Text Leveling System

Let's Investigate Snakes

RDQ:	1406	Max Word/Sent	42	Total Words	1491		Diff Words	533
Lex	810	Ave Word/.Sent	16	Hi Freq occurs	873	58.6%	Hi freq	162	30.4%
AR	4.2	Long Sentences	25.8	Low Freq occurs	150	10.1%	Low freq	116	21.8%

Snakes as Reptiles
Snakes are long 
slender limbless
reptiles.
These animals are very
unique because instead 
of fur like mammals
moist skin like amphibians 
or feathers like 
birds reptiles have dry
skin with scales.]]
As the snake 
grows its
scales simply
dry up and new larger
scales emerge.  The
snake then crawls out of 
its old skin  This process
called ecdysis
occurs about three
times a year.]]
Another trait
that sets
snakes apart as
reptiles is that they are
cold blooded.  This
means that their body
temperature changes
with the temperature
around them.  A snake 
will move in and out of 
the suns rays to regulate
its body temperature.]]
The eyes of a 
snake are also 
unusual because
they cannot blink.
Snakes seem to stare
and while you might
think that a snake's eyes
are always open they 
are really always closed.  
That's because a clear
protective membrane
called a spectacle 
is fused
over the lens
of each eye.]]
How Snakes Hunt
All snakes are carnivorous.
This means they eat such
animals as mice birds
fish frogs worms insects and 
even other snakes.
Snakes can see movement but their distance vision is
poor so they don't use their eyes to hunt for prey.  They 
don't have ears so they cannot listen for their next
meal either.  One way they do hunt is by using small bones in
their heads called columella to feel sound vibrations.]]
Another way 
snakes hunt is 
by using special
nerve endings in 
their skin to detect the 
body heat of prey animals.
For many 
snakes these nerve
endings are in the 
lips.  Pit vipers
however have
a single pair of 
more highly 
developed heat
sensors called pit
organs at the front of 
the head.  When an animal
is close enough the 
organs will alert the 
snake that its next meal
is nearby.]]
Once prey is 
located
snakes flick
their forked tongues to 
catch chemical particles
in the air.  They then use
an organ in the mouth
called the Jacobson's 
organ to identify the 
source of the particles
This helps the snake 
remember the best 
places to hunt.]]
Different kinds of snakes attack prey in different ways.  Constrictors
squeeze and suffocate their prey before swallowing it.  Venomous
snakes poison their prey and wait for it to die before eating it.
Other species such as garter snakes seize and swallow their prey alive.]]
Snakes don't 
chew their food
like many other
kinds of animals.  They 
have jaws made especially
for swallowing prey
whole.  The lower jaw is 
actually two halves that
can be rotated or moved
apart.  Also the entire 
lower jaw can disconnect
from the upper jaw.  This 
allows some snakes to 
swallow food as wide as 
their own heads.
Swallowing prey
can take hours so 
snakes have windpipes
that can move forward
over the tongue.  This 
will allow a snake to still 
breathe while it's eating.]]
Snake venom
Most snakes have small teeth that are used to push food into their
throats but some also have a pair of sharp fangs in their upper jaw.
These fangs are used for one reason:  to inject poisonous venom
into their victims.  Fangs are a sign of a dangerous snake.]]
A few species of snakes have fangs in the back of the mouth.  These fangs
are grooved to carry the venom that spills from glands inside the snake's
mouth.  Usually these "back fanged" snakes are not a danger to humans
but some such as the Southern African boomslang can pose a threat.]]
Most fanged snakes are recognized by having their fangs in the front of 
the mouth.  These fangs are shaped like hollow slender sharp tubes.
Many of these "front fanged" snakes such as rattlesnakes can fold 
their fangs back along the roof of the mouth.  This type of fang can grow to be very
large and is characteristic of snakes that have very potent venom.]]
Snake venom is 
secreted by special
glands in 
the snake's mouth.
Venom can cause damage
to the body tissue of bite
victims.  Some venom
contains poisonous
chemicals that either
paralyze a victim or 
destroy the blood corpuscles.  
This leads to 
the victim bleeding to 
death often slowly
while its destroyed blood
is still in  its body.]]
Whenever snakes bite prey or even people they normally do so because
they are hungry or frightened.  Unless hunting for food or defending
their young most snakes will attempt to avoid contact with people
or other large animals.
Not all snakes are dangerous though.  Many species such as garter snakes and 
some North American tree snakes are no threat to humans.  In fact if in a proper environment like a zoo all snakes including the fanged ones are safe to observe.]]
How snakes move.
There are four basic ways that you might see a snake move its body.  They
are called concertina movements lateral undulation sidewinding and rectilinear
movement.
Concertina movement is when a snake anchors the scales on the front of its
body to the ground and uses its muscles to pull or drag the rest of its body behind
it.  It then anchors the scales on the back part of its body while it moves the front of
its body forward.]]
A snake moves
with lateral
undulation
when it curves itself and
uses the scales on the 
sides of its body to push
against objects like rocks
or grass.
A  dew snake species
move by sidewinding.  To 
do this a snake loops 
its body into an S shape
and pushes itself side ways.
This movement is 
perfect for travel over
soft surfaces like sand.]]
Boas and rattle snakes
travel
by rectilinear
movement.  They slide
their skin back and forth
over their muscles forcing
the friction of their 
scales against the 
ground or a tree to push
their bodies forward.]]
Snake reproduction
Many snake 
species lay
groups of 
eggs or clutches in 
nests but some types of 
snakes can give birth to 
their young in much the 
same way that dogs and 
cats give birth.  The 
number of young
depends on the size and 
species of the snake.
Pythons  for example
may lay as many as 100
eggs and some types of 
vipers may give birth to 
more than 80 babies.
Most other clutches
however range from five
to 50 young.]]
When the young snakes hatch from eggs weeks after being laid they are
described as oviparous.  These baby snakes such as pythons and cobras
escape their shells by using an egg tooth to break out.  Since this is the 
only use of the egg tooth it falls off shortly after the snakes hatch.
There are also certain types of vipers and adders whose young hatch as soon
as the eggs are laid.  These snakes are described as ovoviviparous.]]
Viviparous
snakes develop within sacks
inside their mothers'
bodies and are born
alive.  Rattlesnakes give
birth this way and deliver 
between 8 and 15
young at a time.  Garter 
snakes and a rattlesnake
relative called the fer-delance
deliver as many as 
70 babies at once.]]
All snakes
regardless of 
their species
normally don't  care for 
their young or teach
them how to survive.
Once the babies hatch 
or are born they are on 
their own to learn how 
to hunt and how to protect
themselves from 
their many predators.]]
How snakes protect
themselves
All snakes both 
young and 
adult are constantly 
at risk of becoming
a predator's next 
meal.  Some snakes have
remarkable protection
and defense features
against the birds larger 
mammals and even 
other snakes that try 
to eat them.
One defense feature
that most snakes rely on 
is camouflage or the 
ability to blend in with 
their environment.  For
example the green boa
is colored to resemble
the rain forest.]]
Some snakes take a bolder approach to 
defense.  The coral snake's bright colors
make it easily recognizable to predators as
a highly poisonous meal.
The ground dwelling eastern diamondback
rattlesnake isn't afraid to face its enemies.
It warns would be attackers with a menacing
rattle sound of its tail.]]
Regardless of the 
type of snake
you can be sure
it has some clever way to 
protect itself from enemies. 
It's a good thing 
too because predators
are not the only threat to 
these reptiles.]]
Dangers to snakes
Fewer than 1000
of the nearly
3000 species of
snakes in the world are
poisonous and less than
300 can be fatal to humans.
unfortunately
humans can be fatal to 
snakes as well.  Habitat
destruction and poisoning
has threatened many 
snake species with 
extinction.  The Indian
python once abundant in 
India and Sri Lanka is 
now endangered as are 
the San Francisco garter 
snake and the eastern 
indigo snake of the 
United States.]]
Not only do 
snakes serve
as food for
other larger animals but
their own eating habits
help balance the populations
of their prey.  They 
are a valuable part of the 
world's ecosystem and 
are beautiful and fascinating
animals.  We
should all take the time
to understand the important
roles that snakes
play in nature.]]

