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The
Diversity of The World of Life
Vertebrates
(Vertebrata)
Amphibians
(Amphibia, or Lissamphibia)
Representatives
Frogs
& Toads (Salientia)
Salamanders
& Newts (Caudata)
Caecilians
(Gymnophiona)
Biology
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
4,000 -- many of which are especially endangered worldwide,
apparently because of the thinning atmospheric layer of ozone, which
screens-out the DNA- and otherwise cell-damaging ultraviolet rays from the
sun: Before the ozone layer
was formed, the Earth supported virtually no life on land; and as the most primitive
terrestrial vertebrates (with the exception of lungfish), amphibians are
extremely sensitive to such environmental disruptions.
We have been warned!
ENVIRONMENTS
Amphibians are freshwater and terrestrial species.
Some (such as tree frogs) are "arboreal" (living in trees).
DESCRIPTION
The skin is "naked" (almost never with scales)
and must be kept moist. As the most
primitive living tetrapods, amphibians typically have four legs. There
are often gills, especially on or in the larvae. The skull is flattened
from top to bottom in living species.
The mouth bears jaws, with bony teeth, and an often long tongue.
The eyes have lids and tear glands.
The skeleton of amphibians contains much rubbery
"cartilage", often in the feet.
Salamanders have a long body, a long tail, and usually four
legs.
Caecilians (Tropical) have a long body, a short tail, and
no legs. There may be a few
bony (fish-like) scales in the skin.
Frogs and toads have a body that is short, a tail that is short or absent,
and hind-legs that are adapted for jumping.
Compared to frogs, toads
are generally better adapted to life on dry
land (at least at night), as by having skin that is more horny.
FEEDING HABITS
Various amphibians are scavenging, herbivorous,
and/or carnivorous (especially insectivorous).
MOTION
Without the support of buoyant water, the skeleton of
amphibians and other terrestrial vertebrates is stronger than that of
fish.
The squirming motion used by fish for swimming was
adapted for walking on land by amphibians, their four legs having evolved
from two pairs of fins on their ancestors, similar to modern lobe-finned
fishes (The everyday development of a tadpole into a frog, within a
single generation, is indirect evidence of the prehistoric evolution of
certain fish into amphibians, over a great number of generations).
Tree toads and tree frogs have rough "digital
pads" on their toes, which securely grasp onto tree limbs.
DIGESTION
The typically long tongue of amphibians is shot out quickly
to catch terrestrial or airborne insects, at its sticky tip.
RESPIRATION
Amphibians have a "low metabolic rate" (a
relatively inactive lifestyle). Gases are exchanged through gills on or
in the larvae; in the adults, gases are exchanged through the pulsing mouth,
across the moist naked skin,
and in the lungs -- lungs evolved in primitive amphibians, similar to modern
lungfish,
as out-pouchings of the muscular pharynx" (throat).
Fairly well adapted to life on land, toads have particularly well-developed lungs
(with many folds inside).
Unlike higher terrestrial vertebrates, amphibians inhale
and exhale by the pumping action of the floor of their mouth and pharynx --
like the action employed by a fish to pull water in through the mouth
and then push it out over the gills.
CIRCULATION
In modern amphibians, the "atrium" (the in-coming
chamber of the heart) is divided in two, so the heart is composed of three
chambers in total: Two atria and one "ventricle"
(the pumping, out-going chamber).
Oxygen-depleted blood is returned from throughout the body to the right
atrium and is pumped by the ventricle into the tiny
"capillaries" embedded in the lungs, skin, and mouth, in all of
which the
blood is "oxygenated". The blood returns to the left
atrium of the heart; then the ventricle then pumps the blood out through the
arteries into the rest of
the body. Note how there is some unavoidable, inefficient mixing
of the oxygenated and "de-oxygenated" blood within the
ventricle.
Lower vertebrates are "ectothermic" (so-called
"cold-blooded") -- their body temperature is controlled by the
environment.
Living in the often
harsh environment on land, many amphibians hibernate over winter (some
Tropical species "aestivate" over hot and dry periods).
EXCRETION
Nitrogen-rich and other wastes are
cleansed from the blood of an amphibian by the many tubules within a pair of kidneys and
are excreted into a "cloaca" chamber, which also receives
digestive wastes and opens as the anus.
Excess
nitrogen in amphibians is typically excreted mostly as "urea"
(as in our own, more concentrated urine), which is less toxic than the
ammonia excreted by fish but which is still relatively water-soluble.
Terrestrial vertebrates typically excrete less of their precious
bodily water than do aquatic species.
COORDINATION
As
in other multi-cellular organisms, the growth, development, and activities
of amphibians are under genetic and hormonal control, influenced by the
environment.
Unlike
fish, amphibians (at least as adults) have sense organs adapted for life
out of the water: For example, adult amphibians typically have
a pair of ears with external openings.
The "ribbeting" songs of male frogs and toads are heard
by and instinctively recognized by their potential mates and competitors.
REPRODUCTION
Amphibians typically release swimming sperm directly into
environmental water. A male
amphibian typically grasps the female, underneath, in a hold called "amplexus",
for his fertilization of her eggs released nearby underwater.
The thin-skinned, fertilized eggs of amphibians are
typically laid in watery environments, in which their larvae develop.
However, some Tropical frogs and toads rear their larvae on or within their
moist bodies; and some frogs and toads develop directly into adults,
without the delicate larval stage.
The larvae of salamanders, unlike the tadpoles of frogs and toads,
have external, often feathery gills and bony (not horny) teeth.
Some,
"neotenic" salamanders (such as the "mudpuppy") are
entirely aquatic -- they become sexually mature as larvae and never
develop a terrestrial, adult form.
By
losing their fins and gills and growing legs and lungs, fish-like tadpoles
"recapitulate" (replay) the prehistoric evolution of amphibians
from fish!
Vertebrates
(Vertebrata)
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