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The class name, Amphibian,
means "two lives", indicating that amphibians are able
to survive both in water and on land. They represent an
important step in the phylogeny of vertebrates, the bridge
from water onto land. No amphibians live in the ocean,
but some will tolerate brackish water. Amphibians are
cold-blooded (or poikilothermic) vertebrate
animals. Frogs may be found anywhere in the world (excluding
Antarctica),although most frogs are found in the tropics.
Class characteristics:
 Smooth,
moist skin (no scales)
Toes
lack claws
Gills,
lungs, and skin used in respiration
Metamorphosis
from larval stage to adult
Larvae
heart 2 chambers, adult 3 chambers
Eggs
fertilized externally
Amphibian lungs are poorly efficient. For this reason,
most amphibians also use their moist skin for respiration.
See "Types of Respiration" below. Temperature is
another key factor in the life of amphibians. Being ectothermic,
their body temperature changes with the environment. Temperature
extremes are deadly to amphibians. Toads will burrow below
the frost line to escape winter temperatures and many
frogs and salamanders will hibernate in
the mud at the bottom of rivers and lakes. Some species
actually produce glycerol, a type of "antifreeze", in
their cells to resist ice crystal formation.
Hot, dry conditions are just as deadly. Many species
of amphibian will escape these conditions by estivating,
the summer
equivalent of winter hibernation. They conserve body moisture
by retreating into a moist burrow or crevice, curling
tightly and
remaining inactive, torpid, until better conditions return.
Some even secrete a lipid, fatty substance, and coat their
body to
prevent water loss during this period of inactivity.
See
where amphibians fit into Earth's history. 
Amphibian Orders: There
are three orders of living amphibians containing about
170 genera. The majority of amphibians are found in tropical
regions with abundant rainfall. Correlated with the moisture
requirement of amphibians is the fact that most forms
are nocturnal.
 Apoda
(Gymnophiona) |
Click on images below for additional
pictures.
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Caecilians - legless amphibians (not found
in the U.S.) Caecilians are rarely-seen amphibians
that resemble giant earthworms. They spend most
of their time underground, and on those rare occasions
when they are seen, they are usually crossing
roadways on rainy nights.
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Urodela
- "visible tail"
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Click
on images below for additional pictures.
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Salamanders - elongated bodies with short
legs and a smooth, moist skin. The tail is somewhat
thick and flattened side to side to aid swimming.
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Newts - Newts (Taricha torosa) are large
salamanders reaching total lengths almost up to
8 inches. They are characterized by warty skin
that is not slimy, and are light brown to balck
above with ayellow to orange belly.
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Mud Puppies - Mudpuppies are actually salamanders,
large, permanently aquatic salamanders. Unlike
their amphibian cousins, mudpuppies never form
air-breathing lungs. They rely on the bushy red
gills behind their heads to breathe under water,
like fish.
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Frog Life Cycle:
| Developmental Stages:
The male and female frogs then enter amplexus, where
the eggs leave the female and are externally fertilized.
These eggs are encased in a gelatinous fluid and
laid in water. During these breeding periods, a
female may deposit anywhere from 19 to 30,000 eggs!
Each egg sack alone can, depending on the species,
contain hundreds and hundreds of eggs. Since frog
eggs are a favorite delicacy of many predators,
laying the eggs in large numbers usually assures
that at least a few will hatch into tadpoles, and
an even fewer number will grow to be adults.
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When
the tadpole has had enough time to develop inside
the egg, the envelope holding the egg in the mass
will either disintegrate or a chemical that the
tadpoles secrete will cause the egg to break up.
The tadpole will then emerge, using its external
gills to collect oxygen from the water and its small,
specialized mouth to collect algae. The tadpole
will quickly develop a long, compressed tail which
helps it to maneuver around in the water.
The
tadpole's anatomy then gradually starts changing
in anticipation of its new life on the surface.
Hind legs will start to appear, its digestive tract
changes to prepare for its forthcoming insect diet,
it developes forelegs, and the froglet then hops
onto land. This transformation process can last
anywhere from 12 days to 3 years, again depending
on the particular species.
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last the frog is fully developed. The tail is now
completely off and the frog has changed enough to
let it live on the surface. After 1 to 4 years, this
new frog will be old enough to start breeding, and
the cycle of life continues ever onward.. |
Research Links:
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