ORGANS OF RESPIRATION. 551 



phibia, as in the fishes, the primary free external filiform 

 branchiae are changed at a later period, for more circum- 

 scribed internal forms of these organs. 



In the perennibranchiate amphibia, the branchiae are 

 retained, along with the pulmonary organs, through life; and 

 in the caducibranchiate species, every trace of these organs 

 becomes absorbed, and the respiration is effected in the 

 adult state solely by the developed pulmonary cavities. The 

 branchiae here, as in fishes, are supported by cartilaginous 

 arches connected with the posterior cornua of the os hyoides, 

 and vary in number and form and relative development in 

 different species ; there are three or four of these arches on 

 each side, composed each of one or more pieces, presenting 

 often rudimentary teeth along their inner margin, supporting 

 the free pendent gills along their outer convex edge, and 

 protected anteriorly by a rudimentary cutaneous operculum. 

 The branchiae are retained longer in the larvae of the urode- 

 lous forms, as the tritons, than in those of the anurous species 

 as the frogs and toads ; and the branchial arches, in both 

 tribes, during the metamorphosis, are gradually absorbed 

 with the gills, so as to leave only the os hyoides with its 

 curtailed cornua, in the adult state. In the external forms 

 of the branchiae, the ciliary currents are directed outwards 

 over their surface to the free ends of the branchial laminae, 

 and the ciliary actions continue on portions of the gills, and 

 on portions of the general skin, detached from the body, 

 as the ciliary actions in other classes of animals. 



The branchial arches of amphibia, as of fishes, generally four 

 in number, do not always support effective respiratory gills ; 

 thus there are four branchial arches in the triton, and only 

 three of these are furnished with gills ; the posterior arch is 

 that most generally destitute of branchial laminae. There 

 are likewise only three gills on each side in the siren, the 

 proteus, and the oxolotus, where they are permanent in the 

 adult state, and hang freely exposed from the sides of the 

 neck ; there are four gills on each side in the larvae of anurous 

 amphibia. Muller observed distinct branchiae within a round 

 opening, a line in width, on each side of the neck, in the 

 larvae of ccecilia already three inches long. The branchiae 

 are not composed of closely approximated lanceolate parallel 

 laminae as in fishes, but are here cutaneous penniform ex- 



