DEEP-SEA FISHES. 93 



Owing to the uniformity of the conditions of life in the abyss in 

 different parts of the world, they are probably for the most part 

 wide ranging. 



The greatest depth at which fishes are known to exist is 2,900 

 fathoms. Many littoral fish descend periodically within the 

 limits of the deep-sea fauna, but these are not conspicuously 

 modified. Fishes which habitually live at a depth of 80-120 

 fathoms, have a black lining to the pharynx and large eyes. 

 Fishes which belong to the real deep-sea fauna all present very 

 similar characters, those from 300 fathoms being as much 

 modified as fishes from 2,000 fathoms. The principal changes 

 in external conditions to which deep-sea fishes are subjected 

 as compared with surface forms are (1) absence of light, (2) still- 

 ness of the water, (3) constant low temperature, and (4) increase 

 of pressure. With regard to the latter, it may be stated that 

 pressure increases by one ton on the square inch for every 1,000 

 fathoms of depth. The principal bodily characters are as 

 follows : The eyes are largely developed and luminous organs, 

 or, to speak more correctly, organs the function of which is 

 probably to supply luminosity are present. When the supposed 

 luminous organs are not present the lateral line canals are much 

 dilated, sometimes into wide cavities, and full of mucus. The 

 eyes are, however, in some cases reduced or absent. The osseous 

 and muscular systems are feebly developed ; the bones being 

 light and provided with little calcareous matter, and the muscles 

 thin. When the fish are brought to the surface the bones are 

 found to be but loosely bound together, and the body easily 

 falls to pieces. This is probably due to the expansion of gases 

 within the body. The air-bladder presents no special modifica- 

 tions, and appears to be always without a pneumatic duct, even 

 in Physostomous forms. It is generally ruptured in fishes brought 

 up from the deep sea, and in fishes from 80 fathoms it is much 

 distended, and the eyes protrude and the stomach is everted. 

 Deep-sea fishes are sometimes found floating on the surface in 

 a dead or dying condition, and often with the stomach distended 

 with recently swallowed prey. It is conjectured that such fishes 

 have accidentally ascended too far above their normal depth, 

 possibly during the struggle of swallowing their prey which 

 may be as large as themselves, and then owing to the expansion 

 of gases consequent on the diminished pressure have been 



