, not on the bottom, but at any point during 

 its ascent to the surface. It seems very doubtful, until we get 

 more certain evidence, that any fish can live both at the surface 



