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TWOSIDED FROGMOLD
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Introduction
Frogs have fascinated people since
the beginningof civilization. They’re
everywhere: Frogs can be found on
every continent in the world except
Antarctica. Andnearly everyone
knowssomefamousfictional frog,
whether it’s the Frog Prince from
folklore, or the talking bullfrogs of
TV commercial fame.
Bullfrogs, with their rumblingfoghorn calls and their long, flipping
tongues, are the species of frog many
Americansknowbest. You’ll use this
kit to simulate a bullfrog dissection.
But there are morethan three thousand frog species, and the lives they
lead are endlessly varied. There are
tree frogs that never descendto
earth: their eggsare laid in tiny
pockets of water stored at the base of
leaves. Thereare desert frogs that
live part of their lives underground,
where they stay cool and damp.
Others dwell happily in icy mountain
streams, or in the tropics, wherethey
hatch their eggs in water as hot as 90
degrees F (32°C).
Amphibians
All frogs are amphibians. Amphibians are ancient animals: they’ve been
aroundfor at least 360 million years.
Today, all the world’s amphibians
can be divided into three main
groups: the anurans, whichare the
frogs and toads, the caudates, or
tailed amphibians, which include
salamanders, and the caecilians, or
apoda, whichare blind, legless creatures that primarily live underground
or under water. The word "amphibian" comes from the Greek
amphibios, meaning"with a double
life," and amphibiansall share a very
important characteristic: they spend
part of their life cycle on Imsdand
part in the water.
Most amphibianshave sorry;thing else
in common:they undergo ~etamorphosis, a process of physical change
that alters their anatomyto allow
them to change from an exclusive
water dwellerin the larval r;tage to
one that can breathe air as ~heygrow
froman egg into a mature.flult.
Here is howmetamorpho.,is works:
Adult frogs lay eggs. After they are
hatched from their eggs, they emerge
as larvae called tadpoles. (After
hatching, the tadpoles of most
species swimindependently, although there are a few Sl:ecies where
tadpoles stay attached to their mother’s bodies until they’re ready to
fend for themselves).
Tadpoleslive entirely in water, and
swimby beating a strong tail. They
breathe through gills, muchlike a
fish. Tadpolesare efficient eating
machines,feeding on particles of
plants, animal remains, and algae
that they find in the water
As tadpoles grow and mature into
baby frogs (froglets) or toads
(toadlets), their bodies undergomany
changes. Legs sprout, allowing them
to hop or walk about. Thetails disappear, in a process knownas "resorption." Their gills are also resorbedas
they grow lungs and begin to breathe
air. Bythe end of metamorphosis,the
frogs are no longer just dependenton
water-dwelling alone, but can spend
time on land. The whole process can
take a few days for somespecies, or
as long as four or five years for the
North Americanbullfrog.
Weknowa great deal about frogs,
but some mysteries remain. One of
the biggest is whythe world’s frog
population seems to be diminishing
everywhere.
This decline in global amphibian
populations seems to have begun
around 10 years ago. The first evidence of a problem was anecdotal:
people simply noticed there seemed
to be fewer frogs around. But recently scientists havecollected data to
support this perception. Manyfrog
species do seemto be in danger.
Why?One reason may be human
activities that create dangerouspollution and whichdestroy the habitats
that frogs need to live and grow,
However,it is unlikely that a single
cause wouldaccount for the serious
decline of a wide variety of amphibian species all over the world. It may
take years to solve this mystery.
Meanwhile,we must all work to conserve and protect the natural environments wherefrogs live.
Frogs are an integral part of their
ecosystem. Wehope this toy will
help you improve your knowledge of
frog anatomy, and deepen your understanding of the biology which
makes frogs such an importantand fascinating -- part of the world’s
natural environment.
Bullfrogs
Bullfrogs, Ranacatesbiana, are a
very commonspecies of frog in
North America. They’re large, measuring 100-175 mm,and highly
aquatic: adults rarely travel far from
rivers, lakes or ponds.
Bullfrogs hibernate during the winter.
This is called estivation. After estivation, frogs then emergefor the warm
weatherand begin calling at their
breeding sites during the springtime.
Bullfrog calls are instantly recognizable -- they’re the deep, foghorn-like
calls that can sometimesbe heard
from as far as a kilometer away.
Themales call to attract matesand to
declaretheir territories. Asingle
femalebullfrog can lay 20,000 eggs at
a time; the eggs hatch in four days or
less in warmsummerwaters.
However,bullfrog tadpoles develop
slowly-- it can take up to five years
for themto reach maturity.
