II GILLS 43 



aiul then u])oii the .second arcli. In the G nun. larva the first 

 gill shows four knobs, the second two, the third one knol). They 

 are ahvays delicate and thin, althon(>h sometimes ])igmented, long, 

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

 largest ; well developed and persisting a long time in Hana 

 tcmporaria ; smaller in 7/. escnlnihi and Bnfo vulgaris ; very 

 sliort, scarcely forked, in II. viridis and Nz/Ia arhorea. They are 

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

 descri])tions of these gills will l)e found in the systematic part. 



Great changes take place al)0ut the time when the fourth or 

 last branchial arch and the pulmonary arteries are developed. 

 This occurs in IL esrvlenfa when the larva is about 9 mm. long. 

 The sprouting of the gills extends gradually downwards along the 

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

 loops transform themselves into numerous dendritic bundles, 

 resting in several thickset rows upon the liimler margin of the 

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

 cames no external gill. These " internal gills " look like red 

 bolsters or thick and short-tasselled launches. "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 u|) in an outer branchial chamljer, just like that of Teleostei 

 and other Tectobranch fishes. This is the reason why these ne^\■ 

 gills lia.ve 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 

 l)ranchial arches, and are developed befoi'C 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 fact that the Amphibia ]jossess only external gills throws 

 important light upon their phylogeny. Xot only do the Apoda, 

 Urodela, and Anura agree much more with eaih other than 



