382 COMPARATIVE ANATOMY AND PHYSIOLOGY. 

 cases, as in tlie Rays, movement is almost altogether 



. O 



effected by the undulation of the margins of the enor- 

 mous pectorals. In flat-fishes and eels locomotion is 

 due to the undulations or curvatures of the wholo 

 body. 



In other aquatic Vertebrates, such as the marine 

 turtles, the penguins, or the whales, where the limbs 

 take some share in their movements through the water, 



Fig. 162. Exoccetus volitans. 



the tendency is for the flipper to become a more or 

 less rigid organ, movable only at its point of attach- 

 ment to the body. The series of modifications which 

 lead to this arrangement are very well seen among 

 the Cheloiiia. In the marsh tortoises the digits are 

 united by a web, but each digit has a claw ; in the 

 mud tortoises the limbs are flatter, and there are claws 

 on only three of the digits, while in marine turtles 

 the still more flattened digits are united by a common 

 covering of skin into a more rigid paddle, and only 

 one or two claws are found. In the penguin the 

 wings are converted into firm paddles, movable only 



