Some of them, like the Frogs and Newts, lose their
gills altogether on attaining the adult condition; but others,
such as the living _Proteus_ and _Menobranchus_, retain
their gills even after acquiring their lungs, and are thus fitted
indifferently for an aquatic or terrestrial existence. The name of
"Amphibia," though applied to the whole class, is thus not
precisely appropriate except to these last-mentioned forms
(Gr. _amphi_, both; _bios_, life). The Amphibians also
differ amongst themselves according as to whether they keep
permanently the long tail which they all possess when young (as
do the Newts and Salamanders), or lose this appendage when
grown up (as do the Frogs and Toads). Most of them have
naked skins, but a few living and many extinct forms have
hard structures in the shape of scales developed in the integument.
All of them have well-ossified skeletons, though some
fossil types are partially deficient in this respect; and all of
them which possess limbs at all have these appendages supported
by bones essentially similar to those found in the limbs
of the higher Vertebrates. All the Carboniferous Amphibians
belong to a group which has now wholly passed away--namely,
that of the _Labyrinthodonts_. In the marine strata which
form the base of the Carboniferous series these creatures have only
been recognised by their curious hand-shaped footprints, similar
in character to those which occur in the Triassic rocks, and which
will be subsequently spoken of under the name of _Cheirotherium_.
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