. The biology of the amphibia. Amphibians. 160 THE BIOLOGY OF THE AMPHIBIA branchial arches. In caecilians there may be five of these bran- chial arches in the embryo (Fig. 60), while in all other Amphibia four is the maximum number and there may be less. The reduction in the number of arches is not always correlated with differences in habitat. The brook-dwelling Desmognathus pos- sesses four branchial arches and some species of Eurycea, three. Branchial arches have been described in various fossil Amphibia such as Dwinasaurus, Archegosaurus, and Lysorophus, but they were not known to be more
. The biology of the amphibia. Amphibians. 160 THE BIOLOGY OF THE AMPHIBIA branchial arches. In caecilians there may be five of these bran- chial arches in the embryo (Fig. 60), while in all other Amphibia four is the maximum number and there may be less. The reduction in the number of arches is not always correlated with differences in habitat. The brook-dwelling Desmognathus pos- sesses four branchial arches and some species of Eurycea, three. Branchial arches have been described in various fossil Amphibia such as Dwinasaurus, Archegosaurus, and Lysorophus, but they were not known to be more than four in number. The adult caecilians have at most four functional branchial arches, and hence the fifth may never have been a distinct arch in the adults of any Amphibia. In the larvae of urodeles and caecilians the gills arise from a portion of the outer surface of the first three branchial arches. In the tadpoles of Salientia similar gills appear early in develop- ment and in some species they may become greatly elongated (Chap. III). In the Marsupial Frog, Gastro- theca, the two anterior pairs of external gills may form enormous bell-shaped struc- tures which function as vas- cular wrappings completely surrounding the embryo. In most frog tadpoles the exter- nal gills do not attain the size or complexity of these struc- tures in urodeles. Further, they are soon covered over by the operculum, a fold of integument which grows back from the hyoid arch. They are then replaced by rows of shorter gill processes, which grow from the anterior and posterior margins of the same arches and also from the anterior edge of the fourth branchial arch. These are often considered internal gills, homologous with the ordinary gills of fish, in contradistinction to the early formed gills, which are called "external" and homologized with the larval gills of crossopterygians and dipnoans. Except for their point of origin, there is very little difference between external and interna
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