192 THE ANATOBIT OF VERTEBEATED ANIMALS. 



Before the development of the lungs the heart has only 

 a single auricle ; afterwards, the auricle becomes divided into 

 two. The aortic arches, at first, pass along the visceral and 

 branchial arches to the dorsal aorta, as in other vertebrate 

 embryos. When external gills are developed, each receives 

 a loop from the corresponding arch, much as in Proteus. 



When the internal gills of the Batraclua appear, each aortic 

 arch which belongs to a branchial arch splits into two trunks, 

 ! — one which remains directly connected with the cardiac 

 aorta, and another which opens into the dorsal aorta. The 

 vessels of the branchial filaments constitute loops between 

 these aiferent and efferent trunks, which always remain 

 united by anastomoses. When branchial respiration ceases, 

 and the branchial processes and their vessels disappear, 

 the anastomoses dHate ; the direct communication between 

 the afferent and efferent trunks of the second pair of in- 

 ternal branchiae is re-established ; and they become the 

 permanent arches of the aorta. The anterior branchiee are 

 replaced by the carotid glands, and their afferent vessel is 

 the carotid i^assage of the adult. The afferent and efferent 

 trunks of the third pair of branchiae are converted into the 

 stem of the cutaneous artery, and the afferent trunk of the 

 fourth pair of branchiffi into that of the pulmonary artery. 

 The diagram (Fig. 25, p. 91) is intended to make these 

 changes, and the relations of the various trunks to the 

 embryonic aortic arches, intelligible. 



The alimentary canal of the Tadpole is, at first, long, and 

 coiled up into a close spii-e, like a watch-spring, in the 

 abdomen, but its length becomes relatively less as age ad- 

 vances. At the same time, the diet changes from vegetable 

 to animal — tlie young tadpole being chiefly herbivorous, the 

 adult, insectivorous. 



In the Urodela the tail persists, and developes complete ver- 

 tebrae ; but, in the Batrachia, the caudal part of the spinal 

 column disappears, for the most part, together with the 

 rest of the tail, and only the basal portion of the notochord 

 becomes converted into the urostyle, which eventually 

 ankyloses with the two hindermost neiiral arches. 



