154 A NATURAL HISTORY OF THE SEAS 



Whereas the light of many fishes appears to wax and 

 wane according to health — or emotional state of the fish 

 concerned — in some cases it is completely under its owner's 

 control In some deep-sea fishes the light organ is stationed 

 one just behind each eye, and observation has revealed that 

 the fish possesses a black shutter, like an eyelid, that can 

 be pulled down over the light at will. 



The Angler Fishes not only frequently possess light 

 organs of high development, but also present to an unusual 

 degree the development of tactile organs, which play so 

 important a part in the economy of many fishes. Sense 

 organs of this category are seen in many common fishes, 

 the Cod {Gadus morrhua), Red Mullet (MuIIus barbatus), 

 and all Catfishes having barbels depending from the chin, 

 which restlessly probe the sea-bed and notify their owner 

 of anything edible which may lie in its path. In deep-sea 

 Anglers and some other oceanic fishes, the barbel takes 

 the form of a most complicated beard divided and sub- 

 divided into innumerable ramifications, with the result 

 that a veritable bush is the result. Such organs are of 

 special service to fishes living in muddy waters, when 

 the general density of the surroundings obliges the animal 

 to live in a perpetual state of low visibility. 



Light and sense organs have a very direct bearing upon 

 the faculty of vision. The depth at which a fish lives and 

 its general mode of life may indeed frequently be gauged 

 with some accuracy by the development of its eyes. Fish 

 frequenting a muddy bottom usually have small eyes, sight 

 being of small avail in highly turbid surroundings. 



The Lampreys {Vetromy^on) and the Hag-fishes (Myxim) 

 have likewise poor sight, but for quite another reason. 



