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Animal Origins and the
Evolution of Body Plans
32
Animal Origins and the Evolution of Body Plans
• Animals: Descendants of a Common Ancestor
• Body Plans: Basic Structural Designs
• Sponges: Loosely Organized Animals
• Cnidarians: Two Cell Layers and Blind Guts
• Ctenophores: Complete Guts and Tentacles
• The Evolution of Bilaterally Symmetrical Animals
• Simple Lophotrochozoans
• Lophophorates: An Ancient Body Plan
• Spiralians: Spiral Cleavage and Wormlike Body Plans
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Animals: Descendants of a Common Ancestor
• Evidence indicates that all animals are
descendants of a single ancestral lineage.
• All animals share a set of derived traits:
 Similarities in their small-subunit ribosomal
RNAs
 Similarities in their Hox genes
 Special types of cell–cell junctions: tight
junctions, desmosomes, and gap junctions
 A common set of extracellular matrix molecules,
including collagen
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Animals: Descendants of a Common Ancestor
• Animals evolved from ancestral colonial flagellated
protists.
• Within these ancestral colonies, a division of labor
arose.
• Cells became specialized for different functions, such
as movement, nutrition, and reproduction.
• The specialized units continued to differentiate while
improving their coordination with other working
groups of cells.
• These coordinated groups of cells evolved into
animals.
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Animals: Descendants of a Common Ancestor
• Generalized traits characterize animals:
 They are multicellular organisms that must
take in pre-formed organic molecules.
 They acquire these organic molecules by
ingesting other organisms, living or dead, and
digesting them within their bodies.
 Animals must expend energy to acquire these
organic molecules.
 Most have circulatory systems that carry O2,
CO2, and nutrients.
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Animals: Descendants of a Common Ancestor
• Much of the diversity in the animal kingdom
evolved as animals acquired the ability to capture
and eat many different types of food and to avoid
becoming food for other animals.
• The need to move in search of food has favored
sensory structures that provide animals with
detailed information about their environment.
• Animals expend a considerable amount of energy
to maintain relatively constant internal conditions
while taking in foods that vary chemically.
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Animals: Descendants of a Common Ancestor
• Clues to the evolutionary relationships among
animals are found in the fossil record, patterns of
embryonic development, comparative physiology
and morphology, and the structure of molecules
such as the small-subunit RNAs and
mitochondrial genes.
• The sponges, cnidarians, and ctenophores
separated from the other animal lineages early in
evolutionary history.
• The remaining animals have been divided into two
major lineages: the protostomes and the
deuterostomes.
Figure 32.1 A Current Phylogeny of the Animals
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Animals: Descendants of a Common Ancestor
• Animals form layers of cells during their
development from a single-celled zygote to a
multicellular adult.
• The embryos of diploblastic animals have two cell
layers: an outer ectoderm and an inner endoderm.
• The embryos of triploblastic animals have a third
layer, the mesoderm.
• The existence of three cell layers distinguishes the
protostomes and deuterostomes from simple
animals that diverged earlier.
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Animals: Descendants of a Common Ancestor
• Protostomes and deuterostomes differ in the fate
of the blastopore , the opening of the cavity that
forms in the spherical embryo.
• In the protostomes, the mouth arises from the
blastopore.
• In the deuterostomes, the blastopore gives rise
to the anus.
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Animals: Descendants of a Common Ancestor
• Most protostomes and the deuterostomes exhibit
a pattern of early cell division in the fertilized egg
called radial cleavage.
• In radial cleavage, cells divide along a plane
either parallel to or at right angles to the long axis
of the fertilized egg.
• One major protostome lineage evolved a pattern
of early cell division called spiral cleavage.
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Body Plans: Basic Structural Designs
• The entire structure of an animal, its organ
systems, and the integrated functioning of its
parts are known as its body plan.
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Body Plans: Basic Structural Designs
• Overall shape is referred to as symmetry. A
symmetrical animal can be divided into similar
halves along at least one plane.
• Animals that have no plane of symmetry are said to
be asymmetrical.
• In spherical symmetry body parts radiate out from
a central point. Spherical symmetry is widespread
among the protists.
