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 caecilians 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 



FIG. 249. Dissection of pseudobranchs (ps) and cephalic circle in pike (Esox}, after 

 Maurer. cc, cephalic circle e, vessels to eyes; g, gills; n, 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 the gills, 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- 



