458 ANIMAL RESPIRATION 



shrivel up and the gill-clefts close, while at the same time the 

 lungs grow out from the under side of the gullet. Corresponding 

 changes take place in the heart and blood-vessels, and, as often 

 remarked, the successive stages in the life-history afford a practical 

 lesson in evolution ; for if such a startling series of changes can 

 take place in the life of one animal, it is not difficult to picture a 

 similar sequence of events in the evolution of a group. 