Thebullfrog’s appetite is legendary.
Theyeat (or try to eat!) anythingthat
moves, from bugs to baby ducks to
snakes. Thefrogs in tum are preyed
uponby snakes, raccoon, large birds,
and manyother predators.
A bullfrog can live 7 to 9 years in the
wild, thoughthe record for a frog in
captivity is 16 years.
The frog you’ll be dissecting had a
muchshorter life span, of course -0 years from the day it was molded
to the present. It’s plastic. However,
following the steps belowand reading about the various organs inside
the frog will help you learn about the
complexbiology of real bullfrogs.
Preparing the frog
To familiarize yourself with the preparation process, read steps 1 through
6 first, before actually preparing
the frog.
IMPORTANTNOTE: Lay generous
amountsof newspaperon the table
that you are workingon. Since
preparing the frog can be messy, be
sure to workon top of the newspaper.
1. Unpackthe envelope containing
the frog skeleton parts and internal
organs. Lay aside the dissecting
tools and plastic grid screen.
3. Punchout the center (frag shaped
body) from the gasket cardboard. Set
this cardboard"frog" aside. Use the
outside "frame" (which has the
round holes and slots and frog "cut
out" for the "gasket"). Laythis "gasket" on the top of your clear frog
moldhalf. The"gasket" is prepunchedin such a wayif,at will
allow the buttons and sicts of the
moldto pass through it. This "gasket" is importantsince it will absorb
and prevent most of the skin making
compoundfrom spilling out of the
sides and bottom of the assembled
mold. Since some of the compound
mayoozeout, it is irnpo~tto makesine
that youare workingover rewspaper.
4. Assemblethe mold halves. Make
sure that the skeleton aad organ systems are properly placed inside the
white moldcavity half and that the
cardboard gasket is properly in place
on the clear moldhalf. carefully
press the two halves together with
the cardboardgasket i l between
them. Use careful and solid pressure
(both thumbsworkw~;il) to snap the
round buttons of the clear moldhalf
into the roundwells c,f the white
moldhalf, Install the squaregrid filter into the receptacle provided.(see
Figure A)
DoubleCheck!Are the frog internal
organs and skeleton :n place? Is the
cardboardgasket securely in place
between the white and clear mold
section halves? Are the two mold
sections securely sm~pped
together?
If you do not have everything
securedtightly, the wet skin mixture
2. Nowassemble the frog skeleton
system and organs as shownin the
sequence in the drawing below. After
you have assembled the skeleton and
internal organs, place this internal
structure assemblyinto the cavity of
the white frog moldtray. Be sure to
place skeleton-organ assemblyso that
the ventral (underside)of this assembly faces upwardsin the white mold
tray (belly facing up).
3
will seep out the sides and bottom
of the moldsections. Makesure the
completed mold is standing upright
and the squaregrid filter is in its
proper place in its receptacle at
the top.
is providedfor about three separate
dissection experiments.
Carefully emptythis powderto the
sheet of typing paper or 1/2 sheet of
newspaperjust for a while. Youwill
use the paper to deliver small amounts
of powderto your mixing cup as you
mix the frog skin compound.
Lookat the measuringscale markings on the side of your small plastic
measuringcup. It is scaled in "oz"
(fluid ounces) and "cc" (cubic centimeters). Youwill use the fluid
ounces or "oz" scale. Obtain warm
tap water from the faucet (makesure
not to use water that is too hot nor
too cold as the mixture will become
lumpyand not set correctly).
Fill the small measuringcup to its
rim. This will be 5 oz. (ounces)
water. Emptythis 5 oz. of warmtap
water into the large plastic mixing
cup. Nowadd eight more ounces
(oz.) of warmtap water into the large
mixing cup, to makea total of 13
ounces(oz.) of water in the large
plastic mixingcup.
Deliver the frog skin powderfrom
the paper sheet into the water in the
large plastic mixingcup little by little. Stir the small amountsof powder
into the water, stirring constantly.
Dothis little bylittle, stirring all the
time, until you have mixedall of the
frog skin compoundinto the warm
tap water. Stir quickly with your
woodenstirring stick until powder
completely absorbs the water - about
a minute.
FigureA
Closedmoldwithinternalorgans
andskeleton,gasketandfilter in place.
5. Mixingthe frog skin: Youwill
need both large and small plastic
cups providedin your kit as well as
the woodenmixing stick. One or two
sheets of paper the size of typing
paper or a 1/2 sheet of newspaper
will be needed.
Measureout two cupfuls of frog skin
mixing compound. Use the small
measuringcup provided in your kit.
