400 



Vertehrata. 



bar; tlie efferent branchials unite to form the aorta. In urodelan 

 larvae a single arterial arcli runs to the last branchial bar^ which is 

 destitute of gills ; the pulmonary artery arises from this, or, in 

 the larval Anura, from the last efferent branchial artery; the 

 carotids, vessels to the head, spring from the first efferent 

 branchial artery. 



At the metamorphosis the gill vessels degenerate, and the afferent 

 .and efferent branchial arteries unite* to form simple arterial 

 arches, which, like their precursors, the efferent branchial arteries, 



ea ca 



Pig. 328. Arterial arches of the U rode la: diagramiuatic. A larva, B 

 r adult, ao aorta, br gill (removed from the second and third arches), ca carotid, j) pul- 

 monary artery, st conus arteriosus ; 1 — 1' first, 2 second, 3 third, 4 — V fourth arterial 

 arches, la — 3a first — third afferent branchial arteries ; 16 — 36 first — third efferent branchial 

 . arteries. — Orig. 



unite to form the aorta. The first, however, usually lose their 

 connection with the others, and simply supply the head with blood ; 

 the fourth also generally become independent, and form only the 

 pulmonary arteries ; and the third arterial arches, in many cases, 

 atrophy completely. When this occurs, and if at the same time the 

 first and fourth arches have no connection with it, the aorta is 

 formed by the second arterial arches only, which are better 

 developed than the others. The aorta of Amphibia is sometimes 

 formed by a single pair of arterial arches, sometimes by several. 

 The Gymnophiona in the adult condition are very similar to the 

 others ; the vascular system of the larva is at present unknown. 



* In the larva of the Urodeles the afferent and efferent branchial arteries are con- 

 nected by a small vessel at the base of the gill (anastomosis), and this enlarges at the 

 metamorphosis. 



