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Unit 1: What is Biology?
Unit 2: Ecology
Unit 3: The Life of a Cell
Unit 4: Genetics
Unit 5: Change Through Time
Unit 6: Viruses, Bacteria, Protists, and Fungi
Unit 7: Plants
Unit 8: Invertebrates
Unit 9: Vertebrates
Unit 10: The Human Body
Unit 1: What is Biology?
Chapter 1: Biology: The Study of Life
Unit 2: Ecology
Chapter 2: Principles of Ecology
Chapter 3: Communities and Biomes
Chapter 4: Population Biology
Chapter 5: Biological Diversity and Conservation
Unit 3: The Life of a Cell
Chapter 6: The Chemistry of Life
Chapter 7: A View of the Cell
Chapter 8: Cellular Transport and the Cell Cycle
Chapter 9: Energy in a Cell
Unit 4: Genetics
Chapter 10: Mendel and Meiosis
Chapter 11: DNA and Genes
Chapter 12: Patterns of Heredity and Human Genetics
Chapter 13: Genetic Technology
Unit 5: Change Through Time
Chapter 14: The History of Life
Chapter 15: The Theory of Evolution
Chapter 16: Primate Evolution
Chapter 17: Organizing Life’s Diversity
Unit 6: Viruses, Bacteria, Protists, and Fungi
Chapter 18: Viruses and Bacteria
Chapter 19: Protists
Chapter 20: Fungi
Unit 7: Plants
Chapter 21:
Chapter 22:
Chapter 23:
Chapter 24:
What Is a Plant?
The Diversity of Plants
Plant Structure and Function
Reproduction in Plants
Unit 8: Invertebrates
Chapter 25: What Is an Animal?
Chapter 26: Sponges, Cnidarians, Flatworms, and
Roundworms
Chapter 27: Mollusks and Segmented Worms
Chapter 28: Arthropods
Chapter 29: Echinoderms and Invertebrate
Chordates
Unit 9: Vertebrates
Chapter 30: Fishes and Amphibians
Chapter 31: Reptiles and Birds
Chapter 32: Mammals
Chapter 33: Animal Behavior
Unit 10: The Human Body
Chapter 34: Protection, Support, and Locomotion
Chapter 35: The Digestive and Endocrine Systems
Chapter 36: The Nervous System
Chapter 37: Respiration, Circulation, and Excretion
Chapter 38: Reproduction and Development
Chapter 39: Immunity from Disease
Invertebrates
What is an animal?
Sponges, Cnidarians, Flatworms and
Roundworms
Mollusks and Segmented Worms
Arthropods
Echinoderms and Invertebrate Chordates
Chapter 25 What is an animal?
25.1: Typical Animal Characteristics
25.1: Section Check
25.2: Body Plans and Adaptations
25.2: Section Check
Chapter 25 Summary
Chapter 25 Assessment
What You’ll Learn
You will identify animal characteristics
and distinguish them from those of other
life forms.
You will identify cell differentiation in
the developmental stages of animals.
You will identify and interpret body
plans of animals.
Section Objectives:
• Identify the characteristics of animals.
• Identify cell differentiation in the
development of a typical animal.
• Sequence the development of a typical
animal.
Characteristics of Animals
• Animals are eukaryotic, multicellular
organisms with ways of moving that help
them reproduce, obtain food, and protect
themselves.
Characteristics of Animals
• Most animals have specialized cells that form
tissues and organs—such as nerves and
muscles.
• Animals are composed of cells that do not
have cell walls.
Animals obtain food
• One characteristic common to all animals is
that they are heterotrophic, meaning they
must consume food to obtain energy and
nutrients.
• All animals depend
either directly or
indirectly on
autotrophs for food.
Animals obtain food
• Scientists hypothesize that animals first
evolved in water.
• In water, some animals, such as barnacles
and oysters, do not move from place to place
and have adaptations that allow them to
capture food from their water environment.
Animals obtain food
• Organisms that
are permanently
attached to a
surface are
called sessile.
