ix. i BODY FORM OF FISHES 247 



tendency by the pectorals to produce a rise; the condition is exactly 

 that which would be expected in a flying-fish. 



With these increased opportunities for delicate control of movement 

 without the devices of flattening of the front part of the body and a 

 heterocercal tail, the bony fishes have also been able to make many 

 other improvements in the efficiency of their swimming. The caudal 

 fin has, of course, adopted its symmetrical shape and is used to increase 

 the efficiency in turning. After its amputation a fish is not able to turn 

 in its own length as normally. 



With shortening of the body and its lateral flattening all sorts of 

 new factors in streamlining the body are developed, but the details 

 are difficult to understand. The nature of the turbulence produced 

 by the movements of such a complicated structure is far from clear, 

 but it seems probable that the shape of the higher fishes is such 

 as to reduce the total skin-friction and to increase the efficiency of 

 swimming. Other factors such as the flexibility, which has an impor- 

 tant influence on the efficiency of the propulsive mechanism, have 

 also been changed, again in ways not fully understood, by the special 

 developments of the vertebral column and ribs. The speed that can 

 be reached increases with the length of the fish. Cruising speeds, which 

 are maintained for hours, are of the order of three to six times the 

 body-length per second, the relationship varying with the species. 

 During sudden bursts the speed may be much greater. Thus Bain- 

 bridge found that 10 L/sec could be maintained only for one second, 

 5 L/sec for 10 sec, and 4 L/sec for 20 sec (in dace, goldfish, and trout). 



The locomotion of each type of fish is adapted to its habits. Most 

 freshwater fishes are 'sprinters' but there are varying degrees of 

 staying power. Thus we may distinguish (1) typical sprinters (pike 

 and perch), (2) sneakers (eel) with some staying power, (3) crawlers 

 (rudd, bream), with considerable staying powers for escape, (4) 

 stayers, either for migration (salmon) or for feeding (carp). In the fish 

 with staying powers there is a lateral strip of narrow red muscle fibres 

 in addition to the characteristic broad white fibres of fish muscles. 



Bathypelagic fishes, living in deep waters, below the thermocline 

 at about 75 m, encounter special problems. Here currents and tur- 

 bulence are low, but since the water is cold it is very viscous, making 

 swimming difficult but sinking slow. Many deep-sea fishes have 

 elaborate lures, often phosphorescent. They may be described as 

 'floating fish traps'. They often have no swim-bladder and achieve 

 an almost neutral buoyancy by great reductions of the skeleton and 

 muscles (e.g. Ceratias). The only parts to be well ossified are the 



