CHAPTER II 

 FORM AND LOCOMOTION 



Shape of a typical fish. Fins and their functions. Other animals with a 

 fish-like form. Departures from the ideal form, and compensating 

 factors. Depressed and compressed fishes. Flat-fishes. Fishes with 

 rounded bodies : Globe-fishes, Puffers, Sun-fishes, etc. Elongate fishes. 

 Sea Horses. Methods of locomotion. Muscular movements. Swim- 

 ming of Mackerel, Eel, and Trunk-fish. Locomotion by means of 

 fin-movements: caudal fin, dorsal and anal fins, pectoral fins. Jet 

 propulsion. Speed. Swimming positions. Leaping. Burrowing. 



Of the many and varied forms of animal life found in the seas 

 and in the fresh waters few are more perfectly adapted for 

 dwelling in a liquid medium than the fishes. Many of the 

 invertebrates spend the greater part of their lives attached to 

 the sea bottom, or crawl sluggishly over a small area of its 

 surface; others float more or less passively at the surface or in 

 the middle layers of the water, their movements dependent to 

 a great extent on the action of the tides and currents; the 

 Squids and Cuttle-fishes alone approach the fishes in rapidity 

 of motion and grace of form, but lack their agility in the water, 

 and are generally far inferior to them in mastery of their 

 medium. 



The water in which a fish lives and 'moves is a comparatively 

 dense medium, and in order to attain the most efficient move- 

 ment with the greatest economy of energy a certain form of 

 body is essential, varying somewhat in detail with the actual 

 speed required. The shape of the body, therefore, is not an 

 arbitrary one, but conforms to a number of definite mechanical 

 conditions induced by its environment. These mechanical 

 principles cannot be dealt with here, since a study of this 

 subject would involve the consideration of such theoretical 

 problems as "curves and displacement," "streamlines," "en- 

 tering angles," "runs" and the like, the proper understanding 

 of which entails some knowledge of higher mathematics. It 

 must suffice to point out that the fine form of a typical swift- 

 swimming fish such as the Mackerel {Scomber) (Fig. 5A) or 



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