THE FROG. 197 



of water, and examined under low power of microscope, 

 the circulation of the blood may be beautifully seen in 

 its gills. 



As the tadpole grows, a membranous fold (operculuni) 

 arises in front of the gills, and extends backward until it 

 covers them. On the right side of the body, this fold 

 becomes closely adherent by its posterior margin, thus 

 closing the exit of the water on that side ; but a trans- 

 verse passage is developed below, so that all the water 

 from both gill chambers passes out through the aperture 

 remaining on the left side. As soon as the external gills 

 are covered, they begin to disappear by absorption (atro- 

 phy), and internal gills are developed on the inner side 

 of the gill slits. Compare the structure of the tadpole, 

 at this stage, with that of the fish. 



Later a pair of legs appear on the sides of the base of 

 the tail, at first beneath the skin, but soon becoming free. 

 Another pair is developed beneath the operculum, and 

 lungs begin to be developed within the body cavity. 

 Then, by a final molting of the skin, the opercular mem- 

 brane is shed, the fore legs are set free, and the eyes are 

 fully exposed. Finally the gills are absorbed, and lungs 

 become fully developed, and aquatic is exchanged for 

 aerial respiration. The gill slits close, the tail is ab- 

 sorbed, and the tadpole has become a frog. 



This brief account of the frog's life history touches 

 only a few of the most salient points, all of which, and 

 more than which, the careful student will see for him- 

 self. 



Each student should dissect a large tadpole, and should 

 compare the internal organs with those of an adult frog. 

 Each student should find out what articles constitute the 

 tadpole's vegetable diet, and should note, that, with the 

 change to a diet of animal food in the adult, there is a 

 great decrease in the relative length of the intestine. 



