760 COMPARATIVE ANATOMY 



permit water to enter but not flow out again. In such cases there is 

 a more posterior pair formed by the branchiostegal membrane closing 

 the opercular opening through which the outflow of water may occur. 



In some of the teleosts and in such forms as Polypterus there is an 

 opercular gill with respiratory functions developed on the inner surface 

 of the operculum, while in some of the elasmobranchs (even those in 

 which the spiracle is closed), pseudobranchs composed of vertical folds 

 are developed on the anterior wall of the cleft. These are homologous 

 with gills but they are not respiratory, as they receive only arterial blood 

 which passes from the pseudobranch to the choroid coat of the eye and 

 sometimes even to the brain. 



AMPHIBIA 



In the amphibia, although the gill pouches form just as they do in 

 fishes, the first and fifth never break through, while in nearly all adult 

 forms all the clefts are closed. Exceptions occur in perennibranchs and 

 the derotremes in which from one to three external openings persist. 



In the tailed amphibia and in the caecilians the operculum is merely 

 a fold of integument in front of the gill-area (Fig. 341). The operculum 

 develops without a skeleton support in the larva of tailless amphibians. 

 This fold grows backward over the gills and fuses. Thus there are 

 atrial chambers formed which usually open by a single excurrent pore 

 to the exterior. In a few forms, however, both right and left excurrent 

 openings occur. 



It is usually conceded that the gills of amphibia are of ectodermal 

 origin, and that there may be both external and internal gills present at 

 the same time. In the tailless amphibia, such as frogs, the operculum 

 grows over the gill clefts, and the external gills are folded into the atrial 

 chamber where they are gradually reduced, while the gills which de- 

 veloped from the walls of the clefts become functional. At the time of 

 metamorphosis the clefts are entirely closed and the gills absorbed. 



It has usually been taught that the gills of fishes were entodermal in 

 origin, but if this is true, they cannot be homologues of the amphibian 

 gills. However, the structures are so much alike in appearance, in struc- 

 ture and in function, that it seems they must be homologous. Neverthe- 

 less, more evidence will have to be awaited before positive assertions 

 of value can be made. 



!!f It may be interesting, and it may with further knowledge some time 

 prove of value, to note from the foregoing, that amniotes have visceral 

 pouches in the embryo, though gills are never developed in the adult ; 

 that reptiles have five of these pouches birds and mammals four. In 

 man only the first breaks through to form a cleft, while in many of the 

 higher forms there are grooves on the outside of the neck which show 

 their original position. The manner of obliterating these external 

 grooves is as follows : The arches most cephalad, especially the hyoid, 



