176 THE FROG. 



arch. The arrangement and mode of development of the vessels 

 which are actually present in the hycid arch, agree so closely 

 with those seen in the vessels of the branchial arches as to 

 strongly suggest that frogs must be descended from ancestors 

 in which gills were present on the hyoid arch as well as on the 

 branchial arches. 



f. The vessels of the mandibular arch. These appear later 

 than the vessels of the hyoid arch, and depart even more markedly 

 from the typical branchial arrangement. 



Up to the time of hatching there are no vessels at all in the 

 mandibular arch. Shortly after hatching there appears in the 

 lower or ventral part of the arch a lacunar vessel, which lies 

 parallel to and in front of the similar vessel in the hyoid arch, 

 and, like this, opens into the venous sinuses above the sucker ; it 

 may be spoken of as the mandibular vein. There is also present 

 a very small diverticulum from the dorsal aorta. 



A little later (Fig. 78), both these factors have grown con- 

 siderably. The mandibular vein, VM, has extended dorsal wards, 

 and the aortic diverticulum, EM, ventrahvards ; and the two 

 vessels are now continuous with each other. Shortly after the 

 mouth opens, the two again separate ; the mandibular vein 

 gradually shrinks up, as the sucker degenerates, and the aortic 

 diverticulum grows forwards as the pharyngeal artery of the 

 adult (Fig. 80, AY). 



From the above account it appears that the vessels of the 

 mandibular arch, though still referable to the type of the 

 branchial vessels, are even more modified than those of the 

 hyoid arch ; the afferent lacunar vessel and the diverticulum 

 from the truncus arteriosus are completely absent, and at no 

 time have the vessels any connection with the heart. 



g. The changes in the branchial vessels at the metamorphosis. 

 For some time before the metamorphosis the tadpole breathes by 

 lungs as well as by gills, though the main part of the respiratory 

 work is performed by the latter. 



The condition of the blood-vessels during this period of 

 double respiration is as follows. The mandibular and hyoid 

 vessels may be omitted, as, although these are formed on the 

 type of the branchial vessels, they have no connection with the 

 heart, and no gills are developed in relation with them. 



Gills are present on all four branchial arches, and the 



