552 



ANURA 



[CH 



18 



-14 



digit. It is a common occurrence for the number five to be 

 diminished, but very rare for it to be increased, but if the penta- 

 dactyle limb was derived from a fin like that of the Dipnoi by a 

 shortening of the main axis and a reduction in the number of rays, 

 it would be not unnatural to expect to find in the lower groups 

 of land animals traces of 'extra rays. 



The main differences 

 between the circulatory 

 system of the frog and 

 that of the Newt are to 

 be found in the arterial 

 system. Not only as in 

 the Newt does the fifth 

 arterial arch of the embryo 

 disappear altogether, but 

 the sixth becomes entirely 

 cut off from the aorta and 

 in addition to supplying 

 the lung it sends a large 

 branch to the skin, for 

 which reason it is called 

 the pulmocutaneous 

 arch (Fig. 273). The 

 conus arteriosus, as in 

 Molge, has two transverse 

 rows of pocket valves, one 

 near the heart and one 

 near the outer end, but in 

 the frog there is in ad- 

 dition a longitudinal 

 valve with a free ventral 

 edge running somewhat 

 obliquely from the one 

 row of valves to the other. 

 TV hen the ventricle con- 

 tracts, the blood from the 

 right and left auricles lies 



22 



-24 



FIG. 273. Diagram of arterial arches of Frog 

 viewed from the ventral aspect. 



i, n, in, iv, v, vi. First to sixth arterial 

 arches. 9. Carotid gland. 12. Lingual 

 (ventral carotid). 13. Common carotid 

 (dorsal carotid). 14. Systemic arch. 



17. Dorsal aorta. 19. Pulmocutaneous 

 artery. 22. Subclavian (dorsal type). 



24. Coeliacomesenteric. 



on opposite sides of its 

 cavity, and this cavity is 

 converted into a series of 

 ramifying passages by the numerous muscular trabeculae traversing- 



