Animal Diversity: Chordata

Tetrapoda
The class Amphibia is composed of tetrapods in which the transition from aquatic to
terrestrial life is clearly indicated. Amphibians are the first vertebrates to live on land,
although they lay their eggs in water or in moist situations.
The first tetrapods evolved from rhipidistian crossopterygian fishes. The fossil remains of
primitive tetrapods have been found in the eastern parts of Greenland in Devonian deposits.
These specimens have features intermediate between late crossopterygians and early
amphibians.
The group Tetrapoda is divided into four classes made up of amphibians, reptiles, birds and
mammals. The living representatives of the class Amphibia include salamanders, newts,
frogs, toads, and the caecilians. The amphibians lead a double life, that is, first in the water,
and then on the land. The result of this ambitious attempt is that they present a medley of
makeshift adaptations, which leave them still a long way from vertebrate perfection.
Among the dual adjustments that they make, are those associated with locomotion and
protection against desiccation. In water, an elongated fishlike body, propelled by a muscular
tail, has proved to be the most efficient mechanism for locomotion. However on land, the
weight of the body is no longer supported by the surrounding aqueous medium, so that the
two pairs of appendages become modified into legs, which act as levers to lift the body away
from the ground. Such levers are equipped with adequate muscles without adding excessively
to the body weight. However the amphibians are not particularly successful at locomotion on
land. Even in frogs and toads, where amphibian legs reach their highest development, such
locomotor appendages are so inefficiently anchored to a single vertebra of the supporting
backbone that these animals cannot bear their weight upon them in the sustained manner
necessary for standing or walking, and can progress only by the momentary exertion of
hopping or jumping.
The problem of dessication arises from the fact that the surrounding air, takes up moisture
rapidly from any moist surface. Amphibians not only utilize gills and primitive lungs in
respiration, but also exchange gases to a very large extent directly through the skin.
Consequently, these animals can live only in moist places. In comparison, the higher land
animals, in which an efficient pulmonary system is formed, are not restricted because they
develop a thick, relatively dry integument, which is resistant to dessication. Thus, relatively
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