26-1 Introduction to the
Animal Kingdom
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26-1 Introduction to the Animal
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What Is an Animal?
What Is an Animal?
Animals are multicellular, eukaryotic heterotrophs
whose cells lack cell walls.
Animals have the following types of tissues:
•epithelial
•muscular
•connective
•nervous
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What Is an Animal?
Epithelial tissues cover body surfaces.
Muscle tissue cells contain proteins that enable them
to contract, moving parts of animals’ bodies.
Connective tissues, such as bone and blood, support
an animal’s body and connect its parts.
Nervous tissue contains nerve cells, which carry
information throughout the body.
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26-1 Introduction to the Animal
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What Is an Animal?
Invertebrates make up 95% of all animal species.
Invertebrates do not have a backbone, or vertebral
column.
They range in size from microscopic dust mites to the
giant squid, more than 20 meters in length, and also
include sea stars, worms, jellyfishes, and insects.
The other 5% of animals are vertebrates.
Vertebrates have a backbone.
Vertebrates include fishes, amphibians, reptiles,
birds, and mammals.
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26-1 Introduction to the Animal
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What Animals Do to Survive
What Animals Do to Survive
Animals carry out the following essential functions:
• feeding
• respiration
• circulation
• excretion
• response
• movement
• reproduction
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26-1 Introduction to the Animal
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What Animals Do to Survive
Many body functions help animals maintain
homeostasis, or a relatively stable internal
environment.
Homeostasis is often maintained by internal feedback
mechanisms called feedback loops.
Most feedback loops involve feedback inhibition, in
which the product or result of a process stops or limits
the process.
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26-1 Introduction to the Animal
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What Animals Do to Survive
Feeding
Herbivores eat plants.
Carnivores eat other animals.
Omnivores feed on both plants and animals.
Detritivores feed on decaying plant and animal
material.
Filter feeders are aquatic animals that strain tiny
floating organisms from water.
Animals can also form symbiotic relationships,
where two species live in close association with
each other.
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What Animals Do to Survive
Respiration
Whether they live in water or on land, all animals
respire—they take in oxygen and give off carbon
dioxide.
Some animals rely on diffusion of gases through
their skin, but most animals rely on evolved
complex tissues and organ systems for respiration.
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26-1 Introduction to the Animal
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What Animals Do to Survive
Circulation
Animals transport oxygen, nutrient molecules, and
waste products among all their cells through either
simple diffusion or some kind of circulatory system.
Excretion
Ammonia is a waste product of cells and a poisonous
substance that contains nitrogen.
Most animals have an excretory system that
eliminates ammonia quickly or converts it into a less
toxic substance that is removed from the body.
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26-1 Introduction to the Animal
Kingdom
What Animals Do to Survive
Response
Animals respond to events in their environment
using specialized cells, called nerve cells. In most
animals, nerve cells form a nervous system.
Receptor cells respond to sound, light, and
external stimuli. Other nerve cells process
information and determine how the animal
responds.
The arrangement of nerve cells in the body
changes dramatically from phylum to phylum.
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26-1 Introduction to the Animal
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What Animals Do to Survive
Movement
Some animals stay at a single spot, but most can
move. Most animals have muscles or muscle-like
tissues.
Muscle contraction enables motile animals to move
around by working in combination with a support
structure called a skeleton.
Muscles also help even sedentary animals feed
and pump water and fluids through their bodies.
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26-1 Introduction to the Animal
Kingdom
What Animals Do to Survive
Reproduction
Most animals reproduce sexually. This helps to
create and maintain genetic diversity in populations
and improve species’ abilities to evolve when the
environment changes.
Many invertebrates can also reproduce asexually.
This produces offspring that are genetically
identical to the parent. It allows animals to increase
their numbers rapidly.
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26-1 Introduction to the Animal
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Trends in Animal Evolution
Trends in Animal Evolution
Our survey of the animal kingdom will begin with simple
forms and move through more complicated ones. These
different phyla are related to one another by a common
evolutionary heritage.
Complex animals tend to have:
• high levels of cell specialization and internal body
organization
• bilateral body symmetry
• a front end or head with sense organs
• a body cavity
• embryos develop in layers
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Trends in Animal Evolution
Cell Specialization and Levels of Organization
As animals have evolved, their cells have become
specialized to carry out different functions.
In multicellular organisms, each cell type has a
structure and chemical composition that enable it to
perform a specialized function.
Groups of specialized cells form tissues.
Tissues join together to form organs and organ
systems—all of which work together to carry out a
variety of complex functions.
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26-1 Introduction to the Animal
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Trends in Animal Evolution
Early Development
Animals that reproduce sexually begin life as a
zygote, or fertilized egg.
The zygote undergoes a series of divisions to form
a blastula, a hollow ball of cells.
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26-1 Introduction to the Animal
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Trends in Animal Evolution
The blastula folds in on itself, forming a single
opening called a blastopore.
The blastopore leads into a central tube that runs the
length of the developing embryo. This tube becomes
the digestive tract and is formed in one of two ways.