Animals obtain food
• Some aquatic animals,
such as corals and
sponges move about
only during the early
stages of their lives.
• Most adults are
sessile and attach
themselves to rocks
or other objects.
Animals obtain food
• There is little suspended food in the air.
• Land animals use more oxygen and
expend more energy to find food.
Animals digest food
• In some animals, digestion is carried out
within individual cells; in other animals,
digestion takes place in an internal cavity.
• Some of the food that an animal
consumes and digests is stored as fat or
glycogen, a polysaccharide, and used
when other food is not available.
Animals digest food
• In animals such as planarians and earthworms,
food is digested in a digestive tract.
Mouth
Digestive
tract
Digestive
tract
Extended
pharynx
Anus
Animal cell adaptations
• Most animal cells are differentiated and
carry out different functions.
• Animals have specialized cells that enable
them to sense and seek out food and mates,
and allow them to identify and protect
themselves from predators.
Development of Animals
• Most animals develop from a fertilized egg
cell called a zygote.
• After fertilization, the zygote of different
animal species all have similar, genetically
determined stages of development.
Fertilization
• Most animals reproduce sexually.
• Male animals produce sperm cells and
female animals produce egg cells.
Formation of mesoderm
• The mesoderm cells develop into the
muscles, circulatory system, excretory
system, and, in some animals, the respiratory
system.
Formation of mesoderm
• When the opening in the gastrula develops
into the mouth, the animal is called a
protostome.
• Snails,
earthworms,
and insects are
examples of
protostomes.
Formation of mesoderm
• In other animals, such as sea stars, fishes,
toads, snakes, birds, and humans, the mouth
does not develop from the gastrula’s opening.
Formation of mesoderm
• An animal whose mouth developed not from
the opening, but from cells elsewhere on the
gastrula is called a deuterostome.
Formation of mesoderm
• Scientists hypothesize that protostome
animals were the first to appear in
evolutionary history, and that deuterostomes
followed at a later time.
• Determining whether an animal is a
protostome or deuterostome can help
biologists identify its group.
Cell differentiation in Animal
Development
• The fertilized eggs of most animals follow a
similar pattern of development. From one
fertilized egg cell, many divisions occur until
a fluid-filled ball of cells forms.
• The ball folds inward and continues to
develop.
Sperm cells
Cell
Differentiation
in Animal
Development
Fertilization
Egg cell
Formation of
mesoderm
Endoderm
First cell
division
Mesoderm
Ectoderm
Gastrulation
Additional cell
divisions
Formation of a blastula
Growth and development
• Most animal embryos continue to develop
over time, becoming juveniles that look like
smaller versions of the adult animal.
• In some animals, such as insects and
echinoderms, the embryo develops inside an
egg into an intermediate stage called a larva
(plural larvae).
Growth and development
Growth and development
• A larva often bears little resemblance to the
adult animal.
• Inside the egg, the larva is surrounded by a
membrane formed right after fertilization.
• When the egg hatches, the larva breaks
through this fertilization membrane.
Adult animals
• Once the juvenile or larval stage has passed,
most animals continue to grow and develop
into adults.
• This growth and development may take just a
few days in some insects, or up to fourteen
years in some mammals.
• Eventually the adult animals reach sexual
maturity, mate, and the cycle begins again.
Question 1
Which of the following is NOT a characteristic
of animals?
A. eukaryotic
B. multicellular
C. heterotrophic
D. prokaryotic
The answer is D.
Question 2
Sessile animals _______.
A. live only underground
B. are autotrophs
C. are permanently attached to a surface
D. live only on land
The answer is C.
Sessile animals
are permanently
attached to a
surface.
Question 3
Ingestion is another word for _______.
A. digestion
B. physically responding to a light stimulus
C. breathing
D. eating
The answer is D,
eating.
Question 4
Which of the following is NOT true of animal
fertilization?
