Download Identify the features that animals have in common. • Distinguish

HOLT BIOLOGY
UNIT 7-EXPLORING INVERTEBRATES
CHAPTER 27
INTRODUCTION TO ANIMALS
I. 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.
A. General Features of Animals
1. Heterotrophy
a. Animals are heterotrophs—that is, they cannot make their own food.
b. Most animals move from place to place searching for food.
c. Once food is located, it is eaten and then digested in a cavity inside the animal’s body.
2. Mobility
a. Animals are unique among living things in being able to perform rapid, complex movements.
b. Animals move by means of muscle cells, specialized cells that are able to contract with
considerable force.
c. Animals can swim, crawl, walk, run, and even fly.
d. In fact, flight has evolved four times among animals, in insects, pterosaurs (extinct reptiles from
the time of the dinosaurs), birds, and bats.
3. Multicellularity
a. All animals are multicellular.
b. In spite of differences in body size, there is little difference in the size of most of the cells that
make up these animals.
c. 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.
4. Diploidy
a. With few exceptions, animals are diploid, meaning adults have two copies of each chromosome,
one inherited from their father and one from their mother.
b. Only their gametes (egg and sperm) are haploid.
c. 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.
5. Sexual Reproduction
a. Almost all animals reproduce sexually by producing gametes, as do many plants, fungi, and
protists.
b. The females’ egg cells are much larger than the males’ sperm cells.
c. Unlike the egg cells, the sperm cells of animals have flagella and are highly mobile.
6. Absence of a Cell Wall
a. Among the cells of multicellular organisms, only animal cells lack rigid cell walls.
b. The absence of a cell wall has allowed animals mobility that other multicellular organisms do not
have.
c. You may not realize this, but there are cells moving about in your body all the time.
d. Cells called macrophages, for example, act as mobile garbage collectors, crawling over tissues
and removing debris.
7. Blastula Formation
a. In all animals except sponges, the zygote (fertilized egg cell) undergoes cell divisions that form a
hollow ball of cells called a blastula.
b. Cells within the blastula eventually develop into three distinct layers of cells:
i. Ectoderm
ii. Endoderm
iii. mesoderm.
c. These layers are called the primary tissue layers because they give rise to all of the tissues and
organs of the adult body.
8. Tissues
a. The cells of all animals except sponges are organized into structural and functional units called
tissues.
b. Tissues are groups of cells with a common structure that work together to perform a specific
function.
B. Body Symmetry
1. All animals have their own particular body plan, a term used to describe an animal’s shape,
symmetry, and internal organization.
a. An animal’s body plan results from a pattern of development programmed into the animal’s
genes by natural selection.
b. Sponges have the simplest body plan of all animals.
c. Sponges are asymmetrical, or irregular in shape, and sometimes their shape depends on where
they are growing.
2. Radial Symmetry
a. Animals with radial symmetry have body parts arranged around a central axis, somewhat like the
spokes around a bicycle wheel.
b. A plane passing through the central axis divides the organism into roughly equal halves.
c. Today’s radially symmetrical animals are aquatic.
d. Most move slowly or drift in ocean currents.
3. Bilateral Symmetry
a. The bodies of all other animals show bilateral symmetry, a body design in which there are
distinct right and left halves.
b. A plane passing through the animal’s midline divides the animal into mirror image halves.
c. Most bilaterally symmetrical animals have evolved an anterior concentration of sensory
structures and nerves, a process called cephalization.
C. Internal Body Cavity
1. Bilaterally symmetrical animals have one of three basic kinds of internal body plans.
a. The body plan may include a body cavity, or coelom, a fluid-filled space found between the body
wall and the digestive tract (gut).
b. This space is lined with cells that come from mesoderm.
2. Animals with no body cavity are called acoelomates.
3. Animals called pseudocoelomates have a body cavity located between the mesoderm and endoderm.
4. Coelomates have a true coelom, a body cavity located entirely within the mesoderm.
5. A true coelom provides an internal space where mesoderm and endoderm can be in contact with
each other during embryonic development.
