Lesson 5 introduction to animals

Unit 2 Lesson 5 Introduction to Animals
Copyright © Houghton Mifflin Harcourt Publishing Company

You Are an Animal!
• All animals are multicellular organisms, which
means that they are made up of many cells.
• Animal cells are eukaryotic, so they have a
nucleus. In animals, all of the cells work together
to perform the life functions of the animal.
What characteristics do animals share?

• The cells of a multicellular organism develop into
different kinds of cells in a process called
differentiation.
• Some cells may become skin cells, and others
may become gut cells.
• Each type of cell has a special function in an
organism.

• Animals move in various ways. Some move to find
food, shelter, and mates, while others move
during only part of their life cycle.
• Most animals use sexual reproduction, in which a
male sex cell, the sperm, fertilizes a female sex
cell, the egg.
• The fertilized sex cell, or zygote, divides many
times to form an embryo.

• Some animals, such as hydras and sponges, can
reproduce asexually.
• Offspring of asexual reproduction are genetically
identical to their parent.
• Animals cannot produce their own food, so they are
consumers. A consumer is an animal that eats
other organisms, such as plants, animals, or both,
for energy.

• All animals need to maintain their bodies within a
specific range of temperatures.
• Birds and mammals maintain their own body
temperatures by using some of the energy released
by chemical reactions.
• Other animals rely on their environment to maintain
their body temperature.

Such Diversity!
What groups make up the diversity of
animals?
• Animals are the most physically diverse kingdom
of organisms.
• Animals can be categorized by symmetry or body
plan.

animals?
• Some animals, such as sponges, are
asymmetrical—you cannot draw a straight line to
divide a sponge into equal parts.

animals?
• Animals like the sea anemone have a radial body
plan, organized like the spokes of a wheel.
• Some animals have bilateral symmetry, with two
mirror-image sides.

• Animals can also be categorized by internal traits,
such as whether or not they have a backbone.
• An invertebrate is an animal without a
backbone.
• Invertebrates do not have bones, but some have a
hard, external covering, which supports the body,
called an exoskeleton.
• Asexual reproduction is common in invertebrates.
animals?

• Two special kinds of invertebrates are tunicates
and lancelets, which share some characteristics
with vertebrates.
animals?

• Tunicates, such as sea squirts, are small, sac-
shaped animals. Lancelets are small, fish-shaped
animals.
• Tunicates and lancelets, along with vertebrates,
are part of a group of animals called chordates.
animals?

• Animals with backbones are vertebrates. The
backbone is part of an endoskeleton, an internal
skeleton that supports an animal’s body.
• The backbone is made of bones called vertebrae
that protect part of the nervous system. A
vertebrate also has a braincase, or skull, that
protects its brain.
• Almost all vertebrates reproduce sexually. In a
few species, a female’s egg can develop into an
individual without being fertilized.
animals?

Soft and Squishy?
What are some different kinds of
invertebrates?
• Most animal species are invertebrates. A vast
diversity of animals make up this group.

invertebrates?
• Cnidarians live in oceans and have two body
forms: polyp, like a sea anemone, or medusa, like
a jellyfish.
• Porifera includes sponges with specialized cells
connected by jelly-like material. They live on the
ocean floor and filter food particles from water.

invertebrates?
• Anthropoda includes animals that live on land and
in water with jointed appendages and an
exoskeleton that protects them from predators.
• Mollusca live in water or on land and have soft
bodies. Many, such as snails and clams, have a
protective outer shell and a muscular foot.
• Nematoda are roundworms that live in fresh
water, soil, or other animals. Many of these
animals, such as hookworms, are parasites.

Some Familiar Faces ...
vertebrates?
• Vertebrates are divided into five main groups:
fish, amphibians, reptiles, birds, and mammals.
• Vertebrates live in water, on land, or both. They
can eat plants, animals, or both.
• Both DNA and body form and structure are used
to classify vertebrates.

vertebrates?
• Amphibians live on land and in water. Most have
four limbs and live near fresh water because their
eggs and larvae need water to survive.
• Amphibians have thin skins that must be kept
moist.
• Frogs, toads, and salamanders are examples of
amphibians.

vertebrates?
• Reptiles have bodies covered with scales or plates,
and reproduce by laying eggs.
• Reptiles can live nearly anywhere on land because
they can lay eggs out of water. The eggs are
protected by membranes and a shell.
• Examples include turtles, snakes, lizards, and
crocodiles.

vertebrates?
• Birds have hollow bones, wings, and feathers. They
lay eggs, which they sit on to keep warm. Most
birds can fly. A few, such as penguins, cannot.
• Mammals have hair, a jaw, and three middle-ear
bones, and they produce milk.
• Monotremes are mammals that lay eggs with
shells.

vertebrates?
• Marsupials have embryos that develop in a pouch.
Placental mammals, such as wolves, beavers, and
sloths, have embryos develop inside their bodies.
• Fish live in water. Cartilaginous fish, such as
sharks and stingrays, have a skeleton made of
flexible cartilage. Most fish are bony.
• Some fish reproduce by laying eggs. Other fish
have embryos that develop inside the female.

Lesson 5 introduction to animals

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