• An organism with radial symmetry has one main
axis around which its body parts are arranged.
• Bilaterally symmetric animals can be divided into
mirror images by a single plane.
Figure 32.2 Body Symmetry
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Body Plans: Basic Structural Designs
• Bilateral symmetry is a common characteristic of
animals that move freely through their
environments.
• Bilateral symmetry is often associated with
cephalization: the presence of a head bearing
sensory organs and central nervous tissues at the
anterior end of the animal.
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Body Plans: Basic Structural Designs
• Body cavities are fluid-filled spaces that lie
between the cell layers of the bodies of many kinds
of animals.
• The type of body cavity an animal has influences
how it can move.
• Animals can be grouped into three major categories
based on the type of body cavity they have: the
acoelomates, the pseudocoelomates, and the
coelomates.
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Body Plans: Basic Structural Designs
• Acoelomates lack an enclosed body cavity. The
space between the gut and body wall is filled with
cells called mesenchyme.
• Pseudocoelomates have a pseudocoel, a liquid
filled space in which organs are suspended.
• Coelomates have a coelom that develops within
the mesoderm. It is lined with the peritoneum
and enclosed on the inside and outside by
muscles.
Figure 32.3 Animal Body Cavities (Part 1)
Figure 32.3 Animal Body Cavities (Part 2)
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Body Plans: Basic Structural Designs
• The fluid-filled body cavities of simple animals
function as hydrostatic skeletons.
• When the muscles surrounding fluids contract, the
fluids can be moved to other parts of the body,
causing these body regions to expand.
• Other forms of skeletons developed in many
lineages, including internal skeletons (vertebrate
bones), and external skeletons (crab shells, clam
shells).
• The form of an animal’s skeleton and body
cavities strongly influences the degree to which it
can control and change its shape and thus the
complexity of the movements it can perform.
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Sponges: Loosely Organized Animals
• The lineage leading to modern sponges (phylum
Porifera) separated from the lineage leading to
other animals very early during animal evolution.
• Sponges are sessile—they live attached to the
substratum.
• The body plan of sponges is an aggregation of
cells built around a water canal system.
Figure 32.4 The Body Plan of a Simple Sponge
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Sponges: Loosely Organized Animals
• Sponges have a supporting skeleton, either in the
form of branching spines called spicules or as an
elastic network of fibers.
• Sponges are loosely organized; if a sponge is
completely disassociated, its cells can reassemble
into a new sponge.
• Sponges depend on water movement through their
bodies to obtain food and are often oriented at right
angles to current flow so that they may intercept
water as it flows past.
• Sponges reproduce both sexually and asexually. In
most species, a single individual produces both eggs
and sperm. Asexual reproduction is by budding and
fragmentation.
Figure 32.5 Sponges Differ in Size and Shape
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Cnidarians: Two Cell Layers and Blind Guts
• The cnidarians (phylum Cnidaria) were the next
lineage to split off from the main line of animal
evolution after the sponges.
• They are diploblastic and have a blind gut with
only one entrance.
• Despite their relatively simple structures, the
Cnidarians have structural molecules, such as
actin and collagen, and homeobox genes.
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Cnidarians: Two Cell Layers and Blind Guts
• Cnidarians appeared early in evolutionary history
and radiated in the late Precambrian.
• There are about 11,000 species living today.
• The cnidarian body plan combines a low metabolic
rate with the ability to capture large prey, allowing
cnidarians to survive in environments where prey is
scarce.
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Cnidarians: Two Cell Layers and Blind Guts
• Cnidarians have tentacles with specialized cells
called cnidocytes. These cells contain structures
called nematocysts that can discharge toxins into
their prey.
• The mouth of a cnidarian is connected to a blind
sac called the gastrovascular cavity. It functions
in digestion, circulation, and gas exchange.
• Cnidarians have epithelial cells with muscle fibers
whose contractions allow them to move, as well
as nerve nets that integrate body activities.
Figure 32.7 Nematocysts Are Potent Weapons
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Cnidarians: Two Cell Layers and Blind Guts
• The generalized cnidarian life cycle has two stages:
 The polyp is typically asexual; individual polyps
may reproduce by budding to form colonies.