This is called gastrulation.
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26-1 Introduction to the Animal
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Trends in Animal Evolution
Ectoderm
A protostome is an
animal whose mouth is
formed from the
blastopore.
Mesoderm
Endoderm
Blastopore
Most invertebrate
animals are
protostomes.
Anus
Blastopore becomes mouth
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Trends in Animal Evolution
Ectoderm
A deuterostome is an
animal whose anus is
formed from the
blastopore.
Mesoderm
Endoderm
Blastopore
The anus is the opening
through which wastes
leave the digestive tract.
Mouth
The mouth is formed
second.
Blastopore becomes anus
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Trends in Animal Evolution
Echinoderms and
vertebrates are both
deuterostomes.
This similarity in
embryology may
indicate that
vertebrates have a
closer evolutionary
relationship to
echinoderms than to
other invertebrates.
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26-1 Introduction to the Animal
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Trends in Animal Evolution
During early development, the cells of most animal
embryos differentiate into three layers called germ
layers.
The cells of the endoderm, or innermost germ layer,
develop into the linings of the digestive tract and
much of the respiratory system.
Endoderm
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26-1 Introduction to the Animal
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Trends in Animal Evolution
The cells of the mesoderm, or middle layer, develop
into muscles and much of the circulatory, reproductive,
and excretory organ systems.
Mesoderm
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26-1 Introduction to the Animal
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Trends in Animal Evolution
The ectoderm, or outermost layer, develops into the
sense organs, nerves, and the outer layer of the skin.
Ectoderm
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26-1 Introduction to the Animal
Kingdom
Trends in Animal Evolution
Body Symmetry
Except for sponges, every animal exhibits some
body symmetry in its structure, or anatomy.
Many simple animals, like the sea anemone, have
body parts that repeat around the center of the
body.
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26-1 Introduction to the Animal
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Trends in Animal Evolution
Radial symmetry
These animals exhibit
radial symmetry, in
which any number of
imaginary planes can be
drawn through the
center, each dividing the
body into equal halves.
Planes of
symmetry
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Trends in Animal Evolution
In animals with
bilateral symmetry,
only one imaginary
plane can divide the
body into two equal
halves—left and
right.
Plane of
symmetry
Bilateral symmetry
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26-1 Introduction to the Animal
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Posterior end
The anterior is the front
end.
The posterior is the
back end.
Trends in Animal Evolution
Anterior end
Bilateral symmetry
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26-1 Introduction to the Animal
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The dorsal is the upper
side.
Trends in Animal Evolution
Dorsal side
The ventral is the lower
side.
Ventral
side
Bilateral symmetry
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26-1 Introduction to the Animal
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Trends in Animal Evolution
Bilateral symmetry allows for segmentation, in which
the body is constructed of many repeated and similar
parts, or segments. If those segments are repeated
along the longitudinal axis, it is called metamerism. A
metamere is a segment.
The combination of bilateral symmetry and
segmentation is found in two successful animal
groups—arthropods and vertebrates.
Geneticists are learning how gene interactions during
development control the growth/form of segments.
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26-1 Introduction to the Animal
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Trends in Animal Evolution
Cephalization
Animals with bilateral symmetry usually exhibit
cephalization, which is the concentration of sense
organs and nerve cells at the front end of the body.
Animals with bilateral symmetry usually move with
the anterior end forward, so this end comes in
contact with new parts of the environment first.
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26-1 Introduction to the Animal
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Trends in Animal Evolution
As sense organs have evolved, they have tended to
gather at the anterior end, as have nerve cells that
process information and “decide” what the animal
should do.
In general, the more complex animals become, the
more pronounced their cephalization. The anterior
end is often different enough from the rest of the
body that it is called a head.
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26-1 Introduction to the Animal
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Trends in Animal Evolution
Body Cavity Formation
Most animals have a body cavity, a fluid-filled
space between the digestive tract and body wall.
A body cavity provides a space in which internal
organs can be suspended so that they are not
pressed on by muscles or twisted out of shape by
body movements.
Body cavities also allow for specialized regions to
develop, and they provide room for internal organs
to grow and expand.
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26-1
Homeostasis is often maintained by feedback
loops that involve
a. gastrulation.
b. feedback inhibition.
c. spontaneous generation.
d. equilibrium.
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26-1
Animals respond to events in their environments
using specialized cells called
a. muscle cells.
b. nerve cells.
c. gametes.
d. blood cells.
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26-1
A characteristic that all animals share is being
a. heterotrophic.
b. autotrophic.
c. prokaryotic.
d. anaerobic.
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Excretion is a function of all animals that
involves
a. exchange of oxygen and carbon dioxide.
b. transport of material from one part of the
body to another.
c. digestion and absorption of food molecules.
d. removal of metabolic wastes.
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26-1
One major trend in animal evolution has been
a. the simplification of body organ systems.
b. an increase in the degree of cephalization.
c. a shift from bilateral symmetry to radial
symmetry.
d. disappearance of the blastula stage in early
development.
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