32 MARINE AND AQUATIC TRESPASSERS. 



In fact, even supposing that the whale were to attempt 

 to expel the air from its lungs when below the surface, 

 it would be unable to open the valve, the pressure from 

 above being too strong to yield to any force that the 

 animal could apply from below. 



All the whale tribes possess this structure, but it is 

 developed to the fullest extent in those animals which 

 are obliged to descend to considerable depths below 

 the surface of the water. This valve is formed much 

 after the fashion which is adopted in the heart and 

 veins namely, it is composed of elastic bags, which, 

 when filled, press firmly against each other, but, when 

 empty, fall into wrinkled folds, leaving a clear passage 

 between them. 



A few words as to the general form of the whale 

 tribe. 



It is almost exactly the same as that of the fish, 

 inasmuch as the movements must necessarily be of a 

 similar character. As the whole body is sustained in 

 the water, the limbs are not required for support, and 

 are modified to suit the peculiar life which the animal 

 leads. The fore limbs are very short, very wide, and 

 very flat, so as to act as fins, and are apparently used 

 for the purpose of balancing the animal rather than 

 aiding or even directing its course. 



As to the hinder limbs, they are not needed ; and 

 in some are altogether absent. There is not even a 

 rudiment of them, and the pelvis itself is wanting, 

 being only represented by a small bone lying loose in 

 the tissues. 



Thus, the whole of the body behind the thorax 

 is flexible ; and, unlike that of fishes in general, can 



