242 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
In the amphibia the gill clefts are formed in the same way as in the 
fishes, but the first and fifth never break through, and all are usually 
closed in the adult, the exceptions being in the perennibranchs and 
derotremes where from one to three clefts remain open through life. In 
the urodeles and cecilians there is a reduced operculum -which never 
becomes prominent, being merely a fold of the integument in front of 
the gill area. In the larval anura it is well developed, though skeletal 
Fic. 249.—Dissection of pseudobranchs (gs) and cephalic circle in pike (Esox), after 
Maurer. cc, cephalic circle e, vessels to eyes; g, gills; , vessels to palate and nose; 
I-IV, efferent branchial arteries. 
supports are lacking, as in all amphibia. Before the time of metamor- 
phosis it grows backward over thegills, gill clefts, and the anlagen of 
the fore limbs, and fuses with the sides of the body behind the latter. 
In this way these parts are enclosed in an extrabranchial or atrial 
chamber, the chambers of the two sides being in communication below. 
During larval life the branchial chambers usually communicate with 
the exterior by a single excurrent pore, usually on the left side, but in 
the larval aglossa right and left excurrent pores are found. 
The gills of the amphibia are certainly of ectodermal origin (cf. p. 
237). First to appear are the external gills, covered with ciliated epi- 