 The medusae produce eggs and sperm and
release them into the water.
• A fertilized egg becomes a free-swimming, ciliated
larva called a planula that eventually settles to the
bottom and transforms into a polyp.
Figure 32.8 A Generalized Cnidarian Life Cycle
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Cnidarians: Two Cell Layers and Blind Guts
• Corals are also usually sessile and colonial.
• The polyps of corals secrete a matrix of organic
molecules upon which calcium carbonate is
deposited.
• This matrix forms the eventual skeleton of the
coral colony.
• As coral colonies grow, old polyps die and leave
their calcareous skeletons behind.
• Living members of the colony form a layer on top
of a growing reef of skeletal remains.
Figure 32.9 Corals (Part 1)
Figure 32.9 Corals (Part 2)
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The Evolution of Bilaterally Symmetrical Animals
• A common ancestor of all bilaterally symmetrical
animals is postulated.
• Zoologists use evidence from genes, development,
and the structure of existing animals to infer the
form of ancient bilaterians.
• The development of all bilaterally symmetrical
animals is controlled by homologous Hox and
homeobox genes. It is unlikely that these genes
evolved separately in several animal lineages.
• Fossilized tracks from the late Precambrian suggest
that early bilaterians had circulatory systems,
antagonistic muscles, and a tissue- or fluid-filled
body cavity.
Figure 32.13 The Trail of an Early Bilaterian
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The Evolution of Bilaterally Symmetrical Animals
• The protostomes and the deuterostomes that
dominate today’s fauna have been evolving
separately since the Cambrian period.
• Members of both lineages are bilaterally
symmetrical and have cephalization.
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The Evolution of Bilaterally Symmetrical Animals
• Shared, derived traits that unite the protostomes
include:
 A central nervous system consisting of an
anterior brain that surrounds the entrance to the
digestive tract
 A ventral nervous system consisting of paired or
fused longitudinal nerve cords
 Free-floating larvae with a food-collecting system
consisting of compound cilia on multiciliate cells
 A blastopore that becomes the mouth
 Spiral cleavage (in some species)
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The Evolution of Bilaterally Symmetrical Animals
• The major shared, derived traits that unite the
deuterostomes inlcude:
 A dorsal nervous system
 Larvae, if present, that have a food-collecting
system consisting of cells with a single cilium
 A blastopore that becomes the anus
 Radial cleavage
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Simple Lophotrochozoans
• The flatworms (phylum Platyhelminthes) are the
simplest of the lophotrochozoans.
• The flatworms are bilaterally symmetrical,
unsegmented, acoelomate animals.
• They lack organs for transporting oxygen to internal
tissues.
• They have simple organs for excreting metabolic
wastes.
• Their flattened form allows each body cell to be
near a body surface, a requirement of their body
plan.
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Simple Lophotrochozoans
• The flatworm digestive tract is a mouth opening
into a blind sac.
• The sac is often highly branched, increasing the
surface area available for the absorption of
nutrients.
• Flatworms feed on living or dead animal tissue.
• The motile flatworms move by beating broad
bands of cilia.
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Simple Lophotrochozoans
• The flatworms of the class Turbellaria are
probably most similar to ancestral flatworm forms.
• Turbellarians are small, free-living, marine and
freshwater animals.
• The head has chemoreceptor organs, simple
eyes, and a small brain.
Figure 32.15 Flatworms Live Freely and Parasitically (Part 1)
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Simple Lophotrochozoans
• Most living flatworms are parasitic, such as the
tapeworms (class Cestoda) and the flukes (class
Trematoda).
• Parasitic flatworms lack digestive tracts; they
absorb digested food from their hosts.
• Some species cause serious diseases, such as
schistosomiasis.
• Most parasitic species have complex life cycles
involving one or more intermediate hosts and
several larval stages.
Figure 32.15 Flatworms Live Freely and Parasitically (Part 2)
Figure 32.16 Reaching a Host by a Complex Route
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Lophophorates: An Ancient Body Plan
• The brachiopods (phylum Brachiopoda) are
solitary, marine lophophorate animals that
superficially resemble bivalve mollusks.
