STRUCTURE OF THE VERTEBRATES 



167 



The carnivores show almost every degree of aquatic specializa- 

 tion. Beginning with animals like the otter which spends a large 

 part of its time in the water but has typical five-toed feet, a 

 further degree of change is found in the Alaskan sea otter 

 which spends most of its time floating on its back. The Pinni- 

 pedia (water living carnivores) form a separate sub-order and 

 carrv the modifications still further. The sea-lion and walrus 



Fig. A 

 Dolphin 



Fig. B 



Ichthyosaur 





OOP o^ 



Fig. 99. Aquatic Adaptations of the Pectoral Appendages. The number 



of phalanges is increased, and in the more specialized forms the arm and 



hand form a flipper. 



have flippers, both pairs of which can be used for locomotion on 

 land. In the Atlantic seal the pelvic flippers are enclosed by 

 skin for most of their length, and are functionless on land. The 

 Cetacea and Sirenia (page 90) have carried modifications for 

 water life to the ultimate extent. The front flippers have an in- 

 creased number of digits, and the pelvic appendages have dis- 

 appeared except in rare instances where a small femur is found. 

 Although these animals are externally modified, they are 



