422 THE MECHANICAL TUNCTIONS. 



tortoise are replaced by simple cartilage or mem- 

 brane. 



The enormous weight of the shell of the turtle 

 Avould be a serious impediment to the motion of 

 this animal in the water, were there not some pro- 

 vision made for diminishing the specific gravity of 

 the body. This purpose is answered by the great 

 capacity of the lungs, which, when inflated with 

 air, nearly fill the thorax, and give great buoyancy 

 to the whole mass. Thus, wherever there exists 

 a supposed inconvenience, dependent on the fulfil- 

 ment of one condition, we are certain to meet 

 with a compensation in the structure of some 

 other part, and in the mode of executing some 

 other function. An express provision for giving 

 buoyancy has been made in the construction of 

 the shell of a species of tortoise inhabiting the 

 coasts of the Seychelle Islands. The under sur- 

 face of the shell, instead of being gently concave, 

 as in land tortoises, has a deep circular concavity 

 in the centre, above four inches in depth, which, 

 when the animal goes into the water, retains a 

 large volume of air, buoying up the whole mass 

 while it remains in that element.* The greater 

 size of turtles, when compared with tortoises, is a 

 further instance of the superior facility with which 

 organic growth proceeds in aquatic than in land 

 animals formed on the same model of construction. 



* Home's Lectures, vi. 37. 