A. occurs when a sperm cell penetrates an egg
cell
B. forms a haploid zygote
Question 4
Which of the following is NOT true of animal
fertilization?
C. forms a diploid zygote
D. may be internal or external
The answer is B, forms a haploid zygote.
Question 5
When a zygote divides by mitosis and cell
division to form two cells, the process is called
_______.
A. cleavage
B. fertilization
C. ingestion
D. gastrulation
The answer is A,
cleavage.
Cleavage
Section Objectives:
• Compare and contrast radial and bilateral
symmetry with asymmetry.
• Trace the phylogeny of animal body plans.
• Distinguish among the body plans of
acoelomate, pseudocoelomate, and
coelomate animals.
What is symmetry
• Symmetry is a term that describes the
arrangement of body structures.
• Different kinds of symmetry enable animals
to move about in different ways.
Asymmetry
• An animal that is irregular in shape has no
symmetry or an asymmetrical body plan.
• Animals with no
symmetry often are
sessile organisms that
do not move from
place to place.
• Most adult sponges
do not move about.
Asymmetry
• The bodies of most sponges consist of two
layers of cells.
• Unlike all other animals, a sponge’s
embryonic development does not include
the formation of an endoderm and
mesoderm, or a gastrula stage.
Radial symmetry
• Animals with
radial symmetry
can be divided
along any plane,
through a central
axis, into roughly
equal halves.
Radial symmetry
• Radial symmetry is an adaptation that
enables an animal to detect and capture
prey coming toward it from any direction.
Radial symmetry
• The body plan of a hydra can be
compared to a sack within a sack.
• These sacks are cell layers organized
into tissues with distinct functions.
Radial symmetry
• A hydra develops
from just two
embryonic cell
layers—ectoderm
and endoderm.
Inner cell layer
Outer cell layer
Bilateral symmetry
• An organism
with bilateral
symmetry can
be divided
down its
length into
similar right
and left
halves.
Bilateral symmetry
• Bilaterally symmetrical animals can be
divided in half only along one plane.
• In bilateral animals, the anterior, or head end,
often has sensory organs.
• The posterior of these animals is the tail
end.
Bilateral symmetry
• The dorsal, or upper surface, also looks
different from the ventral, or lower
surface.
• Animals with bilateral symmetry can find
food and mates and avoid predators because
they have sensory organs and good muscular
control.
Bilateral Symmetry and Body Plans
• All bilaterally symmetrical animals developed
from three embryonic cell layers—ectoderm,
endoderm, and mesoderm.
• Some bilaterally symmetrical animals also
have fluid-filled spaces inside their bodies
called body cavities in which internal organs
are found.
Acoelomates
• Animals that develop from three cell
layers—ectoderm, endoderm, and
mesoderm—but have no body cavities are
called acoelomate animals.
• They have a digestive tract that extends
throughout the body.
Acoelomates
• Flatworms are
bilaterally
symmetrical animals
with solid, compact
bodies. Like other
acoelomate animals,
the organs of
flatworms are
embedded in the solid
tissues of their bodies.
Acoelomate Flatworm
Ectoderm
Mesoderm
Endoderm
Body
cavity
Digestive
tract
Acoelomates
• A flattened body
and branched
digestive tract
allow for the
diffusion of
nutrients, water,
and oxygen to
supply all body
cells and to
eliminate wastes.
Acoelomate Flatworm
Ectoderm
Mesoderm
Endoderm
Body
cavity
Digestive
tract
Pseudocoelomates
Pseudocoelomate Roundworm
Ectoderm
Mesoderm
Endoderm
Body
cavity
Digestive
tract
• A roundworm is an
animal with
bilateral symmetry.
• The body of a
roundworm has a
space that develops
between the
endoderm and
mesoderm.
Pseudocoelomates
Pseudocoelomate Roundworm
• It is called a
pseudocoelom
—a fluid-filled
body cavity
partly lined
with
mesoderm.
Ectoderm
Mesoderm
Endoderm
Body
cavity
Digestive
tract
Pseudocoelom
Pseudocoelomates
• Pseudocoelomates can move quickly.
