Chapter 30: Animals: Part I
30-1
Evolution and Classification of
Animals
Animals are multicellular heterotrophs that
ingest their food.
Animals belong to the Eukarya and
kingdom Animalia.
In general, animals have some form of
locomotion and have tissues and organs.
The adult is typically diploid and practices
sexual reproduction.
An embryonic stage undergoes
development.
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Evolution of Animals
It is difficult to trace the complete
evolutionary tree of animals because
soft-bodied animals are poorly
preserved as fossils.
All animals probably evolved from a
protistan ancestor.
All the major animal phyla include some
invertebrates, animals without
backbones.
The phylum Chordata is mainly composed
of vertebrates.
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Animal diversity
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Criteria for Classification
The classification of animals is based on
the level of organization or number of
germ layers, symmetry, type of coelom,
body plan, and presence or absence of
segmentation.
An evolutionary tree based on these
features depicts a possible
evolutionary relationship between the
animals.
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Evolutionary tree
30-6
Level of Organization
Animals can have the cellular level, the
tissue level, or the organ level of
organization.
One of the main events during animal
development is the establishment of
germ layers.
If two germ layers (ectoderm and
endoderm) are present, then the animal
has the tissue level of organization; if
all three germ layers are present, the
organ level of organization is attained.
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Type of Body Plan
Two body plans are present in the animal
kingdom: the sac plan and tube-withina-tube plan.
Animals with a sac plan have an
incomplete digestive system with only
one opening.
Animals with the tube-within-a-tube plan
have a complete digestive system.
Two openings allows for specialization
along the length of the tube.
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Type of Symmetry
Animals can be asymmetrical, radially
symmetrical, or bilaterally symmetrical.
Asymmetrical animals have no particular
symmetry.
Radial symmetry means the animal is
organized similar to a wheel.
Bilateral symmetry means the animal has
definite right and left halves.
Bilateral symmetry leads to cephalization.
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Type of Coelom
A true coelom (in coelomates) is an
internal body cavity completely lined
with mesoderm, where internal organs
are found.
Coelomates are either protostomes or
deuterostomes.
Acoelomates have mesoderm but no body
cavity.
Animals that have a pseudocoelom have a
body cavity incompletely lined with
mesoderm.
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Segmentation
Segmentation is the repetition of body
parts along the length of the body.
Animals can be segmented or
nonsegmented.
Segmentation leads to specialization of
parts because the various segments
can become differentiated for specific
purposes.
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Introducing the Vertebrates
Sponges are asymmetrical.
Cnidarians have radial symmetry.
All other phyla contain bilaterally
symmetrical animals.
Flatworms have three germ layers but no
coelom.
Roundworms have a pseudocoelom and
a tube-within-a-tube body plan.
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Sponges
Sponges are mainly marine animals at
the cellular level of organization.
The sponge body wall has an outer layer
of epidermal cells; a middle layer
consisting of semi-fluid matrix where
amoeboid cells transport nutrients,
produce spicules, and form sex cells;
and an inner layer of collar cells with
flagella that wave water through pores
and out an osculum.
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Collar cells also engulf food particles and
pass them to amoeboid cells.
Sponges are classified according to type of
spicules.
Chalk sponges have spicules made of
calcium carbonate; glass sponges have
spicules that contain silica.
Most sponges also contain spongin fibers
made of collagen.
Sponges are sessile filter feeders.
They reproduce asexually by budding or by
fragmentation.
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Sponge
30-16
Cnidarians
Cnidarians are mostly coastal marine
animals with a tissue level of
organization and radial symmetry.
They may be a polyp or a medusa or may
alternate between the two forms.
They have cnidocytes that discharge
stinging nematocysts, long threads that
may have spines and contain a poison.
Cnidarians are diverse and include sea
anemones, coral, and jellyfishes.
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Cnidarian diversity
30-18
Hydra
A hydra polyp has an outer layer of
epidermis derived from ectoderm and an
inner layer called gastrodermis derived
from endoderm.
Mesoglea lies between the two layers and
contains a nerve net that communicates
with muscle fibers so that the animal is
able to move.
Digestion begins in a gastrovascular cavity
and finishes in gastrodermal cells.
Nutrients and gases are distributed from
layer to layer by diffusion.