Subjective Criteria

Illustrations: Little support

Simple graphics supplementary to text comprehension

Vocabulary: Many difficult words

Language: Many descriptive words

Sentences: Many Long Complex

Text structure: Formal

Inferences: Some implications

Complexity: Several ideas - closely related

Inserts/Side bars

Diagrams

Headings

Features

Topic

Snakes

Reptiles

Nature

*Animals

Low Frequency Words

ability	abundant	adders	amphibians	anchors
approach	boa	boas	boomslang	camouflage
carnivorous	characteristic	chemical	chemicals	columella
concertina	constantly	coral	corpuscles	curves
delance	destruction	diamondback	disconnect	eastern
ecdysis	ecosystem	emerge	endangered	environment
extinction	fascinating	fatal	features	fer
fewer	forked	friction	fused	glands
grooved	habitat	halves	highly	hollow
identify	indigo	inject	jacobson's	lanka
lens	limbless	membrane	menacing	movement
movements	nature	objects	observe	occurs
organ	organs	oviparous	ovoviviparous	paralyze
particles	poisoning	poisonous	pose	potent
predator's	predators	process	protection	protective
python	pythons	recognizable	recognized	rectilinear
regulate	relative	rely	remarkable	reproduction
resemble	rotated	secreted	seize	sensors
shortly	slender	source	southern	sri
suffocate	surfaces	survive	swallow	swallowing
threat	threatened	tissue	tongues	trait
undulation	unfortunately	unique	valuable	venom
venomous	vibrations	vipers	vision	viviparous
windpipes