474 THE MECHANICAL FUNCTIONS. 



according to the diversity of their habits. Tor- 

 toises which live on land, require more com- 

 plete protection by means of their shell than 

 turtles, or Emydes, which d\vell only in the 

 water: hence the convexity of their carapace, 

 the solidity of its ossification, its immoveable 

 connexion with the plastron, and the complete 

 shelter it affords to the head and limbs. Turtles, 

 on the other hand, receiving support from the 

 element in which they reside, require less pro- 

 vision to be made for these objects. Their ca- 

 rapace is smaller, has a more flattened form, 

 and cannot afford protection to the head and 

 limbs. These latter organs are proportionally 

 larger, present a greater developement of the 

 radius and ulna, and are compressed into a flat 

 expanded surface. Previously to the retraction 

 of the head and limbs within the shell, the air is 

 expelled from the large cavities of the lungs, by 

 the vigorous actions of the abdominal muscles, 

 which exist in these animals as well as in all the 

 vertebrata, although here they are covered by 

 the bones, and compress the lungs by pushing 

 the abdominal viscera against them. This sud- 

 den expulsion of air is the cause of the long 

 continued hissing sound which the tortoise emits 

 while preparing to retreat into its strong hold. 



The ribs, though they at first assume the form 

 of broad plates immoveably united to the spine, 

 when they have proceeded a certain distance, 



