42 STORY OP THE AMPHIBIANS 



in damp weather, and hence the tree-toads rejoice 

 at the prospect of rain. 



ClECULATION 



Amphibians, as a rule, are above fishes in that 

 they have better hearts. n 



Although the heart is usually three-chambered, the 

 blood is piped away from it in such a way that only a 

 portion of it passes through the lungs or gills, the re- 

 mainder going the round of the body again without 

 reaching any aerating surface. They are not, there- 

 fore, warm-blooded. Before the lungs of tadpoles are 

 used, the heart has only two chambers, as in fishes, 

 while the blood runs from the heart through a pipe for 

 each of the three gills on each side ; but when the frogf 

 is grown, two of these tubes go to each lung and one 

 other is absorbed. This is a noticeable step upward,' 

 since the warm-blooded creatures have only one of all 

 these six tubes left, while the earliest fishes had eight. 



In the frog which may sit part of the time with 

 his rear parts in the water, and his foreparts in the 

 air, there is a beautiful arrangement of pipes, valves 

 and obstructing glands whereby Nature seems to com- 

 promise with him, and make the part of him in the 

 air warmer-blooded than that in the water. Only in 

 the crocodile (a reptile) elsewhere is there any such 

 arrangement, and that is not just like this. 



Lymph Ciectjlation 



The body of all creatures has a special fluid for 

 carrying material for repair to the muscles, etc., and 



