HOW THE FISHES LEARNED TO SWIM HEINTZ 227 



case the anterior, the anterior and posterior, or more rarely the pos- 

 terior extremities alone serve in the capacity of oars. They force 

 the short and plump body through the water by powerful strokes, 

 which may perhaps best be compared with the wing strokes of a 

 bird. These forms are as a rule without special organs of equilib- 

 rium or for steering purposes, as the extremities alone serve in all 

 three capacities, that is both for propulsion, balancing, and steering 

 of the body. In some cases, however, the posterior extremities or 

 the tail may serve as a lateral rudder. As the best examples of this 

 type one might mention the sea-turtles (E), the extinct large presio- 

 saurs, and the penguins. 



The third type must be regarded as even less adapted to swimming. 

 It is the snakelike type (fig. 2, F, G). The animal swims by means 

 of a wavelike motion of the entire body. As balancing organs there 

 are as a rule more or less well-developed median fins. Special organs 

 for steering are usually absent, as the very flexible body takes care of 

 the steering without any special fins. As examples of this type, one 

 can mention the recent cyclostomes or lampreys (F), eels, and a 

 number of other fishes. Many of the recent or extinct amphibians 

 and reptiles also approach this type more or less closely. 



The fourth group is of a more distinct type. It has, so far as 1 

 know, not previously been separately defined. In its purest form it 

 is not found either among adult fishes, amphibians, or reptiles, but 

 we encounter it among the larvae of tunicates and amphibians. It is 

 best represented by the tadpole (fig. 2, H) . The large, somewhat flat- 

 tened, round " cephalothorax " (head and anterior part of the body) 

 is without any indication of fins or any other appendages or organs 

 of equilibrium. The tail is strongly compressed, fairly long, with a 

 slightly thickened central core and wide, finlike rims above and 

 below. The tail of the tadpole combines in itself all the three func- 

 tions of locomotion: Propulsion, balancing, and steering. The 

 heavy and clumsy " cephalothorax " participates only quite passively 

 in the swimming. Perhaps it may have some function as a gliding 

 plane. Anybody who has seen a tadpole will know that it cannot be 

 considered among the good swimmers. Their adaptations to swim- 

 ming are very imperfect, and we also see that they spend the greatest 

 part of their time resting upon the bottom. But as soon as the pos- 

 terior extremities develop and start to grow, the tadpole begins using 

 them as organs of equilibrium and steering, and the swimming then 

 immediately becomes much more efficient. 



The tadpole shape with a large head and anterior part of the body 

 (cephalothorax) and a thin, flat tail is not very rare among bottom 

 fishes, but they then always have paired and unpaired fins to help 



111666—35 16 



