125 THE EVOLUTION OF MAM. 



ture reseml:)les the tail of a Fish (s). At first it has no 

 limbs. Respiration is accomplished solely by gills, which 

 are at first external (k) and afterwards internal. Corre- 

 spo'ndingly, the heart is also of the same form as in the 

 Fishes, and consists of only two compartments — an auricle, 

 which receives the venous blood of the body, and a ven- 

 tricle, which drives it through the arterial bulb into the 

 gills. 



Numbers of these fish-like Frog larvae, or " tadpoles," as 

 they are called, swim about every spring in all ponds and 

 pools, using their muscular tails for propulsion, just as is 

 done by Fishes and larval Ascidians. The remarkable 

 transformation of the fish-like form into that of the Froji 

 does not take place till after the tadpole has grown to n 

 certain size. From the throat grows a closed sac which 

 develops into a pair of large sacs; these are the lungs. 

 The simple chamber of the heart is divided into two auricles, 

 owing to the formation of a partition wall, and simul- 

 taneously considerable changes of structure occur in the 

 main arterial trunks. Previously all the blood passed from 

 the heart-chamber through the aorta arches into the gills ; 

 but only part of it now passes to the gills, while another 

 part passes thiough the newly formed lung arteries into the 

 lungs. From the lungs arterial blood returns into the left 

 auricle of the heart, while the venous blood of the body 

 collects in the right auricle. As both of the auricles open 

 into the simple ventricle, the latter contains mixed blood. 

 The fish-like form has now passed into the Dipneusta form. 

 During the further course of modification the gills, with 

 the gill- vessels, are entirely lost, and respiration is now pei- 

 formed by the lungs alone. Yet later, the long tail is also 



