Amphibians
Amphibian Evolution:
 Arose from lobe-fined ancestor called
Crossopterygians
 Land plants & insects provided new food
source
 Had primitive lungs & short, limb like fins
for short periods on land
 Appeared during late Devonian
 Icthyostega early amphibian with 4 limbs,
lungs, & a tail for swimming
Icthyostega
 Well developed limbs (*showed
adaptations for life on land but probably
spent most of their time in water)
 Large tail fin
 Lateral line canals on head
 Large sharp teeth, indicating a diet of fish
Icthyostega
Adaptations:
 Metamorphosis
 Moist, thin skin with no scales (the structure of
amphibian skin indicates the habitats in which they can
survive)
 Most use gills, lungs, and skin in respiration
 Feet, if present, lack claws and are often webbed
 Both internal & external nares (nostrils)
 Three chambered heart (two atria & one ventricle)
 Double loop blood circulation to lungs & rest of body
cells
 Eggs lack multicellular membranes or shells.
 They are usually laid in water or in moist places and are
usually fertilized externally
Double loop blood
circulation
Adaptations: cont…
 Skin with keratin (protein) to prevent water loss
 Necks to more easily see & feed
 Most with smooth, moist skin to take in dissolved
oxygen
 Some with oral glands to moisten food they eat
 Webbed toes without claws
 Ectothermic - body temperature changes with
environment
 Show dormancy or torpor (state of inactivity during
unfavorable environmental conditions)
 Hibernate in winter and aestivate in summer
 Aquatic larva called tadpole goes through
metamorphosis to adult Metamorphosis controlled by
hormone called thyroxine
Adaptations: cont…
 External fertilization with amplexus (male clasps
back of female as sperm & eggs deposited into
water)
 Eggs coated with sticky, jelly like material so
they attach to objects in water & do not float
away
 Eggs hatch into tadpoles in about 12 days.
 Males with vocal sacs to croak
 Digested system adapted to swallow prey
whole
 Well developed muscular system
Classification:
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Anura - frogs & toads
Urodela - salamanders & newts
Apoda - caecilians
Trachystoma - sirens or mud eels
Anuran Characteristics:
 Both terrestrial & freshwater species
 Tadpole with tail, gills, & two-chambered
heart
 Adults without a tail, four limbs, & lungs
 Frog skin smooth & moist for
cutaneous respiration, while toads is
rough & warty (poison glands)
 Long hind limbs for jumping
 Long, forked tongue hinged at front of
mouth
Anuran Characteristics:
Urodela Characteristics:
 Includes salamanders & newts
 Have elongated bodies with a tail &
four limbs
 Smooth, moist skin for cutaneous
respiration
 Less able to stay on dry land than
anurans
Urodela Characteristics:
 Size from a few centimeters long to 1.5
meters
 Nocturnal when live in drier areas
 Newts are aquatic species
 Lay eggs in water or damp soil
 Some bear live young
 May or may not go through tadpole
stage (some hatch & look like small adult)
Urodela Characteristics:
Apodan Characteristics:
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Includes caecilians
Tropical, burrowing, worm like amphibians
Legless
Small eyes & often blind
Eat worms & other invertebrates
Average length 30 centimeters, but can grow
up to 1.3 meters
 internal fertilization
 Female bear live young
Apodan Characteristics:
Trachystoma
Characteristics:
 Includes mud eels or sirens
 Known as "rough mouth" amphibians
 Found in eastern U.S. & southern
Europe
 Have minute forelimbs & no hindlimbs
Trachystoma
Characteristics:
External Frog Anatomy:
 Live double life on land & water
 Powerful hind legs for jumping & swimming
fold under body when at rest
 Bulging eyes to stay submerged but still see
predators
 Blinking eyelids protect eyes from dust &
dehydration
 Nictitating membranes clear to moisten eye &
see underwater
 Internal nostrils or nares allow frog to
breathe underwater
External Frog Anatomy:
 Tympanic membranes or eardrums behind
each eye transmit sound through bone called
columella to inner ear
 Eustachian tubes connect mouth & middle
ear to equalize pressure
 Males croak or make sound to attract females
& ward off other males
 Have protective coloration from cells called
chromatophores
 Granular glands secrete foul tasting or
poisonous substance
 Mucus glands lubricate skin for oxygen to be
dissolved & absorbed
External Frog Anatomy:
Internal Frog Anatomy:
 Skeletal System (adaptation for life on land)
 Rigid spine to bear weight of body
 Nine spinal vertebrae (1 cervical in neck, 7
trunk, & 1 sacral supporting hind legs)
 Urostyle long, slim bone connecting sacral
vertebrae & trunk
 No rib cage, but pectoral girdle forms
shoulders & connects front legs
 Pelvic girdle connects to hind legs
 A fused lower forelimb (radio-ulna) and a hind
limb (tibiofibula)
Internal Frog Anatomy:
 Digestive System
 Tongue sticky, forked, & hinged at front of
mouth so can be extended out to catch insects
 Can pull eyes inward to help swallow food
 Two, sharp, backward-pointing vomerine teeth
in roof of mouth help prevent prey from escaping
 Maxillary teeth line the edge of the upper jaw
 Alimentary canal (mouth, pharynx, esophagus,
stomach, small & large intestines, and cloaca) is
where food is digested, absorbed & wastes
eliminated
 Stomach makes gastric juices to break down
food
Internal Frog Anatomy:
 Pyloric sphincter muscle controls movement
of food from stomach into first part of small
intestine called duodenum
 Liver makes bile to digest fats; stored in gall
bladder
 Pancreas makes pancreatic juice to digest
food in small intestine
 Ileum is coiled mid portion of small intestine
 Mesentery is a fanlike membrane holding the
intestine in place
 Wastes collect in large intestine & then move
into cloaca along with eggs, sperm, & urine
until they leave body through the anus or vent
Internal Frog Anatomy:
 Circulatory System
 Need more oxygen to burn increased amount
of food needed to live on land
 3 chambered heart (right atrium receives
deoxygenated blood from body, left atrium
receives oxygenated blood from lungs, &
ventricle pumps blood to lungs & rest of the
body)
 Double loop blood circulation (pulmonary
from heart to lungs & systemic from heart to
rest of body)
 Conus arteriosus carries blood from ventricle to
body cells
Internal Frog Anatomy:
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1.
