. Un manuel de zoologie. Zoologie. IV. Vertébrés : AMPHIBIA. 519 Le développement de l'intérêt spécial possède Amphibia, puisqu'il offre la seule facilement observable des instances d'une métamorphose chez les vertébrés. Cette métamorphose est le plus marqué le plus les adultes diffèrent des poissons. Dans les anoures une larve, le têtard (fig. 4) s'échappe de l'oeuf. Il n'a pas les poumons, mais comporte trois paires de branchies externes, pas de jambes, mais d'une queue avec un fin-comme double. Dans la métamorphose les branchies externes et la queue-organes-larvaire sont absorbés, alors que les poumons et les jambes sont formées. Une complication est

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. A manual of zoology. Zoology. 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

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