D. Body Segmentation
1. Segmented animals are composed of a series of repeating, similar units called segments.
a. Segmentation underlies the organization of all “advanced” animals and is easy to observe in
some animals, such as earthworms.
b. In vertebrates, segments are not visible externally, but there is evidence of segmentation in a
vertebrate embryo.
E. Kinds of Animals
1. Kingdom Animalia contains about 35 major divisions called phyla (singular, phylum), depending on
how certain organisms are classified.
a. To visually represent the relationships among various groups of animals, scientists often use a
type of branching diagram called a phylogenetic tree.
b. A phylogenetic tree shows how animals are related through evolution.
2. The animal kingdom is often divided into two groups:
a. invertebrates (animals without a backbone)
b. vertebrates (animals with a backbone).
II. 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.
A. Tissues and Organs
1. Digestion
a. Single-celled organisms and sponges digest their food within their body cells.
b. All other animals digest their food extracellularly (outside of their body cells) within a digestive
cavity.
c. Simple animals, such as the hydra and flatworms, have a gastrovascular cavity, a digestive cavity
with only one opening.
d. Other animals have a digestive tract (gut) with two openings, a mouth and an anus.
e. A hydra has a gastrovascular cavity, while a roundworm has a digestive tract in which food
travels in one direction only.
2. Respiration
a. In simple animals, oxygen gas and carbon dioxide gas are exchanged directly with the
environment by diffusion.
b. The uptake of oxygen and the release of carbon dioxide, called respiration, can take place only
across a moist surface.
c. Some aquatic (and a few terrestrial) animals respire with gills, very thin projections of tissue that
are rich in blood vessels.
3. Circulation
a. In complex animals, oxygen and nutrients must be transported to these body cells by a
circulatory system.
b. In an open circulatory system, a heart pumps fluid containing oxygen and nutrients through a
series of vessels out into the body cavity.
c. In a closed circulatory system, a heart pumps blood through a system of blood vessels.
4. Conduction of Nerve Impulses
a. Nerve cells (neurons) are specialized for carrying messages in the form of electrical impulses
(conduction).
b. Bilaterally symmetric animals have clusters of neurons called ganglia.
c. More-complex invertebrates, such as the grasshopper, have brains with sensory structures, such
as eyes, associated with them.
d. The hydra has a simple nerve net, while the flatworm and the grasshopper have more-complex
nervous systems.
5. Support
a. Many soft-bodied invertebrates have a hydrostatic skeleton.
b. A hydrostatic skeleton consists of water that is contained under pressure in a closed cavity, such
as a gastrovascular cavity or a coelom.
c. 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.
d. An endoskeleton is composed of a hard material, such as bone, embedded within an animal.
6. Excretion
a. The term excretion refers to the removal of wastes produced by cellular metabolism.
b. Simple aquatic invertebrates and some fishes excrete ammonia into the water through their skin
or gills by diffusion.
c. Other animals, especially terrestrial animals, convert ammonia to nontoxic chemicals, like urea.
d. As the excretory system eliminates these wastes, water and other useful substances are returned
to the body.
B. Reproductive Strategies
1. Asexual Reproduction
a. Reproduction that does not involve the fusion of two gametes is called asexual reproduction.
b. An unusual method of asexual reproduction is parthenogenesis, in which a new individual
develops from an unfertilized egg.
c. Animals that reproduce asexually are usually able to also reproduce sexually.
2. Sexual Reproduction
a. In sexual reproduction, a new individual is formed by the union of a male and a female gamete.
b. Gametes are produced in the sex organs.
c. The testes produce the male gametes (sperm), and the ovaries produce the female gametes
(eggs).
d. Some species of animals, called hermaphrodites, have both testes and ovaries.
e. Most aquatic animals simply release the male and female gametes near one another in the water,
where fertilization occurs.
f. This method is called external fertilization because the egg is fertilized outside of the female’s
body.
g. Most terrestrial animals reproduce sexually by means of internal fertilization.
h. In internal fertilization, the union of the sperm and egg occurs within the female’s body.