2.
3.
4.
5.
6.
7.
8.
Circulatory System cont…
Sinus venosus – deoxygenated blood enters first
Right atrium – deoxygenated blood from the sinus
venosus enters
Ventricle – deoxygenated and oxygenated blood
enters the ventricle
Conus Arteriosus – both kinds of bood is expelled into
this chamber
Lungs – deoxygenated blood leaves the conus
arteriosus and travels to the lungs
Left atrium – oxygenated blood from the lungs enters
the left atrium
Ventricle - oxygenated and deoxygenated blood is
forced into the ventricle, then to the conus arteriosus
where it is sent separate ways
Body – oxygenated blood leaves the conus arteriosus
and travels to the body
Circulatory System
Internal Frog Anatomy:
 Respiratory System
 Tadpoles use gills to breathe
 Adult frogs breathe through lungs &
moist skin (cutaneous respiration)
 Glottis is the opening into throat & lungs
Respiratory System
 Positive-pressure Breathing
 See figure 42 – 9, pg 826
 The floor of the mouth drops and the
nostrils open (air rushes in)
 The floor of the mouth is raised and the
nostrils close ( air is pushed into the
lungs)
Internal Frog Anatomy:
 Excretory System
 Carbon dioxide excreted through skin &
lungs
 Kidneys filter blood & store urine in
urinary bladder until leaves cloaca
Internal Frog Anatomy:
 Nervous System
 Olfactory lobes at base of brain detect smells
 Cerebrum behind olfactory lobes controls
muscles (voluntary activities of the body)
 Optic lobes detect sight
 Cerebellum controls balance & coordination
 Medulla oblongata controls heart rate &
breathing
 Cranial nerves connect brain & spinal cord,
 Spinal nerves branch off the spinal cord to
muscles & sensory receptors
 Tympanic membrane and columella detect
vibrations in the air
Life cycle of a frog
including metamorphosis
Mating embrace (amplexus); male’s sperm
fertilizes female’s eggs externally
 Eggs hatch and the tadpole is released
 Tadpole metamorphosis (stimulated by the
hormone thyroxine)
 Metamorphosis:
1. First lives off yolk stored in its body
2. Gradually grows larger and develops three
pairs of gills

Life cycle of a frog
including metamorphosis
 Metamorphosis cont…
3. Tadpole’s mouth opens, allowing it to feed
4. Legs grow from the body, and the tail and gills
disappear
5. The mouth broadens, developing teeth and
jaws
6. The lungs become functional
7. A small adult frog emerges from the water onto
dry land
Parental care in
Amphibians
 Some species of poison dart frog lay eggs on the forest
floor and protect them, guarding the eggs from
predation and keeping them moist. The frog will urinate
on them if they become too dry. After hatching, a
parent (the gender depends upon the species) will
move them, on its back, to a water-holding bromeliad.
The parent then feeds them by laying unfertilized eggs
in the bromeliad until the young have metamorphosed
 The females of some species sit on their eggs to keep
them moist
Poison Dart Frog
Parental care in
Amphibians
 Other frogs carry the eggs and tadpoles
on their hind legs or back (e.g. the
midwife toads, Alytes spp.).
 Some frogs even protect their offspring
inside their own bodies. The male
Australian Pouched frog (Assa
darlingtoni) has pouches along its side in
which the tadpoles reside until
metamorphosis.
Parental care in
Amphibians
 The female Gastric-brooding frog (genus
Rheobatrachus) from Australia, now
probably extinct, swallows its tadpoles,
which then develop in the stomach. To
do this, the Gastric-brooding Frog must
stop secreting stomach acid and
suppress peristalsis (contractions of the
stomach).
Parental care in
Amphibians
 Darwin’s frog(Rhinoderma darwinii) from
Chile puts the tadpoles in its vocal sac for
development.