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Introduction to
Animals
• Chapter 27
I. Characteristics of Animals
A. General Features of Animals:
• 1. 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.
• In some animals, digestion
is carried out within
individual cells; in other
animals, digestion takes
place in an internal cavity.
2. Mobility• Can perform rapid, complex movements
• 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.
• Animals are
organisms with
ways of moving that
help them
reproduce, obtain
food, and protect
themselves.
3. Multicellularity:
• 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.
• The body size does not matter all of the cells are
similar in size.
• Advantage : individual cells can specialize in
one life task.
4. Diploidy
• Meaning adults have two copies of each
chromosome.
• One inherited from mother and one from
father.
• Only their gametes are haploid.
• Advantage- 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/Fertilization
• Most animals reproduce sexually by
producing gametes.
• Male animals produce sperm cells (which are
smaller and have a flagella for moving) 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.
6. Absence of a Cell Wall
• Animals do not have a cell wall.
• This characteristic has allowed animals
mobility that other multi-cellular organisms do
not have.
• Cells move about in animals bodies all the
time.
• Cells called macrophages , act as mobile
garbage collectors, crawling over tissues and
removing debris.
7. Blastula Formation/Cell division
• The zygote
divides by
mitosis and cell
division to form
two cells in a
process called
cleavage.
• Once cell division has begun, the
organism is known as an
embryo.
cleavage
Cell division
• The two cells that result from cleavage then
divide to form four cells and so on, until a cellcovered, 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,
nervous tissue,
sense organs such
as eyes of the
animal.
Ectoderm
Gastrulation
• The endoderm cells
develop into the
lining of the
animal’s digestive
tract and into
organs associated
with digestion.
Endoderm
Formation of mesoderm
• Mesoderm is found in the middle of the embryo;
the term meso means “middle.”
• The mesoderm is
the third cell layer
found in the
developing
embryo between
the ectoderm and
the endoderm.
Mesoderm
Formation of mesoderm
• The mesoderm cells develop into the muscles,
circulatory system, excretory system, and, in
some animals, the respiratory system.
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
8. Tissue
• Cells are organized into structural and
functional units called tissues. (except in
sponges)
• Tissue- are groups of cells with a common
structure that work together to perform a
specific function.
• Ex: Adipose tissue- store fat
Muscle tissue- contract- producing
movement.
Nerve tissue- conduct signals.
B. Body Symmetry
• Symmetry is a term that describes the
arrangement of body structures.
• Different kinds of symmetry enable animals to
move about in different ways.
1 . 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
have the simplest
body form and 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.
2. Radial symmetry
• Animals with radial
symmetry can be
divided along any
plane, through a
central axis, into
roughly equal
halves.
• Radial symmetry is an
adaptation that enables an
animal to detect and capture
prey coming toward it from any
direction.
Radial symmetry
• A hydra
develops from just
two embryonic cell
layers—ectoderm
and endoderm.
• Most are aquatic,
move slowly or
drift in ocean
currents.
Inner cell layer
Outer cell layer
3. 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.
• The dorsal, or upper surface, also looks
different from the ventral, or lower surface.
Bilateral Symmetry and Body Plans
• Animals with bilateral symmetry have become
specilized in different ways, for example , they
have an anterior concentration of sensory
structures and nerves, a process called
cephalization.
• With sensory organs concentrated in the front,
such animals can more easily sense food and
danger.
• All bilaterally symmetrical animals developed from
three embryonic cell layers—ectoderm, endoderm,
and mesoderm.
C. Internal Body Cavity:
• Some bilaterally symmetrical animals also have
fluid-filled spaces inside their bodies called body
cavities (coelom) in which internal organs are
found.
• 1. acoelomate- Animals that develop from
three cell layers—ectoderm, endoderm, and
mesoderm—but have no body cavities .
• 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
2. 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
3. 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.
D. Body Segmentation
• Segmented animals are composed of a series of
repeating, similar units called segments.
• You can observe segmentation in some animals
such as earthworms, crustaceans, spiders and
insects.
• In vertebrates, segments are not visible
externally, but there is evidence in a vertebrae
embryo.
• Vertebrae muscles develop from repeated
blocks of tissue called somites.
• The backbone consists of a stack of very similar
vertebrae.
Body Segmentation
• In segmented animals each segment can move
independently. However, they are not totally
independent of each other because materials pass
from one segment to another through a circulatory
and nervous system that connects them.
• Therefore, they have great flexibility and mobility.
• Each segment repeats many of the organs in the
adjacent segment, as a result an injured animal
can still perform vital life functions.
• Segmentation also offers evolutionary flexibility.
E. Kinds of Animals
• Kingdom Animalia- contain about 35 major
divisions called phyla. (singular Phylum)
• Scientists use a Phylogenetic Tree – to show
how animals are related through evolution.
• They compare:
• 1. anatomy and physiology
• 2. patterns of development in embryos
• 3. DNA
• The animal Kingdom is often divided into two
groups Invertebrates and Vertebrates.
