246 ZOOLOGY. 



jaws and suckers go with it ; the mouth at the same time 

 becomes wider ; the gill-slits close and the gill-filaments are 

 also absorbed. The fore limbs come out, the left one 

 through the opercular opening, the right one bursting 

 through the operculum. Many other changes take place 

 at the same time, and when the animal recovers its appetite 

 it prefers insects to plants. It takes to the land and 

 gradually becomes a frog. 



19. Changes in the Vascular System. We have seen 

 that the tadpole's circulation is just that of a fish, though 

 the branchial vessels have a much simpler arrangement 

 than in the dogfish. When the gill-slits close, the blood 

 begins to flow more and more through the direct communica- 

 tion between afferent and efferent arteries, until these 

 become simply one arch. There are thus four pairs of 

 arches; of these the first becomes the carotid arch, its 

 connexion with the aorta of the same side persisting in a 

 closed condition as the duct of Botallus (chap, xiii., 5). 

 The second becomes the systemic arch. The third dis- 

 appears altogether. The fourth, which already had 

 pulmonary and cutaneous branches, loses its connexion 

 with the dorsal aorta and becomes the pulmo-cutaneous 

 arch. About the same time the originally single auricle 

 becomes divided into two by a septum so disposed that only 

 the pulmonary veins enter the left auricle. 



The heart has been gradually shifting back as the 

 larynx and lungs developed, and so the wall of the peri- 

 cardium projects back into the rest of the ccclom and 

 assumes its bag-like character (fig. 91). The transverse 

 Cuvierian veins are thus made to slope back to the heart, 

 and are now called the precaval veins. The posterior 

 cardinals disappear (though there are recorded cases of 

 frogs in which they persisted). The anterior cardinals form 

 the innominate and internal jugular veins. 



