SUPPORT, MOTION, AND SENSATION 



333 



The feet of animals show many remarkable adaptations. Foot 

 posture involves more than fallen arches ; it determines the speed 

 at which an animal can travel. 

 If the wrist and ankle are raised 

 from the ground the result is a 

 longer leg capable of a longer 

 stride, which means covering 

 more ground in the same inter- 

 val of time. Anatomists dis- 

 tinguish three types of feet : 

 plantigrade, the primitive fiat- 

 footed type found in man and the 

 bear ; digitigradc, characteristic 

 of cats or dogs that are literally 

 "on their toes " all the time ; and 

 the unguligradc, restricted to 

 forms which walk on their nails, 

 like horses, cows, and camels. 



Functions of Skeletons 



digitigixxcCe -unguligracCe^ 



Types of mammalian feet. State the 

 advantages and disadvantages of each 

 type of foot. (After Pander and D'Al- 

 ton.) 



Skeletal devices usually serve 

 one of three functions, namely, 

 support, protection, or movement. Examples of each type will be given, 

 although it is sometimes difficult to separate these functions. 



Support 



It is quite apparent that organisms living in water have much less 

 necessity for a supporting framework than land-inhabiting animals. 

 This is due to the fact that the body approximates more closely the 

 density of the surrounding medium and is consequently buoyed up by 

 it. Cuttlefishes and jellyfishes maintain their shape in their natural 

 environment but out of water collapse more or less completely. 



In like manner, the bivalve shells of clams and mussels form a 

 supporting skeleton, to which is attached the mantle that in turn 

 encloses the viscera. Crayfish and lobsters offer still another ex- 

 ample of skeletal support, for their movement is largely brought 

 about through the interaction of a well-developed exoskeleton and 

 inside muscles. 



In land-inhabiting forms, the function of the skeleton as a sup- 

 porting device becomes most apparent. It is hard to envisage any 