II. Animal Body Systems:
• A. Tissues and Organs – have evolved to
carry out and specialize to perform specific
functions.
6 - Important functions of these tissues and
organs are: Digestion
Respiration
Circulation
Conduction of nerve impulses
Support
Excretion
1. Digestion
a.) Single celled organisms and Sponges digest their food
within their body cells. intracellularly
b.) Other simple animals digest their food
extracellularly (outside their body cells) within a
digestive cavity.
• Gastrovascular cavity – a digestive cavity with
only one opening. ( hydra and flatworm)
• There can be no specialization within a gastrovascular cavity because
every cell in exposed to all stages of food digestion.
c.) Other animals have a digestive tract (gut) with
two openings, a mouth and an anus.
• This one way digestive tract allows for specialization.
(food storage, breaking down, chemical , absorption.)
2. Respiration- the uptake of oxygen and the release
of carbon dioxide.
a.) Can take place only across a moist surface, such as
damp skin of an earthworm. (diffusion)
b.) Larger, more complex animals have specialized
structures.
• gills-very thin projections of tissue that are rich in blood
vessels. (provide a large surface area for gas exchange)
• Lungs –allow terrestrial animals to respire on dry land.
3. Circulation-transports oxygen and nutrients to
body cells.
a.) Open Circulatory System- a heart pumps fluid
containing oxygen and nutrients through a series
of vessels out into the body cavity.
• Fluid washes across the bodies tissues. The
fluid collects in open spaces and flows back to the
heart. Ex: crayfish
b.) Closed Circulatory System-a heart pumps
blood through a system of blood vessels.
• The blood remains in the vessels and does not come
in direct contact with the body’s tissues.
• Materials pass into and out of the blood by diffusing
through the walls of the blood vessels.
4. Conduction of Nerve Impulses
a.) Nerve Cells (neurons) are specialized for
carrying messages in the form of electrical
impulses. (Conduction)
• These cells coordinate the activities in an
animals body.
• 1. Nerve net- all nerve cells are similar and linked
together in a web. (ex: hydra,jellyfish)
• 2. ganglia- clusters of neurons that form a brain like
structure.( ex: flatworm)
• 3. brains- more complex invertebrates have brains with
sensory structures, such as eyes, associated with them.
• These cephalized animals interact with their environment
in more complex ways. (ex: grasshopper – human)
5. Support
• An animals skeleton provides a framework
that supports its body and is vital to
movement.
a.Hydrostatic skeleton- consists of water
that is contained under pressure in a closed
cavity. It is formed by the gastrovascular
cavity. (ex: hydra, earthworm.)
b.) Exoskeleton- a rigid external skeleton
that encases the body of an animal.
• The muscles are attached to the inside of the skeleton,
which provides a surface for them to pull against.
• Exoskeletons also protect soft body tissues,
prevent water loss, and provide protection from
predators.
• 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.
• C. Endoskeleton- is composed of a hard
material, such as bone, embedded within an
animal.
• 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.
• The endoskeleton protects internal organs and
provides an internal brace for muscles to pull
against.
• An endoskeleton may be made of calcium
carbonate, as in sea stars; cartilage, as in
sharks; or bone.
Calcium carbonate
cartilage
• Bony fishes, amphibians, reptiles, birds, and
mammals all have endoskeletons made of bone.
bone
• A vertebrate is an animal with an endoskeleton
and a backbone. All vertebrates are bilaterally
symmetrical.
• 6. Excretion- the removal of wastes produced
by cellular metabolism. (ammonia)
• a.)Diffusion-through skin or gills (fish and
some aquatic invertebrates)
• This is effective but they lose a lot of water.
• Terrestrial animals need to minimize water lossthey do so by:
• b.) Converting ammonia to nontoxic
chemicals, like urea.
– Water and other useful substances are returned to
the body in this process.
– Kidney’s filter fluid from the blood and excrete
them as concentrated urine.
• B Reproductive Strategies
• 1. Asexual Reproduction- does not involve the
fusion of two gametes.
• a.) fragments-ex: starfish and sponges
• b.) fission- sea anemone
• c.) parthenogenesis- in which a new individual develops
from an unfertilized egg. (ex: honeybees, a few species
of fish, amphibians and lizards.)
• 2. Sexual Reproduction- a new individual is
formed by the union of a male and a female gamete.
• Testes- produce the male gametes (sperm)
• ovaries- produce the female gametes (egg)
• Hermaphrodites- have both (ex; earthworm, some
fish, slugs.
• a.) External fertilization- the egg is fertilized
outside the female body.
– Large numbers of gametes are released due to %
that actually get fertilized.
– Ex: fish
• b.) Internal fertilization- the union of the sperm and
egg occurs within the female’s body.
– the male places semen directly into the the female’s
body.
– In this way fertilization takes place in a moist
environment, and the gametes are protected from
drying out.
– All developing eggs must be kept moist.