234

CHAPTER IV

AMPHIBIANS

Ir is not an easy task to draw the frontier line
between the birds and the reptiles, but it is still more
difficult to separate the amphibia from the fishes.
The amphibia and the reptiles used to be grouped
together, but between them in their living forms a
great gulf is fixed. The reptiles breathe by lungs
all their life, the amphibia breathe by gills in their
youth ; while some of them breathe by gills all along,
and some of them take to lung-breathing in their
mature age.

The amphibians are a very old class. They were
numerous on the earth as long ago as the formation
of the coal measures. They are now generally con-
sidered as divisible into four well-marked orders, one
of which, Stegocephala, is extinct. The three living
orders are: Hcaudata, without tails; Caudata, with
tails ; and Agoda, without either tails or limbs. The
first two of these used to bear the names of Anxoura
and Urvodela, but these are now discarded. The
Apoda are occasionally called Gymunophiona, just as
the Stegocephala are perhaps as well known as the
Labyrinthodontia. Yet one other change in nomen-