XIII 



LOCOMOTION 



351 



forced through the water. It is obvious, however, that the 

 extension or effective stroke must have a considerable surplus of 

 power over the flexion or non-effective stroke, and how this result 

 is achieved will now be briefly considered. Experiment, and the 

 observation of Fishes like the Sturgeon, which habitually move 

 with sufficient slowness to allow the phases of their swimming 

 movements to be followed without much difficulty, show that in 

 swimming a Fish throws its body into two opposite and comple- 

 mentary curves, a cephalic curve formed 

 by the anterior half of the body and a 

 caudal curve by the tail. The double 

 curve enables the Fish always to present 

 a convex, less resisting or non-biting sur- 

 face to the water during the flexion of 

 the tail to the right or left as the case 

 may be, and a concave or biting surface 

 during extension, that is when the tail is 

 straightening itself during the effective 

 stroke. 



Fig. 204, which represents a Fish in 

 two successive positions while swimming, 

 will serve to illustrate these conclusions. 

 A Fish in the position A has its body 

 thrown into a cephalic concavity directed 

 to the right and a caudal concave surface 

 facing the left. The tail is bent to the 

 right of the line a b, which corresponds 

 to the axis of the Fish when at rest and 

 to the course pursued by the animal 

 when swimming, and is in the position 

 which it assumes during a flexion stroke, 

 with its convex non-biting surface directed 



outwards and its concave biting surface inwards. The tail is 

 now ready for an extension stroke, and while this is in 

 progress it is clear that the "concave biting surface of the tail 

 will meet the water, while at the conclusion of the stroke 

 the tail will be in a line with a b. At the same time the 

 cephalic curve has so far diminished that the long axis of 

 the body for a momentary period will also coincide with a b, 

 and the Fish is free to advance without impediment. The tail, 



FIG. 204. To illustrate the 

 mode in which the tail of 

 an ordinary Fish is used 

 in swimming. See the 

 text for the lettering. 

 (Slightly altered from 

 Pettigrew.) 



tail 



