28 A HISTORY OF FISHES 



scarcely ever for propulsion (Fig. 4). In fishes, turning when in 

 motion would appear to be effected largely by appropriate 

 movements of the fins, and particularly of the pectorals, but 

 body movements as well as jets of water from the gill-openings 

 also play their part. Stops are nearly always made by using 

 the pectorals in the manner of brakes, but some forms pull up 

 by reversing their primary locomotor apparatus. 



Finally, there remains the third primary method of loco- 

 motion, namely, by means of jets of water squirted from the 

 gill-openings during respiration. Recent experiments have 

 shown that these exhalations may play an important part in 

 driving the body forward, but the effect varies with different 

 fishes, being of considerable importance to some and of little 

 or none to others. This method is probably always brought 

 into play for high-speed travelling, and assists the muscular 

 activities of the body. The jets reach their maximum strength 

 between the flexures of the body when the fish is straight for- 

 ward, and when they would clearly produce the best effect. 

 When used in conjunction with movements of the pectoral 

 fins, the motions of the fins are timed so that they do not get 

 in the way of the jets. A particularly powerful jet is usually 

 expelled when a fish commences any swimming movement, 

 thus giving an added impetus to the initial muscular efforts of 

 the body in getting under way. By holding down tightly the 

 gill-opening on one side, and forcing all the water out of the 

 opposite one, the fish may make use of this locomotor method 

 to perform turning movements in any given direction. 



Unless a fish is actually resting on the bottom, it is by no 

 means as easy as it would appear to maintain a stationary 

 position in the water. Breathing cannot be suspended for a 

 moment, and although this respiration may be comparatively 

 slow as compared with that taking place when the fish is 

 swimming, the exhalant jets of water are of sufficient strength 

 to cause the body to move forward, and some sort of action is 

 necessary to counteract their force. Observation of a fish 

 resting in mid-water in an aquarium shows that the pectoral 

 fins are in more or less constant motion, and it has been 

 freely stated that they are employed in balancing the body. 

 Experiments, consisting of removing one or both of these fins 

 from a living fish, have shown that they play very little part 

 in maintaining stability, and it seems that they are engaged 

 in constantly backing water, to counteract the forward thrust 

 engendered by the respiratory jets. 



