44 AMPHIBIA 



would be the case if the Anura possessed both internal and 

 external gills, but the Amphibia 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 Amphibia 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 embryonic conditions and to prolonged larval, 

 occasionally perennibranchiate, aquatic life (of the chapter on 

 Xeoteny, p. 63). 



There is no valid reason for supposing that tlie Stegocephali 

 had true internal gills. We know their branchial skeleton, and 

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

 occur also, although but feebly developed, in Urodela. 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 complicated basket-work which acts as a straining apparatus 

 or filter, 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 hyoidean apparatus for the support of the generally very 

 movable and much-specialised tongue. The fact that the hyoid 

 apparatus of the Aglossa, especially that of Xenopus, is con- 

 structed upon the same lines, is a strong indication that these 

 creatures have arrived at their tongueless condition through the 

 loss of this organ, and this is intelligible in correlation with 

 their absolutely aquatic life. 



The opercular folds assume great dimensions in all tadpoles. 

 They cover the whole gill-region, thereby producing on either 

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

 gradually become continuous with the rest of the surface of the 

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

 usually called the spiracle. This condition prevails in the tadpoles 

 of the Aglossa. In the Discoglossidae the two canals gradually 

 converge and combine into one median opening on the middle of 

 the belly. In all the other Anura the right opening becomes 

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



