II GILLS 43 



and then upon the second arch. In the 6 mm. larva the first 

 gill shows four knoljs, the second two, the third one knob. They 

 are always delicate and thin, although sometimes pigmented, long, 

 and much -ramified structures. The first pair is always the 

 largest ; well developed and persisting a long time in Rana 

 temporaria ; smaller in B. escuhnta and Bnfo vulgaris ; very 

 short, scarcely forked, in i>. viridis and Hi/la arborea. They are 

 relatively largest in Alytes, while still in the egg. Numerous 

 descriptions of these gills will be found in the systematic part. 



Great changes take place about the time when the fourth or 

 last branchial arch and the pulmonary arteries are developed. 

 This occurs in 11. escuhnfa when the larva is about 9 mm. long. 

 The sprouting of the gills extends gradually downwards along the 

 arches upon their ventral halves, and these new gill-filaments or 

 loops transform themselves into numerous dendritic bundles, 

 resting in several thickset rows upon the hinder margin of the 

 first to the third arch, one row only on the fourth arch, which 

 can-ies no external gill. These " internal gills " look like red 

 [>olsters or thick and short-tasselled bunches. Whilst they are 

 developing the dorsal, older gills become arrested in their growth 

 and disappear, and at the same time a right and left opercular 

 fold grows out from the head and covers these new gills, shutting 

 them up in an outer branchial chaml jer, just like that of Teleostei 

 and other Tectobranch fishes. This is the reason why these new 

 gills have been called internal, and the mistaken notion has 

 sprung up that they are comparable with the true internal gills 

 of fishes. In reality Amphibia have only external gills. They 

 are always covered by ectoderm, are restricted to the outside of the 

 Ijranchial arches, and are developed l)efore the formation of the 

 clefts. These gills are in many cases directly continuous with 

 the more dorsally and more superficially placed earlier external 

 gills : but although nearly every one who has studied their 

 development has observed this agreement, the old error still 

 prevails. They are morphologically as little internal as the true 

 internal gills of Elasmobranch embryos are external gills, because 

 these have become so elongated that they protrude out of the 

 gill-clefts. 



The tact that the Amphibia possess only external gills throws 

 important light upon their phylogeny. Not only do the Apoda, 

 Urodela, and Anura agree much more with each other than 



