132 MARKETABLE BRITISH MARINE FISHES CHAP. 



on in the daytime. The whiting is another instance. An upper 

 jaw longer than the lower is generally found in fishes which feed 

 on the ground, and in the fishes of the cod family a barbel on 

 the chin is associated with this condition. The barbel signifies 

 that the sense of touch is called to the aid of that of sight and 

 smell in finding prey, and it carries with it the possibility of 

 feeding in the dark. Thus fishes with barbels may be chiefly 

 nocturnal in their habits ; this is the case with the ling and the 

 rocklings, the latter having several barbels. To watch a rockling 

 hunting in the aquarium reminds one forcibly of a dog following 

 a scent, and it is a matter of observation that they remain con- 

 cealed in daytime, except when induced to come out by the 

 presence of food, if they are hungry, and that they move about 

 freely at night. The fishes of the cod family which feed in 

 the upper waters have usually the lower jaw longer than the 

 upper, and no barbel ; as for instance the pollack, but in the 

 hake the jaws are equal in length. 



Among the flat-fishes there are some which feed on other 

 fishes, and others which feed on lower animals. The correspond- 

 ing difference in the jaws is interesting ; the fish-eating forms 

 have the mouth large, the jaws strong, and equal on the two 

 sides of the body, with sharp teeth ; as in the turbot, brill, 

 megrim, long rough dab, and halibut. In the plaice, dab, lemon 

 dab, and sole, on the other hand, the mouth is small, and the 

 jaws and teeth very feeble on the upper side of the head, strong 

 on the lower side. These fish bite with the lower corner of the 

 mouth. The fish-eating flat-fishes also exemplify the process of 

 obtaining prey by lying in ambush, concealed, instead of pursu- 

 ing and overtaking it ; they lie partly or wholly buried in sand or 

 gravel, with their changeable colouring where it is exposed 

 closely resembling the appearance of the surrounding ground, 

 and when other fish come near enough they rise suddenly and 

 seize them. The other flat-fishes are also well concealed, but 

 the result in their case is escape from enemies, not deception of 

 victims. 



The sole is one of the most retiring of fish ; its burrowing 

 habits and power of changing its colour to resemble the ground 

 make it almost or quite invisible, and it seldom moves or un- 

 covers itself except at night. In the aquarium this is constantly 

 observed, although it will when hungry feed in the daytime. It 



