96 REPTILIAN WATER TRESPASSERS. 



lower plate, so that when the mouth is opened the 

 pressure of the water forces one flap against the other, 

 and effectually closes the aperture. And, as is in- 

 variably the case with such valves, the resistance is 

 increased in exact proportion to the pressure, so that 

 the deeper the Crocodile dives, and the greater the 

 pressure of the water trying to force its way down the 

 throat, the more firmly are the two parts of the valves 

 squeezed against each other. 



The valves of the heart, of the veins, and that 

 which has been described on page 31 as aiding the 

 respiration of the whale tribe, are all constructed on 

 the same principle. At present, with all our advances 

 in science, and having the advantage of such models, 

 we cannot make such simple and yet such perfect 

 valves as those which have existed ever since the rep- 

 tiles took their place on earth. 



Another point in the respiration of these creatures 

 is yet to be mentioned. It may be that the animal is 

 forced to respire, and yet cannot come into the open 

 air. As in the case of many animals which pass much of 

 their time in the water, the nostrils are placed at the 

 extremity of the muzzle, so that the creature can keep 

 itself entirely below the surface of the water, with the 

 exception of an inch or two of nostril. The exposed 

 portion is so small, that even in the open water it is 

 not easily detected, while it can be entirely concealed 

 by choosing some situation where there are reeds, or 

 other aquatic vegetation. 



The mode of progress through the water is exactly 

 the same as that which is adopted by the fishes 

 namely, sweeping the body from side to side. In these 



