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Chapter 27
Introduction to Animals
Table of Contents
Section 1 Characteristics of Animals
Section 2 Animal Body Systems
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Chapter 27
Section 1 Characteristics of Animals
Objectives
• Identify the features that animals have in common.
• Distinguish radial symmetry from bilateral symmetry.
• Summarize the importance of a body cavity.
• Identify how scientists determine evolutionary
relationships among animals.
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Chapter 27
Section 1 Characteristics of Animals
Characteristics of Animals
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Chapter 27
Section 1 Characteristics of Animals
General Features of Animals
Heterotrophy
• Animals are heterotrophs—that is, they cannot make
their own food.
• Most animals move from place to place searching for
food.
• Once food is located, it is eaten and then digested in
a cavity inside the animal’s body.
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Chapter 27
Section 1 Characteristics of Animals
Comparing Autotrophs and Heterotrophs
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Chapter 27
Section 1 Characteristics of Animals
General Features of Animals, continued
Mobility
• Animals are unique among living things in being able
to perform rapid, complex movements.
• Animals move by means of muscle cells, specialized
cells that are able to contract with considerable force.
• Animals can swim, crawl, walk, run, and even fly. In
fact, flight has evolved four times among animals, in
insects, pterosaurs (extinct reptiles from the time of
the dinosaurs), birds, and bats.
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Chapter 27
Section 1 Characteristics of Animals
General Features of Animals, continued
Multicellularity
• All animals are multicellular.
• In spite of differences in body size, there is little
difference in the size of most of the cells that make
up these animals.
• The cells on the skin of your hand are roughly the
same size as the cells in the heart of a whale or in
the wing muscle of a hummingbird.
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Chapter 27
Section 1 Characteristics of Animals
Comparing Organisms That Are Unicellular
and Multicellular
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Chapter 27
Section 1 Characteristics of Animals
General Features of Animals, continued
Diploidy
• With few exceptions, animals are diploid, meaning
adults have two copies of each chromosome, one
inherited from their father and one from their
mother.
• Only their gametes (egg and sperm) are haploid.
• A great advantage of diploidy is that it permits an
animal to exchange genes between the two copies
of a set of chromosomes, creating new
combinations of genes.
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Chapter 27
Section 1 Characteristics of Animals
Comparing Haploid and Diploid Cells
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Chapter 27
Section 1 Characteristics of Animals
General Features of Animals, continued
Sexual Reproduction
• Almost all animals reproduce sexually by producing
gametes, as do many plants, fungi, and protists.
• The females’ egg cells are much larger than the
males’ sperm cells.
• Unlike the egg cells, the sperm cells of animals have
a flagella and are highly mobile.
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Chapter 27
Section 1 Characteristics of Animals
General Features of Animals, continued
Absence of a Cell Wall
• Among the cells of multicellular organisms, only animal cells
lack rigid cell walls.
• The absence of a cell wall has allowed animals mobility that
other multicellular organisms do not have.
• You may not realize this, but there are cells moving about in
your body all the time. Cells called macrophages, for example,
act as mobile garbage collectors, crawling over tissues and
removing debris.
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Chapter 27
Section 1 Characteristics of Animals
General Features of Animals, continued
Blastula Formation
• In all animals except sponges, the zygote (fertilized
egg cell) undergoes cell divisions that form a hollow
ball of cells called a blastula.
• Cells within the blastula eventually develop into three
distinct layers of cells—ectoderm, endoderm, and
mesoderm.
• These layers are called the primary tissue layers
because they give rise to all of the tissues and
organs of the adult body.
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Chapter 27
Section 1 Characteristics of Animals
Origin of Animal Tissues and Organs
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Chapter 27
Section 1 Characteristics of Animals
Cleavage and Blastula Formation
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Chapter 27
Section 1 Characteristics of Animals
Cleavage and
Blastula Formation
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Chapter 27
Section 1 Characteristics of Animals
General Features of Animals, continued
Tissues
• The cells of all animals except sponges are
organized into structural and functional units called
tissues.
• Tissues are groups of cells with a common structure
that work together to perform a specific function.
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Chapter 27
Section 1 Characteristics of Animals
Body Symmetry
• All animals have their own particular body plan, a
term used to describe an animal’s shape, symmetry,
and internal organization.
• An animal’s body plan results from a pattern of
development programmed into the animal’s genes by
natural selection.
• Sponges have the simplest body plan of all animals.
Sponges are asymmetrical, or irregular in shape,
and sometimes their shape depends on where they
are growing.
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Chapter 27
Section 1 Characteristics of Animals
Body Symmetry, continued
Radial Symmetry
• Animals with radial symmetry have body parts
arranged around a central axis, somewhat like the
spokes around a bicycle wheel.
• A plane passing through the central axis divides the
organism into roughly equal halves.
• Today’s radially symmetrical animals are aquatic.
