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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



instance consider the mackerel as an example of this type. The 

 body is compressed (B), torpedo-shaped (A), and entirely smooth. 

 The most important organ of locomotion is the caudal fin. Power- 

 ful strokes of the tail caused by a wavelike motion of the entire 

 body (C), drives the fish forward with great speed. As organs of 

 equilibrium we find the dorsal and anal fins and frequently also 

 the ventrals. The pectoral fins and partly also the ventrals per- 



FiGUEH 2. — Four different types of adaptations to movement in water. First type, the 

 most perfect type. Torpedo-shaped body with caudal fin as propulsive organ ; A and 

 B, mackerel seen from the side and from the front ; C, movements of the fish while 

 Bwimming. Second type: Plump, broad body, extrejnities as organs of locomotion ; D, 

 Plesiosaurus, from the upper Cretaceous ; E, recent sea turtle. Third type: Snakelike 

 form, the entire body serves as organ of locomotion ; F, lamprey ; G, eel. Fourth type: 

 The tadpole type, the least efficient form ; H, tadpole from above and from the side. 



form the function of steering. Thanks to all these highly developed 

 adaptations the nectonic, that is the free-swimming, fishes, are the 

 most perfect swimmers we know, moving in all directions with an 

 astounding elegance and ease. Almost the same type of adapta- 

 tions are also found among the recent whales and the extinct 

 Ichthyosaurs. 



The second type, which can best be compared with a rowboat, 

 is practically unknown among fishes, but is frequently seen in marine 

 reptiles (fig. 2, D, E) and diving birds. It is less perfect. In this 



