552 



AMPHIBIA. 



By the 



side by opercular folds, which form gill chambers 



continued growth of these folds 



the gill chambers are closed, 



with the exception of a single 



exhalent aperture on the left 



side. Through this opening 



the water which is taken in by 



the mouth in respiration passes 



outwards, having washed the 



gills on its way. 



Shortly afterwards the rudi- 

 ments of the limbs appear. 

 The fore limbs are concealed 

 within the gill chambers, and 

 so are not obvious until a much 

 later stage ; but the hind legs 

 may be watched in the pro- 

 gress of development from small 

 papillae to the complete limb. 



The lungs are developed as 

 outgrowths from the oesophagus, 

 even before hatching, but in- 

 crease in size very slowly. After 

 the appearance of the hind legs, 

 the larvse come to the surface 

 of the water to breathe, showing 

 that the lungs are now to some 

 extent functional. At this stage 

 the tadpoles, now about two 

 months old, are at the level of 

 Dipnoi. 



The changes in the relations 

 of the blood vessels, which 

 accompany the successive 

 changes in the methods of 

 respiration, and render these 

 possible, are somewhat com- 

 plicated. 



When respiration is by the 

 gills only, the circulation is 

 essentially that of a fish. From the two-chambered heart 



Fig. 1S7. — Dissection of 

 Tadpole. (After Milnes 

 Marshall and Bles.) 



DL.^ lower lip; i/., ventricle of 

 heart; DE.^ ccsophagus ; XA.^ 

 head l^idney ; --J., aorta; A'., kid- 

 ney; KU., ureter; DO., cloaca; 

 LH.. hind limb; KV.. opening of 

 ureter into cloaca ; GR.. genital 

 ridge ; GF., fatty body : LF.. fore 

 limb ; OG.. internal gill. 