Most move slowly or drift in ocean currents.
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Chapter 27
Section 1 Characteristics of Animals
Body Symmetry, continued
Bilateral Symmetry
• The bodies of all other animals show bilateral
symmetry, a body design in which there are distinct
right and left halves.
• A plane passing through the animal’s midline divides
the animal into mirror image halves.
• Most bilaterally symmetrical animals have evolved an
anterior concentration of sensory structures and
nerves, a process called cephalization.
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Chapter 27
Section 1 Characteristics of Animals
Radial and Bilateral Symmetries
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Chapter 27
Section 1 Characteristics of Animals
Symmetry in Body Structure
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Chapter 27
Section 1 Characteristics of Animals
Cephalization
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Chapter 27
Section 1 Characteristics of Animals
Internal Body Cavity
• Bilaterally symmetrical animals have one of three
basic kinds of internal body plans.
• The body plan may include a body cavity, or coelom,
a fluid-filled space found between the body wall and
the digestive tract (gut). This space is lined with cells
that come from mesoderm.
• Animals with no body cavity are called acoelomates.
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Chapter 27
Section 1 Characteristics of Animals
Internal Body Cavity, continued
• Animals called pseudocoelomates have a body
cavity located between the mesoderm and
endoderm.
• Coelomates have a true coelom, a body cavity
located entirely within the mesoderm.
• A true coelom provides an internal space where
mesoderm and endoderm can be in contact with
each other during embryonic development.
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Chapter 27
Section 1 Characteristics of Animals
Body Cavity
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Chapter 27
Section 1 Characteristics of Animals
Three Body Plans of Symmetrical Animals
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Chapter 27
Section 1 Characteristics of Animals
Body Segmentation
• Segmented animals are composed of a series of
repeating, similar units called segments.
• Segmentation underlies the organization of all
―advanced‖ animals and is easy to observe in some
animals, such as earthworms.
• In vertebrates, segments are not visible externally,
but there is evidence of segmentation in a vertebrate
embryo.
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Chapter 27
Section 1 Characteristics of Animals
Segmentation
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Chapter 27
Section 1 Characteristics of Animals
Kinds of Animals
• Kingdom Animalia contains about 35 major divisions called
phyla (singular, phylum), depending on how certain organisms
are classified.
• To visually represent the relationships among various groups of
animals, scientists often use a type of branching diagram called
a phylogenetic tree. A phylogenetic tree shows how animals
are related through evolution.
• The animal kingdom is often divided into two groups:
invertebrates (animals without a backbone) and vertebrates
(animals with a backbone).
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Chapter 27
Section 1 Characteristics of Animals
Evolutionary Relationships in the Animal
Kingdom
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Chapter 27
Section 1 Characteristics of Animals
The Animal
Body: An
Evolutionary
Journey
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Chapter 27
Section 1 Characteristics of Animals
Animal Body Features and Phylogeny
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Chapter 27
Section 1 Characteristics of Animals
Phylogenetic Tree
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Chapter 27
Section 2 Animal Body Systems
Objectives
• Summarize the functions of the digestive,
respiratory, circulatory, nervous, skeletal, and
excretory systems.
• Compare a gastrovascular cavity with a one-way
digestive system.
• Differentiate open from closed circulatory systems.
• Distinguish asexual from sexual reproduction.
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Chapter 27
Section 2 Animal Body Systems
Tissues and Organs
Digestion
• Single-celled organisms and sponges digest their
food within their body cells. All other animals digest
their food extracellularly (outside of their body cells)
within a digestive cavity.
• Simple animals, such as the hydra and flatworms,
have a gastrovascular cavity, a digestive cavity with
only one opening.
• Other animals have a digestive tract (gut) with two
openings, a mouth and an anus.
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Chapter 27
Section 2 Animal Body Systems
Tissues and Organs, continued
Digestion
A hydra has a gastrovascular cavity, while a roundworm
has a digestive tract in which food travels in one
direction only.
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Chapter 27
Section 2 Animal Body Systems
Tissues and Organs, continued
Respiration
• In simple animals, oxygen gas and carbon dioxide
gas are exchanged directly with the environment
by diffusion.
• The uptake of oxygen and the release of carbon
dioxide, called respiration, can take place only
across a moist surface.
• Some aquatic (and a few terrestrial) animals
respire with gills, very thin projections of tissue
that are rich in blood vessels.
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Chapter 27
Section 2 Animal Body Systems
Parts of the Human Respiratory System
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Chapter 27
Section 2 Animal Body Systems
Fish Gills
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Chapter 27
Section 2 Animal Body Systems
Tissues and Organs, continued
Circulation
• In complex animals, oxygen and nutrients must be
transported to these body cells by a circulatory
system.
• In an open circulatory system, a heart pumps fluid
containing oxygen and nutrients through a series of
vessels out into the body cavity.