• Although the roundworm has no bones, it
does have a rigid, fluid-filled space, the
pseudocoelom.
• Its muscles attach to the mesoderm and brace
against the pseudocoelom.
Pseudocoelomates
• Pseudocoelomates have a one-way digestive
tract that has regions with specific functions.
• The mouth takes in food, the breakdown and
absorption of food occurs in the middle
section, and the anus expels waste.
Mouth
Intestine
Round body shape
Anus
Coelomates
Coelomate Segmented Worm
• The body cavity of an
earthworm develops
from a coelom, a fluidfilled space that is
completely surrounded
by mesoderm.
• The greatest diversity
of animals is found
among the coelomates.
Ectoderm
Mesoderm
Endoderm
Body
cavity
Digestive
tract
Coelom
Coelomates
• In coelomate animals, the digestive tract and
other internal organs are attached by double
layers of mesoderm and are suspended within
the coelom.
• The coelom cushions and protects the internal
organs. It provides room for them to grow and
move independently within an animal’s body.
Animal Protection and Support
• Over time, the development of body cavities
resulted in a greater diversity of animal
species.
• Some animals, such as mollusks, evolved hard
shells that protected their soft bodies.
• Other animals, such as sponges, evolved
hardened spicules between their cells that
provided support.
Animal Protection and Support
• Some animals developed exoskeletons. An
exoskeleton is a hard covering on the outside
of the body that provides a framework for
support.
Animal Protection and Support
• Exoskeletons also protect soft body tissues,
prevent water loss, and provide protection
from predators.
Animal Protection and Support
• As an animal grows, it secretes a new
exoskeleton and sheds the old one.
• Exoskeletons are often found in
invertebrates. An invertebrate is an
animal that does not have a backbone.
Animal Protection and Support
• Invertebrates, such as sea urchins and sea stars,
have an internal skeleton called an
endoskeleton. It is covered by layers of cells
and provides support for an animal’s body.
Animal Protection and Support
• The endoskeleton protects internal organs and
provides an internal brace for muscles to pull
against.
Animal Protection and Support
• An endoskeleton may be made of calcium
carbonate, as in sea stars; cartilage, as in
sharks; or bone.
Calcium carbonate
cartilage
Animal Protection and Support
• Bony fishes, amphibians, reptiles, birds, and
mammals all have endoskeletons made of
bone.
bone
Animal Protection and Support
• A vertebrate is an animal with an endoskeleton
and a backbone. All vertebrates are bilaterally
symmetrical.
Origin of Animals
• Most biologists agree that animals probably
evolved from aquatic, colonial protists.
• Scientists trace this evolution back in time to
late in the Precambrian.
Origin
of
Animals
Origin of Animals
• Many scientists agree that all the major animal
body plans that exist today were already in
existence at the beginning of the Cambrian
Period, 543 million years ago.
• All known species have variations of the
animal body plans developed during the
Cambrian Period.
Question 1
A sea star
exhibits
_______.
A. radial symmetry
C. bilateral – anterior
B. asymmetry
D. bilateral – posterior
The answer is A, radial symmetry.
Question 2
Which of the following animals does NOT
exhibit radial symmetry?
A. jellyfish
B. starfish
C. octopus
D. sea urchin
The answer is C. An octopus exhibits bilateral
symmetry.
Question 3
As you look at the cross sections of animals in
the following figure, give the reason why
animals with the basic cross section in the
middle and on the far right will tend to be larger
than animals with the far-left cross section.
Question 3
Acoelomate
Flatworm
Pseudocoelomate
Roundworm
Coelomate
Segmented Worm
Pseudocoelom
Coelom
Ectoderm
Mesoderm
Endoderm
Body cavity
Digestive tract
The development of fluid-filled body cavities
made it possible for animals to grow larger
because it allowed for the efficient circulation
and transport of fluids, and support for organs
and organ systems.
