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 
Fic. 250.—External gills of young Amphiuma, partially covered by opercular fold. 
of cecilians, 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 
Fic. 251.—Diagram of the relations of external and internal gills in the anuran tad- 
pole, after Maurer. ab, eb, afferent and efferent branchial arteries; #, heart; 0, ear cavity; 
ph; pharynx; ra, radix aorte. 
perennibranchs the external gills persist through life (they are said to 
be absorbed and reformed in Siren), but in other urodeles and in cecil- 
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 