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Anatomy of Hydra
30-20
Flatworms
Flatworms are characterized by the
tissue level of organization and a sac
body plan.
These acoelomates have three germ
layers, and have all organs except
respiratory and circulatory organs.
The flat body facilitates diffusion of
oxygen and other molecules from cell
to cell.
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Planarians
Planarians are freshwater, free living,
flatworms.
Flame cells function in excretion.
The small brain extends to a ladder
arrangement of nerves.
Light-sensitive organs (eyespots) are in
the head; planarians exhibit
cephalization.
One organism has both male and female
sex organs – they are hermaphroditic.
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Planarian
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Parasitic Flatworms
Flukes and tapeworms are two classes of
parasitic flatworms; both have
intermediate hosts.
Flukes are oval to elongate and have
suckers at the anterior end.
Blood flukes cause schistosomiasis; other
flukes infect the digestive tract, bile duct,
and lungs.
A tapeworm has an anterior scolex with
hooks and suckers to hold itself inside
the gut.
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Schistosomiasis
30-25
Roundworms
Roundworms have the tube-within-atube plan; they are prevalent in soil and
some parasitize animals and plants.
The pseudocoelom is a body cavity
incompletely lined with mesoderm.
The fluid-filled interior forms a
hydrostatic skeleton.
Most species of roundworms have
separate males and females.
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Coelom structure and function
30-27
Ascaris
Ascaris larvae are swallowed and burrow
through the intestinal wall and make
their way through various organs until
they reach the lungs.
In the lungs, they grow in size for 10
days, then move up to the throat, and
are then swallowed.
After they mature in the intestine,
females produce eggs that pass out
with feces.
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Roundworm anatomy
30-29
Other Roundworms
Trichinosis is a roundworm infection
from eating undercooked pork
containing encysted Trichinella larvae.
The filarial worm is carried by
mosquitoes and causes elephantiasis
by blocking lymphatic drainage.
Pinworms are common infections in
children.
Hookworm is a more serious infection
seen in the southern United States.
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Molluscs
Molluscs, along with annelids and
arthropods, are protostomes because
the first embryonic opening becomes
the mouth.
Because the true coelom form by the
splitting of the mesoderm, protostomes
are also schizocoelomates.
Many protostomes also have
trochophore (top-shaped) larvae.
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Protostomes versus
deuterostomes
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Characteristics of Molluscs
A mollusc body typically contains a
visceral mass, a mantle, and a foot.
Molluscan groups are distinguished by a
modification of the foot.
In gastropods, the foot is ventrally
flattened.
In cephalopods, the foot has evolved into
tentacles about the head.
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Squids are cephalopods that display
marked cephalization, move rapidly by
jet propulsion, and have a closed
circulatory system.
The camera-type eye of cephalopods
evolved separately from the eye of
vertebrates.
In cephalopods, the brain is formed from
a fusion of ganglia, and nerves leaving
the brain supply the body.
Rapid secretion from an ink gland helps
cephalopods escape enemies.
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Molluscan diversity
30-35
Bivalves
Bivalves, such as clams and relatives,
have a hatchet foot and are filter
feeders.
Water enters by an incurrent siphon.
Food trapped on the gills is swept toward
the mouth.
A coelom is present but reduced.
The circulatory system pumps blood
through sinuses.
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In bivalves, there is no head and three
pairs of ganglia control the bivalve.
The digestive system of a clam includes
a mouth with labial palps, an
esophagus, a stomach, and an
intestine, which coils about the visceral
mass and then is surrounded by the
heart as it extends to the anus.
The anus empties at an excurrent siphon.
Sexes are usually separate and the
gonad is located around the coils of the
intestine.
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Clam
30-38
Annelids
Annelids are segmented both externally,
and internally by partitions called septa.
Annelids have a hydrostatic skeleton, and
partitioning of the coelom permits each
body segment to move independently.
The tube-within-a-tube body plan allows
the digestive tract to have specialized
organs.
30-39
Annelids have an extensive closed
circulatory system with blood vessels
that run the length of the body and
branch to every segment.
The brain is connected to a ventral solid
nerve cord with ganglia in each
segment.
The excretory system has nephridia in
each segment.
A nephridium is a tubule that collects
wastes and excretes through an
opening in the body wall.
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Marine Worms
Polychaetes are marine worms with
paddlelike parapodia at the side of each
segment.
