586 CHORDATA. 



The development of the Amphibia possesses special interest, 

 since it affords the only easily observable instances of a metamor- 

 phosis among the vertebrates. This metamorphosis is the more 

 marked the wider the adults are from the fishes. In the Anura a 

 larva, the tadpole (fig. 4) escapes from the egg. It lacks lungs, 

 but has three pairs of external gills, no legs, but a swimming tail 

 with a fin-like fold. In the metamorphosis the gills and tail larval 

 organs are lost, while lungs and legs are formed. A complica- 

 tion is introduced into the metamorphosis in that, for a time after 

 the loss of the external gills, internal branchiae, lying in gill slits, 

 occur. These, however, are not visible from the exterior, since a 

 fold of skin grows back over them, thus forming a cavity, the 

 atrium, into which the gill slits open, and which in turn opens to 

 the exterior by an opening (rarely paired), usually on the left side 

 (fig. 617). In the tailed forms the metamorphosis is simplified, 



Fia. 617. Side view of tadpole, e, eye; g, opening of atrium; I, hind leg; w, mouth; 



u, vent. 



usually consisting in the loss of the external gills and sometimes 

 in the change of form of the tail, which may lose its fin fold and 

 become cylindrical. The last traces of a metamorphosis disappear 

 in the perennibranchs, where lungs occur and the gills persist 

 (Siren is said to lose the external gills and then re-form them). In 

 the Anura the metamorphosis is lost when, as in Hylodes mar- 

 tinicensis, the whole development occurs in the egg, the young 

 hatching in the adult form. 



Order I. Stegocephali. 



Extinct forms with well-developed tail, numerous membrane 

 bones in the skull, and frequently a bony armor, at least on the 

 ventral surface. Some were of gigantic size, and some from the 

 folded condition of the enamel of the teeth are known as Laby- 

 rinthodonta. The group appears in the carboniferous (footprints 

 in the Devonian), and died out in the trias. 