Simplyfill the small measuringcup
up to its rim with the powder. Empty
the small measuringcup into the
larger plastic mixingcup provided.
Youwill put two cupfuls of powder
into your large plastic mixingcup.
(Markthis level with a pencil or
marker for the next time measurement.) Enough frog skin compound
Pour the mixture into the mold. Do
this immediately after mixing for
4
one minute! Look at Figure B to
observe tim pouring procedure. As
you pour the mixture into the mold,
gently shake the mold with two
hands. This gentle shaking will help
distribute the mixtureinto all parts of
the frog mold.
6. Wait 15 minutes. Whenthe frog
skin compoundhas set up into a
fleshlike plastic consistencyit is
ready to be removedfrom the mold.
If you wish, you mayleave the frog
in the white plastic moldhalf since
this is your dissection tray, or you
may remove the complete molded
frog and pre-examineit as a whole
frog. Youmaywish to cut off any
excess material stuck to the edges of
the frog wherethe moldsand gasket
cametogether. Cut this excess away
with your scalpel. See Figure C.
FigureC
Remove
the excessmaterialthat has
formedaroundthe edges.
7. Nowyour frog is ready to dissect. Place the frc,g backinto the
white moldedtin/(your dissection
tray). Sincethe li-og is not real, it is
not possible to dissect exactly as a
real frog. Howe’:er, using your
scalpel and forceps (tweezers) and
probe, cut sections of skin to reveal
the internal organs and skeleton of
your frog.
Usuallythe first cut should be in the
middleof the ventral side (underside)
of the frog beginningjust below(posterior to) the backof the jaw all the
wayto the tail (caudal) end of the
frog.
Side incisions raay be madeto the
side of this central (midline) incision
towardeach side (distally) to allow
flap of skin to be laid backto expose
the position and location of the
internal organs.
THE FROG ORGAN SYSTEMS
Here are the major organ systems of the frog which you will explore
in your "dissection."
SKIN (Integumentary System)
As you cut through the artificial skin
of your frog which you have producedyouwill find that it is a lot
like actual amphibianskin: very
thin, muchthinner than that of reptiles, birds, or mammals.
Frogsuse their skin for manyvital
bodily functions. Manyspecies can
breathe through their skins, even
whenthey are under water. They
don’t needto drink with their
mouths,since their skins absorb
water.
Amphibianshave glands in their
skin which produce mucus. This
mucushelps keep their bodies moist
- that’s whyso manyfrogs and toads
feel "slimy" whenyou touch them.
Poisonous frogs and toads secrete
toxins as a defense against predators. Usually, these toxins are mild:
they might cause burning in the
mouthor eyes of an attacker. But a
few frogs are so poisonousas to be
deadly.
Native Americansin northwestern
South America even arm their blowdarts for hunting with the toxins
from Poison Dart Frogs.
Beneaththe skin you will find the..
DORSAL VIEW
~ Scapula
"Forearm"
/~ i~-’)~
(Radius
& Ulna)
emur
H~
Figure 1
Thefrog’s skeleton is composed
of bone
and cartilage. Musclesattach to the bones
and enable the frog to move.As you
inspectthe skeletal structure, please note
that manyof the bonesin a frog are very
similar in structure andfunctionto those in
humansand other mammals;these include
such bonesas the scapula (shoulder blade),
humerus(proximal arm bone), radius and
ulna (forearm bones), femur(hip bone)
andtibia (shin bone).
Theskull protects the brain and eyes.
In the frog’s chest is the heart.
Righ
Left atrium
entricle
Skeletal System.
Like humans,frogs have a strong
skeleton that provides the body’s
structure and supports and protects
the internal organs.
Liver
Figure
Thefrog’s heart is dividedinto three
chambers:2 atria and1 ventricle.
(Comparethis to the four-chambered
human
heartwith2 alria a.rtl 2 ventricles).
Thefrog’s heart circulates its blood
throughoutthe body.Hereis howthe
process works:blood passes through
the venacava and the ventral abdominal vein, andenters the fight atriumof
the heart, It’s pumped
into the fight side
of the ventricle by contractionof the
Theventricle then contracts, andthe
bloodis sent throughthe pulmonary
artery to the lungs, wherecarbondioxide is removedand oxygenis replaced
in the blood. This oxygenrich bloodthen retttms to the heart -to the iett atrium, whereit is pumped
into the left side of the ventricleand
then ba~kout throughthe aorta to the
rest of the body.
Lungs
Figure 3
Ventralto the lungs is the liver.
