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: What Is a Plant?
Chapter 22: The Diversity of Plants
Chapter 23: Plant Structure and Function
Chapter 24: 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

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

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.
25.1 Section Objectives – page 673

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
Extended pharynx
Digestive tract
Anus
Digestive tract

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.
• Fertilization occurs when a sperm cell penetrates the egg cell, forming a new cell called a zygote.
• In animals, fertilization may be internal or external.

Cell division
• The zygote divides by mitosis and cell division to form two cells in a process called cleavage.
cleavage

Cell division
• Once cell division has begun, the organism is known as an embryo.

Cell division
• The two cells that result from cleavage then divide to form four cells and so on, until a cell-covered, fluid-filled ball called a blastula is formed.
• The blastula is formed early in the development of an animal embryo.

Gastrulation
• After blastula formation, cell division continues.
• The cells on one side of the blastula then move inward to form a gastrula —a structure made up of two layers of cells with an opening at one end.

Gastrulation
• The cells at one end of the blastula move inward, forming a cavity lined with a second layer of cells.
• The layer of cells on the outer surface of the gastrula is called the ectoderm .
• The layer of cells lining the inner surface is called the endoderm .

Gastrulation
• The ectoderm cells of the gastrula continue to grow and divide, and eventually they develop into the skin and nervous tissue of the animal.
Ectoderm

Gastrulation
Endoderm
• The endoderm cells develop into the lining of the animal’s digestive tract and into organs associated with digestion.

Formation of mesoderm
• Mesoderm is found in the middle of the embryo; the term meso means “middle.”
Mesoderm • The mesoderm is the third cell layer found in the developing embryo between the ectoderm and the endoderm.

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
Formation of mesoderm
Endoderm
Mesoderm
Ectoderm
Fertilization
Egg cell
First cell division
Gastrulation
Cell
Differentiation in Animal
Development
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? (TX Obj 2; 8C, 10A, 10B)
A. eukaryotic
B. multicellular
C. heterotrophic
D. prokaryotic
The answer is D.

Question 2
Sessile animals _______.
(TX Obj 2; 8C, 10A, 10B)
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 _______.
(TX Obj 2; 8C, 10A, 10B)
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? (TX Obj 2; 8C, 10A, 10B)
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? (TX Obj 2; 8C, 10A, 10B)
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
_______. (TX Obj 2; 8C, 10A, 10B)
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.
25.2 Section Objectives – page 680

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
• A roundworm is an animal with bilateral symmetry.
Endoderm
Body cavity
Digestive tract
• 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.
Intestine
Mouth
Anus
Round body shape

Coelomates
• The body cavity of an earthworm develops from a coelom , a fluidfilled space that is completely surrounded by mesoderm.
Coelomate Segmented Worm
Ectoderm
Mesoderm
Endoderm
Body cavity
Digestive tract
• The greatest diversity of animals is found among the coelomates.
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
_______.
(TX Obj 2; 8C,
10A, 10B)
A. radial symmetry
B. asymmetry
C. bilateral – anterior
D. bilateral – posterior

The answer is A , radial symmetry.

Question 2
Which of the following animals does NOT exhibit radial symmetry? (TX Obj 2; 8C, 10A,
10B)
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.
(TX Obj 2; 8C, 10A, 10B)

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? (TX Obj 2; 8C, 10A, 10B)
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? (TX Obj 2; 8C, 10A, 10B)
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.

What is the difference between a blastula and a gastrula? (TX Obj 2; 8C, 10A, 10B)

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

The layer of cells on the outer surface of the gastrula is called the _______.
(TX Obj 2; 8C, 10A, 10B)
A. endoderm
B. ectoderm
C. mesoderm
D. blastula

The answer is B , ectoderm.
Endoderm
Ectoderm
Mesoderm

Which of these organs develops from the endoderm? (TX Obj 2; 8C, 10A, 10B)
A. digestive system
B. skin
C. muscles
D. circulatory system
The answer is A.

Which of the following is NOT a deuterostome?
(TX Obj 2; 8C, 10A, 10B)
A. shark
B. dolphin
C. frog
D. honey bee
The answer is D.

List the following stages in the order of their occurrence. (TX Obj 2; 8C, 10A, 10B)
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

Why can an octopus squeeze through spaces much smaller than the width of its body?
(TX Obj 2; 8C, 10A, 10B)

An octopus has no endoskeleton or exoskeleton to maintain a rigid shape for the animal.
Therefore, it can modify its shape as necessity demands.

Which of the following animals does NOT have an exoskeleton? (TX Obj 2; 8C, 10A, 10B)
A. horseshoe crab
B. ant
C. turtle
D. tarantula

The answer is C. A turtle is a vertebrate animal with an internal skeleton.

Why are sessile animals more likely to live in water than on land? (TX Obj 2; 8C, 10A, 10B)
There is little suspended food in the air for these animals to eat.

Why are sessile animals more likely to live in vigorously moving water than in still water? (TX Obj 2;
8C, 10A, 10B)

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.

Describe the way somatic cell nuclear transfer produces stem cells. (TX Obj 2; 4B, 8C)
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.

• Digital Stock
• NOAA
• PhotoDisc
• USDA- ARS
• Alton Biggs
Photo Credits

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