HIGH SCHOOL ZOOLOGY. 



95 





Fig. 69.— Pipa Americana. (After Bichm). 



Structui-ally the tadpoles (Fig. 70) differ from the adult chiefly 

 in the presence of the tail, the want of limbs and the nature of 

 the respiratory and circulatory organs. They possess adhesive 

 discs near the mouth by which they attach themselves to aquatic 

 plants and other objects for support. The fii-st gills are 

 external, but these soon disappear, and give place to internal 

 gills on the four gill-arches ; these are concealed by a gill-cover, 

 which grows over them in such a way as to leave only a single 

 aperture on the left side. Underneath this gill-cover the fore 

 limbs etra tirst budded out and the hind limbs make their aji- 

 pearance immediately afterwards ; both are fully formed before 

 the tail shrivels up. Eventually the gills disappear, and the 

 heart and vessels undergo such an alteration that the venous 

 blood is sent to the lungs and skin to be aerated and the arterial 

 Vjlood to the body generally. 