Theliver synthesizes ,~r stores many
of the vital substance., used throughout the frog’s body. (See Figure 2)
It also absorbs substaaces from the
bloodthat maybe to~.ic to the animal, and breaks them downinto
harmless component:,.
Liver ceils producebile, whichis
ca~ed by a system of bile ducts to
the gallbladder, whereit is stored.
Nowlook for the lungs.
The lungs are divided into two
lobes, the right andleft.
Whenthe frog breathes, it inhales
oxygeninto the lungs. This oxygen
is exchangedfor carbon dioxide in
the blood within the lung’s tiny
blood vessels knownas capillaries.
The carbon dioxide is expelled from
the bodywhenthe frog exhales.
Spleen
Mesentery
Gallbladder
Figure 4A
Whenfrogs hibernate, their metabolism slows downand nearly comes
to a halt:frogs stop breathing with
their lungs, and absorb mostof their
oxygenthrough the skin.
Next comes the stomach.
Digestion begins here. Thoughas tadpoles they are usually vegetarian, all
adult amphibiansare carnivores: they
eat other animals.
intestine and mixes with powerful
digestive enzymessecreted by the
pancreas. (See Figure 4A) The enStomach
zymes break the food downinto very
small particles that can be absorbed
by the intestinal wails. Thenthe particles are secreted into the blood stream
and carried to the liver for processing.
The bile producedby the gallbladder
(See Figure 4A) is released into the
small intestine. Bile aids in digestion
and helps the frog absorb fat.
Food material that is not broken down
in the small intestine passes to the
large intestine, or colon..
Figure5
The colon contains manybacteria and
Most frogs eat insects, ~orms, and
other invertebrates. Large frogs may protozoa whichhelp to break the
also eat birds, mice, snakes, or even food down even more. The frog and
the bacteria and protozoa exist in a
other frogs.
symbiotic relationship -- the frog’s
Although some species may have
body provides a homefor these microprimitive forms of teeth, most amscopic creatures, and in turn they help
phibians don’t have teeth and cannot the frog digest its food.
chewtheir food. Prey is swallowed
Anymaterial left in the colon is passed
whole,often whileit is still alive,
out of the body as feces through the
and sent to the stomach.
cloaca.
Cloaca
VENTRAL
VIEW
~
VENTRAL
Ttntestinal
ract
VIEW~
Thecloaca is the terminal part of three
different body systems.
Fecesfromthe intestinal tract is excreted here. So is urine from the
urinary bladder.
~ DORSAL VIEW
~
Intestinal
Figure6A
la~stinal tract
Mostdigestion occurs in the small
intestine. Partially digested food
from the stomach enters the small
CIoac~.~-~
’~ Figure
6b
Duringmatingseason,eggsare produced
in the ovariesof the female~rog(See
F~gure4,b) andpass into the oviducts.
Theyare then releasedfromher oviducts
into thecloacabeforebeingreleased
into
theenvironment
for fertilization bythe
malefrog. Likewise,malefrogsrelease
sementhough the cloaca.
t Bodies
aVII~~
,. Ocular
~
/~
~ /
~ary
~~
Figure 4b
Unlikemostmammals,
frogs and other
amphibians
havethe ability to re-absorb
much
of the waterin their bladdersif it
becomesnecessary. Frogs andtoads may
also emptytheir bladdersas a defensemechanismagainst predators -- as anybodywho
has ever caughta wildtoad has surelyexperienced!
Brain and Eyes
Frogsdonot havelarge brains. Alarge part
of whattheydohaveis devotedto sight. This
helps themfled foodandavoidpredator.
Muscles
Eye~~.~"
~,~,.~~_._~
~-]
Urinary bladder
DORSAL
F~OGEYES
ry
olfacto
Bul~sof BrainANDBRAIN
~ ]
Optic
Nerve
B~in /
n conclusion
~
Nexttime youre in an a~e~with ponds,
streamsor lakes, Io~kandlisten for frogs.
Youalready knowa goodc~eal abouttheir
appearance,
inside andout: witha little
practice, youcanlearn to identify males
fromfemales,andbullfrogsfromother
species. Youcan evensoy. themout by
theircalls.
Whenyouspy a frog or o~heramphibian,
watchits behavior-ho~it interacts with
the land andthe water, howit breathes,how
it swims,hops,eats andvocalizes.
Ourappreciationof nature is heightenedby
our understanding
of howdifferent animals
interact with their envirorment,
andthe more
wediscoveraboutfrogs andthe environmental pressurethey’reexper~encing,
the more
welearn howto help procotthese animals,
andall of nature,for the-:uture.
SMITH$ONIAN FROG LABTM - 74.q-OSR
Amt.
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