674 



PHYLUM CHORDATA : CLASS AMPHIBIA 



SO are not obvious unti 



DL 



Fig. 387. — Dissection of 



tadpole. — After Milnes 



Marshall and Bles. 



DL., Lower lip ; H., ventricle of 

 heart; DE., oesophagus; NA., 

 head kidney; A., aorta; A'., 

 kidney; KU., ureter; DO., 

 cloaca ; LH., hind-limb ; KV., 

 opening of ureter into cloaca ; 

 G.R., genital ridge ; GF., fatty 

 body ; LF., fore-limb ; OG., 

 gills ; a, epidermis ; b, dermis. 



efferent branchial. At 



1 later ; but the hind-legs may be 

 watched in the progress of de- 

 velopment from small papillae to 

 the complete limb. 



The lungs are developed as 

 outgrowths from the oesophagus, 

 even before hatching, but grow 

 very slowly. After the appear- 

 ance of the hind-legs, the larvae 

 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 ac- 

 company the successive changes 

 in the methods of respiration, 

 and render these possible, are 

 somewhat complicated. 



When respiration is by the gills 

 only, the circulation is essentially 

 that of a fish. From the two- 

 chambered heart the blood is 

 driven by afferent branchials to 

 the gills ; from these it collects 

 in efferent vessels which unite on 

 each side to form two aortae. 

 The aortae send arteries to the 

 head, and passing backwards 

 unite to form the single dorsal 

 aorta which supplies the body. 

 For a time there are two dorsal 

 aortas. When the first set of gills 

 is replaced by the second set, new 

 gill-capillaries are developed, but 

 the circulation remains the same. 

 As in Ceratodus, a pulmonary 

 artery arises from the fourth 

 the time when the hind-legs begin 



