IV. VERTEBRATA: AMPHIBIA. 



519 



The development of the Amphibia possesses special interest, since it 

 affords the only easily observable instances of a metamorphosis among the 

 vertebrates. This metamorphosis is the more marked the more the adults 

 differ 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 ex- 

 ternal gills and tail — larval organs — are absorbed, while lungs and legs are 

 formed. A complication is introduced into the metamorphosis in that, 

 for a time after the loss of the external gills, internal branchiae, lying 

 in gill slits, occur (p. 516). In the tailed forms the metamorphosis 

 is simplified, 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 disap- 

 pear 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 

 martinicensis, Pi pa and several other forms, 

 the whole development occurs in the egg, the 

 young hatching in the adult form. 



Order I. Stegocephali. 



Extinct forms with well-developed tail, numer- 

 ous 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 Labyrinthodonta. The group ap- 

 peared in the carboniferous (footprints in the 

 Devonian), and died out in the trias. 



Order II. Gymnophiona (Caecilise, Apoda). 



These are the nearest of living forms to the 

 Stegocephali, but fossils are entirely unknown. 

 The group is exclusively tropical, occurring in 

 Ceylon, African islands, and America, a discon- 

 tinuous distribution indicative of great age. 

 They are burrowing animals and feed on small invertebrates. As a result 

 of this subterranean life the eyes are small and concealed under the skin, 

 the legs are entirely lost, so that the animals are snake-like in appearance. In 

 the skin there are usually small bony scales; the drum of the ear is lacking; the 

 vertebras are amphiccelous. Inside the egg many species have three pairs of 

 feathered gills (fig. 57T), a proof of their pertinence to the Amphibia. Later, 

 there is an external gill opening which finally closes. Ichthyophis, Ceylon; 

 Hypogeophis, Seychelles; Cacilia, America. 



Order III. Urodela (Gradientia). 



Of recent Amphibia the urodeles are the most fish-like. The vertebral 

 column consists of numerous vertebras, and of these a large part belong to the 



Fig. 571. — Larva of Ichthy- 

 ophis glutinosus (from Boas, 

 after Sarasins). 



