258 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



form, being pectinate, bipinnate, dendritic, etc. (fig. 277), and in one 

 species of caecilian (where but a single pair occurs) they are large 

 leaf-like lobes. In the perennibranchs these external gills persist 

 throughout life (they are said to be absorbed and reformed in Siren), 

 and no gills are developed in the clefts . Elsewhere, in the amphibia, in 

 when the clefts break through, there is an ingrowth of ectoderm into 

 each cleft, from which the internal gills are developed, so that, for a 

 time, in the anura there coexist both external and internal gills (fig. 

 278, right side). In all derotreme and salamandrine urodeles the 

 external gills are absorbed at the time of metamorphosis (the process 

 in the derotremes, fig. 277, being accompanied by an overgrowth by 



Fig. 278. — Diagram of the relations of external and internal gills in the anuran 

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

 cavity; pk, pharynx; ra, radix aortae. 



the opercular fold). In the anura (fig. 278) as the operculum grows 

 over the clefts, the external gills, so prominent in the earlier stages, 

 are folded into the atrial chamber where they are gradually reduced, 

 while those developed from the walls of the clefts become functional, 

 water taken in by the mouth, passing over them in its way to the 

 exterior via the atrial chamber. Then, when the metamorphosis is 

 completed, the clefts are closed and the gills are absorbed, and at the 

 same time the tail is reduced and the fore legs are freed and the tad- 

 pole (larva) assumes the adult shape. 



For many years the gills of the fishes were regarded as entodermal 

 in origin, and from this it followed that the gills of fishes and those of 

 the amphibia were different in character — were not homologous. 



