THE ORIGIN OF MAN 51 



terminating in a long and very flexible tail, which 

 they wriggle in swimming, as do the fishes. 

 These tadpoles have gills which at first project 

 from the sides of the head, but are later covered 

 by an operculum. In about two months they at- 

 tain a stage which is the equivalent of the lung- 

 fishes. They then undergo a marked metamor- 

 phosis, the hind legs appearing first and later the 

 front pair, which are hidden under the opercu- 

 lum. The long tail shortens through internal ab- 

 sorption and the gills gradually are also absorbed 

 or drop off. The lungs then appear, and for a 

 time the young creatures are fully amphibious, 

 breathing water through their gills and extract- 

 ing the free oxygen from it, and also taking in air 

 through the lungs; but soon the small frogs or 

 toads take to the land and breathe air only. 



All Amphibia in their youth are provided with 

 two or three pairs of external gills or internal 

 ones with external gill-clefts, soft feathery out- 

 growths situated at the back part of the head and 



