RESPIRATORY ORGANS. 



243 



thelium. Three pairs of these usually arise, before the gill clefts break 

 through, on the outer surface of the third, fourth and fifth arches, and 

 they are supplied by the corresponding (aortic) arches of the blood 

 system. They are without any skeletal support and are of varying form, 

 -pectinate, bipinnate, dendritic, etc. (fig. 250) and in one species 



FIG. 250. -External gills of young Amphiuma, partially covered by opercular fold. 



of csecilians, where but a single pair occurs, they are large leaf-like 

 lobes. When the gill clefts break through there is an ingrowth of ecto- 

 derm into each cleft, from which (except in perennibranchs) gill fila- 

 ments are developed on the sides of the septa, so that for a time there 

 may be both external and internal gills (fig. 251, right side). In the 



FIG. 251. Diagram of the relations of external and internal gills in the anuran tad- 

 pole, after Maurer. ab, eb, afferent and efferent branchial arteries; h, heart; o, ear cavity; 

 ph; pharynx; ra, radix aortae. 



perennibranchs the external gills persist through life (they are said to 

 be absorbed and reformed m Siren), but in other urodeles and in caecil- 

 ians they are absorbed at the time of metamorphosis. In the anura 

 (fig. 251), as the operculum grows back over the clefts, the external 

 gills, which are so prominent in the earlier stages, become folded into 

 the extrabranchial chamber, where they are gradually reduced, ? while 



