44 AMPHIBIA CHAP. 



would be the ease if the Auura possessed both internal and 

 external gills, Ijut the xVniphiljia reveal themselves also in this 

 point as connected with the Crossopterygii and the Dipnoi, some 

 of which fishes also possess external gills. It is of course quite 

 possible that the Amphiljia have developed these organs in- 

 dependently, but we understand now that the latter are accessory, 

 and not the primitive respiratory organs ; they are developed 

 in adaptation to emlnyonic conditions and to prolonged larval, 

 occasionally perennibranchiate, aquatic life (cf tlie chapter on 

 Neoteny, p. 03). 



There is no valid reason for supposing that tlie Stegocephali 

 hail true internal gills. "VVe know their branchial skeleton, and 

 we can discern even gill-rakers on the arches. Such gill-rakers 

 occur also, although but feebly developed, in Trodela. The 

 whole branchial framework of the Urodela and Apoda undergoes 

 simple reductions during metamorphosis (see p. 86), but in the 

 Anura these arches are in early tadpole life transformed into a 

 most eomplicated basket-work which acts as a straining apparatus 

 or lilter, to prevent any particle of food or other foreign matter 

 from finding its way into the delicate gills, the current of water 

 passing from the mouth through the filter, past the gills and out 

 of the clefts. During metamorphosis this whole elaborate 

 apparatus is again transformed, almost beyond recognition, into 

 the hyoidoan a})paratus for the support of the generally very 

 luovalilc and mueli-specialised tongue. The fact that the hyoid 

 appaiatus (jf the Aglossa, especially that of A'enojJus, is con- 

 structed u})on the same lines, is a strong indication that these 

 ereatures Jiave arrived at their tongueless condition through the 

 loss of this or<ian, and this is intelliuible in correlation with 

 their absolutely aquatic life. 



The opercular folds assume great dimensions in all tadpoles. 

 They cover the wbole gill-region, thereby ])roducing on either 

 side an outer gill-chamber. The posterior margins of the folds 

 gradually liecome continuous with the rest of the surface of the 

 body. Each gill -chamber opens at first by one lateral canal, 

 usually callcul the sjiiracle. I'his condition ])revails in the tadpoles 

 of tlu' .Vglossa. In the Discoglossidae the two canals gradually 

 converge and combine into one median opening on the middle of 

 the l)elly. In all the other Anura the right opening becomes 

 closed, or rather its canal passes over to and joins that of the 



