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Classifying Organisms
Scientist classify organisms based on
many factors. Let’s learn!
Organisms can be grouped, or classified, according to similarities
and differences in their physical characteristics
.
Classifying Animals
Physical characteristics of animals that can be used for grouping
include:
•color
•size
•shape
•male or female
•body covering—skin, fur, hair, scales, or feathers
•number and type of eyes
•number and type of wings, arms, or legs
•presence or absence of tail
•type of ears
•type of mouth and teeth
Classifying
A dog could be grouped with a wolf and a coyote based
on similar body covering (fur), number of legs (four), and
similar shape.
Animals can be grouped using other characteristics as
well. Their sleep habits, what they eat, where they live,
how they move, and many other characteristics can be
used to sort them into groups.
Classifying Plants
Plants can be put into different groups based on the characteristics of their roots, stems,
leaves, flowers, fruits, or seeds. For example, the plants below are put into two groups
based on the shape of their leaves.
All of the leaves in the picture collect sunlight for the plant. So the leaves could also have
been put into the same group based on what they are used for.
Classifying Animals
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Animals are living things that cannot make their own food. They must consume other organisms for
energy. Most animals also have the ability to move from one place to another for at least part of their
lives.
Animals are classified based on similarities in their characteristics. There are many physical characteristics
that can be used to classify animals into specific groups. Animals are grouped with other animals that they
are similar to.
There are two major characteristics used to classify animals.
Does the animal have a backbone?
– Vertebrates are animals that have backbones. Vertebrates include mammals, reptiles, fish,
amphibians, and birds.
– Invertebrates are animals that do not have backbones. Insects, arachnids, and mollusks are all
invertebrates.
Does the animal regulate its own body temperature?
– Warm-blooded animals have a constant internal body temperature. Their internal temperature is not
dependent on the temperature of their surroundings. Mammals and birds are warm-blooded.
– Cold-blooded animals regulate their body temperature externally. They are greatly influenced by the
temperature of their surroundings. Reptiles, fish, amphibians, insects, arachnids, and mollusks are
cold-blooded.
Some of the major animal groups are mammals, fish, birds, reptiles, amphibians, insects, arachnids, and
mollusks.
Mammals
Mammals are animals that
• are warm-blooded.
• have backbones.
• live on land and in water.
• breathe air with lungs.
• have fur or hair.
• Female mammals produce milk from mammary glands to feed their
young. Almost all give birth to live offspring.
• Dolphins, elephants, and wolves are all mammals.
• Mammals can be found living on land and in water. Humans, bears, dogs,
cats, elephants, dolphins, whales, squirrels, foxes, giraffes, rabbits, and
wolves are all examples of mammals.
Birds
Birds are animals that
• are warm-blooded.
• have backbones.
• live mostly on land.
• breathe air with lungs.
• have feathers wings, and a toothless beak.
• Female birds lay hard-shelled eggs in nests. Most take care of the eggs and
offspring for at least some period of time.
• Examples of birds include parrots, eagles, penguins, hawks, chickens,
ducks, turkeys, and ostriches.
Reptiles
Reptiles are animals that
• are cold-blooded.
• have backbones.
• live on land and in water.
• have lungs that breathe air.
• have a bodies covered with dry, scaly skin or plates.
• Most female reptiles lay soft-shelled (leathery) eggs on land, but
some give birth to live young. Rattlesnakes are an example of this.
Some reptiles that lay eggs also build nests. Crocodiles and
alligators build nests on the ground where they lay their eggs.
Others, like turtle, dig holes on land and bury the eggs.
Amphibians
Amphibians are animals that
• are cold-blooded.
• have backbones.
• usually live at least part of their lives in water.
• have smooth, moist skin without scales.
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Most female amphibians lay jelly-like eggs in water, but some have very odd ways of
reproducing. Some frogs lay eggs in plants, some carry the eggs inside of their bodies,
and some frogs carry their eggs around on their backs. Eggs that are laid in water
develop into larvae, such as tadpoles, first. Then they later change into an adult form,
such as a frog, that looks very different.
Some amphibians spend only portions of their lives in water. These usually have gills
when they hatch, but develop lungs as adults. Some types of amphibians live their
entire lives in water. Many of these are salamanders that keep gills for their entire lives.
Some salamanders that live on land as adults never develop lungs. These salamanders
exchange gases through their skin instead. Even amphibians that have gills or lungs
exchange gases with their environments through their skin.
Frogs, including toads, and salamanders are two of the major kinds of amphibians.
(Caecilians, a group of worm-like amphibians, is the other group.)
Fish
Fish are animals that
• are cold-blooded.
• have backbones.
• live in water.
• use gills to "breathe" air dissolved in water.
• have fins and scales.
• Salmon, clownfish, tuna, sharks, trout, snapper, and
swordfish are all classified as fish.
Insects
Insects are animals that
• are cold-blooded.
• have a rigid external skeleton and no backbone.
• have antennae and wings.
• have compound eyes and three pairs of hollow, jointed legs.
• have bodies with three segments: head, thorax (middle section, where
wings and legs are attached) & abdomen (body
• Insects are the most common animals found on Earth. Insects live in all
kinds of environments and climates. Most reproduce by laying eggs.
• Grasshoppers, ants, beetles, butterflies, bees, fleas, and cockroaches are
all kinds of insects.