Question 4
Which of the following pairs of terms is not
related?
A. sponge – spicule
B. mollusk – shell
C. flatworm – coelom
D. coral – larvae
The answer is C.
Question 5
Which of the following is NOT a vertebrate
feature?
A. endoskeleton
B. backbone
C. bilaterally symmetrical
D. pseudocoelom
The answer is D.
Typical Animal Characteristics
• Animals are multicellular eukaryotes whose
cells lack cell walls. Their cells are
specialized to perform different functions.
• All animals are heterotrophs that obtain
and digest food.
• At some point during its life an animal can
move from place to place. Most animals
retain this ability.
Typical Animal Characteristics
• Embryonic development of a fertilized egg
cell by cell division and differentiation is
similar among animal phyla. The sequence of
developmental stages is:
1. formation of a blastula—a cell-covered,
fluid-filled ball;
Typical Animal Characteristics
2. gastrulation—the inward movement of cells
to form two cell layers, the endoderm and
ectoderm;
3. formation of the mesoderm—the
development of a cell layer between the
endoderm and ectoderm.
Body Plans and Adaptations
• Animal adaptations include asymmetry, radial
symmetry, or bilateral symmetry.
• Flatworms and other acoelomates have
flattened, solid bodies with no body cavities.
• Animals such as roundworms have a
pseudocoelom, a body cavity that develops
between the endoderm and mesoderm.
Body Plans and Adaptations
• A coelom is a fluid-filled body cavity that
supports internal organs. Coelomate animals
have internal organs suspended in a body
cavity that is completely surrounded by
mesoderm.
• Exoskeletons provide a framework of support
on the outside of the body. Endoskeletons
provide internal support.
Question 1
What is the difference between a blastula and
a gastrula?
A blastula is a cellcovered, fluid-filled
ball. When the cells
on one side of the
blastula move
inward, they form a
gastrula, which is a
structure made up
of two layers of
cells with an
opening at one end.
Gastrula
Question 2
The layer of cells on the outer surface of the
gastrula is called the _______.
A. endoderm
B. ectoderm
C. mesoderm
D. blastula
The answer is B, ectoderm.
Endoderm
Mesoderm
Ectoderm
Question 3
Which of these organs develops from the
endoderm?
A. digestive system
B. skin
C. muscles
D. circulatory system
The answer is A.
Question 4
Which of the following is NOT a deuterostome?
A. shark
B. dolphin
C. frog
D. honey bee
The answer is D.
Question 5
List the following stages in the order of their
occurrence.
A. embryo formation
B. gastrulation
C. fertilization
D. blastula formation
The answer is c,a,d,b.
C. fertilization
A. embryo formation
D. blastula formation
B. gastrulation
Question 6
Why can an octopus squeeze through spaces
much smaller than the width of its body?
An octopus has no endoskeleton or exoskeleton
to maintain a rigid shape for the animal.
Therefore, it can modify its shape as necessity
demands.
Question 7
Which of the following animals does NOT
have an exoskeleton?
A. horseshoe crab
B. ant
C. turtle
D. tarantula
The answer is C. A turtle is a vertebrate animal
with an internal skeleton.
Question 8
Why are sessile animals more likely to live in
water than on land?
Answer
There is little suspended food in the air for
these animals to eat.
Question 9
Why are sessile
animals more
likely to live in
vigorously moving
water than in still
water?
Vigorously moving
water is much more
likely to bring food
particles past sessile
animals where they
can capture it than
water that is standing
still. Also, moving
water has more
oxygen suspended in
it than still water.
Question 10
Describe the way somatic cell nuclear transfer
produces stem cells.
Answer
The nucleus is removed from a normal
animal egg cell. A somatic cell is placed next
to the egg cell without a nucleus and the two
cells are made to fuse. The new cell
undergoes many cell divisions and forms a
blastocyst from which stem cells are taken.
Photo Credits
• Digital Stock
• NOAA
• PhotoDisc
• USDA- ARS
• Alton Biggs
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