204 A HISTORY OF FISHES 



developed in certain fishes, is the so-called schooling or shoaling 

 sense, the mass control which is so apparent in gregarious birds 

 and mammals. Thus, if the hindmost individuals of a shoal 

 are attacked, the remainder will instantly scatter in every 

 direction, although those in front cannot possibly have seen 

 any cause for alarm, and the feeling of danger must have been 

 communicated to every member of the shoal with incredible 

 rapidity. It is said that two individuals of a gregarious species, 

 when brought close enough together to perceive each other 

 clearly, approach to within a certain distance and then change 

 their course so as to swim forward side by side. The usual 

 movements of a shoal of fishes seem to be due to similar visual 

 reactions. If some influence tends to stop the forward progress, 

 whether it be the proximity of a rock, shallower water, or the 

 approach of an enemy, it has been shown that the advance 

 members of the shoal turn abruptly backwards, and a sort of 

 ''milling" movement is set up, in which the shoal turns in a 

 circular path and continues in the new direction until some other 

 obstacle or disruption upsets it. 



Another question frequently raised concerns the habit of 

 sleeping, and the absence of eyelids in the vast majority of fishes 

 leads many to suppose that sleep is unknown to them. This 

 is untrue, however, and although it is impossible for them to 

 shut their eyes to impressions from the outside world, there can 

 be little doubt that all fishes spend at least a part of the day 

 or night in a state of suspended animation. This has been 

 verified in a number of species, and it has been found possible 

 in many cases to approach a sleeping fish and to remove it 

 from the water with the hand. Mr. Boulenger has noted that 

 in the aquarium at the Zoological Gardens the position adopted 

 by a fish when sleeping varies a good deal, not only in the 

 diflferent groups of fishes, but in closely related species of the 

 same genus. When suddenly disturbed by flashing an electric 

 torch on to the tanks, some were found to be resting in a 

 horizontal position on the bottom, others, like the Wrasses 

 (Labridae), were lying at the bottom on their sides, and others 

 were sleeping in a horizontal position but entirely surrounded 

 by water. It was found that fish which normally sleep when 

 darkness arrives will remain awake and active if hungry, and 

 it is suggested that Trout taking a fly at night are hungry 

 individuals that remain awake owing to the abnormal nocturnal 

 activity of their insect prey. Flat-fishes such as Plaice {Pleuro- 

 nectes) and Dabs (Limanda) were found just above the bottom 