Some polychaetes are sessile tube worms.
A clam worm is a predaceous marine worm
with a defined head region.
During breeding seasons, some worms
form sex organs in special segments and
shed these segment during breeding.
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Polychaete diversity
30-42
Earthworms
Earthworms are oligochaetes having few
setae per segment.
Most scavenge for food in the soil and the
moist body wall functions in gas
exchange.
When muscles contract in each segment,
setae anchor in the soil, and aid
locomotion.
Five “hearts” pump blood and a branch
blood vessel reaches each segment.
These worms are hermaphroditic.
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Segmentation in earthworms is
evidenced by:
Body rings
Coelom divided by septa
Setae on most segments
Ganglia and lateral nerves in each
segment
Nephridia in most segments
Branch blood vessels in each segment
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Earthworm, Lumbricus
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30-46
Leeches
Most leeches are fluid feeders that attach
themselves to open wounds using
suckers.
Bloodsuckers, such as the medicinal
leech, can cut through tissue.
An anticoagulant (hirudin) in their saliva
keeps blood from clotting.
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Arthropods
Arthropods are the most varied and
numerous of animals.
The success of arthropods is largely
attributable to a flexible exoskeleton,
jointed appendages, and specialization
of body regions.
Three body regions – head, thorax, and
abdomen – with specialized appendages
in each region, and a well-developed
nervous system characterize this group.
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Arthropod diversity
30-49
Crustaceans
Crustaceans are largely marine and have
a head that bears compound eyes, two
pair of antennae, and specialized mouth
parts.
Five pairs of walking legs include a first
pair of pinching claws.
In the crayfish, head and thorax are fused
into a cephalothorax which is covered
on the top and sides by carapace.
The abdominal segments have
swimmerets.
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The crayfish has an open circulatory
system in which the heart pumps blood
into a hemocoel consisting of sinuses
where the hemolymph flows about the
organs.
Respiration takes place by gills under the
hard carapace, and there is a ventral
solid nerve cord.
Sexes are separate in the crayfish.
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Male crayfish, Cambarus
30-52
Insects
The head of an insect usually bears a
pair of antennae, compound eyes, and
simple eyes.
The thorax bears three pairs of legs and
up to two pairs of wings, and the
abdomen contains most of the internal
organs.
The insect exoskeleton is lighter and
contains less chitin than that of many
other arthropods.
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Insect diversity
30-54
Grasshoppers are examples of insects
adapted to a terrestrial life; they respire
by tracheae and have wings that allow
them to evade enemies; the third pair of
legs is suitable for jumping.
There is a tympanum for the reception of
sound waves and a male penis for
passing sperm to the female without
desiccation.
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Malpighian tubules function in excretion
in grasshopper.
Grasshoppers undergo gradual
metamorphosis from nymph to adult.
Butterflies undergo complete
metamorphosis, changing from larva to
pupa to adult.
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Female grasshopper
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Arachnids
The arachnids include terrestrial spiders,
scorpions, ticks, and mites.
The cephalothorax bears six pairs of
appendages: the chelicerae and the
pedipalps, and four pairs of walking
legs.
Scorpions are the oldest terrestrial
arthropods.
Ticks and mites are parasitic.
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Spiders are well-adapted to life on land
and have Malphigian tubules – they
secrete uric acid, helping to conserve
water.
Spiders spin silk used in various ways.
Where spiders spin webs, the type of
web is a feature that demonstrates the
evolutionary relationship among
spiders.
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Arachnid diversity
30-60
Chapter Summary
Animals are multicellular heterotrophs
exhibiting at least some mobility.
Animals are grouped according to level
of organization, symmetry, body plan,
pattern of embryonic development, and
presence or absence of segmentation.
Sponges are multicellular with limited
mobility and no symmetry.
30-61
Cnidarians are radially symmetrical with
true tissue layers.
Planarians are bilaterally symmetrical
with a definite head region.
Roundworms have a pseudocoelom and
a tube-within-a-tube body plan.
Molluscs have a muscular foot (variously
modified) and a visceral mass
enveloped by a mantle.
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Annelids are segmented with a well
developed true coelom.
Arthropods have jointed appendages and
a water-repellent exoskeleton that must
be periodically shed.
Each of the five major groups of
arthropods contains species that are
adapted to terrestrial life.
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