• In a closed circulatory system, a heart pumps
blood through a system of blood vessels.
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Chapter 27
Section 2 Animal Body Systems
Open and
Closed
Circulatory
Systems
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Chapter 27
Section 2 Animal Body Systems
Comparing Open and Closed Circulatory
Systems
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Chapter 27
Section 2 Animal Body Systems
Tissues and Organs, continued
Conduction of Nerve Impulses
• Nerve cells (neurons) are specialized for carrying
messages in the form of electrical impulses
(conduction).
• Bilaterally symmetric animals have clusters of
neurons called ganglia.
• More-complex invertebrates, such as the
grasshopper, have brains with sensory structures,
such as eyes, associated with them.
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Chapter 27
Section 2 Animal Body Systems
Tissues and Organs, continued
Conduction of Nerve Impulses
The hydra has a simple nerve net, while the flatworm
and the grasshopper have more-complex nervous
systems.
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Chapter 27
Section 2 Animal Body Systems
Tissues and Organs, continued
Support
• Many soft-bodied invertebrates have a hydrostatic
skeleton. A hydrostatic skeleton consists of water
that is contained under pressure in a closed cavity,
such as a gastrovascular cavity or a coelom.
• Other invertebrates, such as insects, have a type of
skeleton known as an exoskeleton, which is a rigid
external skeleton that encases the body of an animal.
• An endoskeleton is composed of a hard material,
such as bone, embedded within an animal.
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Chapter 27
Section 2 Animal Body Systems
Comparing Exoskeletons and Endoskeletons
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Chapter 27
Section 2 Animal Body Systems
Tissues and Organs, continued
Excretion
• The term excretion refers to the removal of wastes produced by
cellular metabolism.
• Simple aquatic invertebrates and some fishes excrete ammonia
into the water through their skin or gills by diffusion.
• Other animals, especially terrestrial animals, convert ammonia to
nontoxic chemicals, like urea. As the excretory system eliminates
these wastes, water and other useful substances are returned to
the body.
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Chapter 27
Section 2 Animal Body Systems
Reproductive Strategies
Asexual Reproduction
• Reproduction that does not involve the fusion of two
gametes is called asexual reproduction.
• An unusual method of asexual reproduction is
parthenogenesis, in which a new individual develops
from an unfertilized egg.
• Animals that reproduce asexually are usually able to
also reproduce sexually.
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Chapter 27
Section 2 Animal Body Systems
Reproductive Strategies, continued
Sexual Reproduction
• In sexual reproduction, a new individual is formed by
the union of a male and a female gamete.
• Gametes are produced in the sex organs. The testes
produce the male gametes (sperm), and the ovaries
produce the female gametes (eggs).
• Some species of animals, called hermaphrodites,
have both testes and ovaries.
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Chapter 27
Section 2 Animal Body Systems
Reproductive Strategies, continued
Sexual Reproduction
• Most aquatic animals simply release the male and
female gametes near one another in the water, where
fertilization occurs. This method is called external
fertilization because the egg is fertilized outside of
the female’s body.
• Most terrestrial animals reproduce sexually by means
of internal fertilization. In internal fertilization, the
union of the sperm and egg occurs within the
female’s body.
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Chapter 27
Section 2 Animal Body Systems
Sexual Reproduction
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Chapter 27
Standardized Test Prep
Multiple Choice
The diagram on the right shows a
phylogenetic tree of animal phyla.
The letters A–D on the diagram
represent milestones in the
evolution of animals. Use the
diagram to answer questions 1–3.
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Chapter 27
Standardized Test Prep
Multiple Choice, continued
1. Which letter on the phylogenetic tree represents the
presence of tissues?
A.
B.
C.
D.
A
B
C
D
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Chapter 27
Standardized Test Prep
Multiple Choice, continued
1. Which letter on the phylogenetic tree represents the
presence of tissues?
A.
B.
C.
D.
A
B
C
D
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Chapter 27
Standardized Test Prep
Multiple Choice, continued
2. Which phylum contains animals that lack body
symmetry?
F.
G.
H.
J.
Echinodermata
Porifera
Cnidaria
Mollusca
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Chapter 27
Standardized Test Prep
Multiple Choice, continued
2. Which phylum contains animals that lack body
symmetry?
F.
G.
H.
J.
Echinodermata
Porifera
Cnidaria
Mollusca
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Chapter 27
Standardized Test Prep
Multiple Choice, continued
3. Which phylum contains animals that have a
pseudocoelom?
A.
B.
C.
D.
Porifera
Platyhelminthes
Nematoda
Annelida
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Chapter 27
Standardized Test Prep
Multiple Choice, continued
3. Which phylum contains animals that have a
pseudocoelom?
A.
B.
C.
D.
Porifera
Platyhelminthes
Nematoda
Annelida
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