• The shell differs from that of mollusks in that its
two halves are dorsal and ventral rather than
lateral.
• Brachiopods are either attached to a solid
substrate by a short, flexible stalk or embedded in
soft sediment.
• Most species release gametes into the water,
where they are fertilized.
• More than 26,000 fossil species have been
described, but only 350 species survive today.
Figure 32.20 Brachiopods
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Spiralians: Spiral Cleavage and
Wormlike Body Plans
• Ribbon worms (phylum Nemertea) are
carnivorous spiralians.
• They are similar in structure to the flatworms, but
they have a complete digestive tract.
• Small ribbon worms move by beating their cilia;
larger ones move by waves of contraction of body
muscles.
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Spiralians: Spiral Cleavage and
Wormlike Body Plans
• A body cavity that is segmented allows an animal
to alter the shape of its body in complex ways and
to control its movements precisely.
• Segmentation evolved several times among
spiralians.
• The annelids (phylum Annelida) are a diverse
group of segmented worms.
• Annelid species can be found in marine,
freshwater, and terrestrial environments.
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Spiralians: Spiral Cleavage and
Wormlike Body Plans
• Nerve cord is found on the ventral side.
• Each segment in an annelid is controlled by a
separate nerve center called a segmented
ganglion. All the ganglia are connected by nerve
cords that coordinate their function.
• The coelom in each segment is isolated from
those in other segments.
• Most species lack a rigid, external protective
surface.
• The thin body wall serves as a surface for gas
exchange and also limits annelids to moist
environments, as they lose body water rapidly in
dry air.
Figure 32.22 Annelids Have Many Body Segments
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Spiralians: Spiral Cleavage and
Wormlike Body Plans
• The mollusks (phylum Mollusca) range in size
from small snails to giant squids that can be more
than 18 meters long.
• Mollusks have a unique body plan with three
major structural components: foot, mantle ( a hard
skeleton structure), and visceral mass that covers
the internal organs.
• The molluscan foot is a large, muscular structure
that originally was both an organ of locomotion
and support for the internal organs.
Figure 32.25 Molluscan Body Plans (Part 1)
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Spiralians: Spiral Cleavage and
Wormlike Body Plans
• The bivalves (class Bivalvia) have a hinged, twopart shell that extends over the sides and top of
their body.
• Bivalves are largely sedentary.
• They have greatly reduced heads.
• Feeding is accomplished by bringing water in
through an opening called an incurrent siphon and
extracting food from the water using their gills.
• Water and gametes exit through another opening,
the excurrent siphon.
Figure 32.26 Diversity among the Mollusks (Part 2)
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Spiralians: Spiral Cleavage and
Wormlike Body Plans
• The gastropods (class Gastropoda) are mostly
motile, using their large foot to move across a
substrate or to burrow through it.
• The gastropods are the most species-rich and
widely distributed of the molluscan classes.
• Some gastropods can crawl, whereas others have
a modified foot that functions as a swimming
organ.
• Gastropods are the only terrestrial mollusks. They
have a mantle cavity that is modified into a highly
vascularized lung.
Figure 32.25 Molluscan Body Plans (Part 4)
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Spiralians: Spiral Cleavage and
Wormlike Body Plans
• The cephalopods (class Cephalopoda) have a
modified excurrent siphon.
• This modification allowed early cephalopods to
control the water content of the mantle cavity.
• The modification of the mantle into a device for
forcibly ejecting water from the cavity enabled
cephalopods to move rapidly through the water.
• It also allows the animals to control their
buoyancy.
• Their greatly enhanced mobility allowed some
cephalopods, such as squids and octopuses, to
become the major predators in open ocean
waters.
Figure 32.25 Molluscan Body Plans (Part 5)
Figure 32.26 Diversity among the Mollusks (Part 4)
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Spiralians: Spiral Cleavage and
Wormlike Body Plans
• Cephalopods include the squids, octopuses, and
nautiluses.
• They appeared near the beginning of the
Cambrian period about 600 million years ago.
• They were the first large, shelled animals able to
move vertically in the ocean.
• Nautiloids are the only cephalopods with external
chambered shells that survive today.
Figure 32.26 Diversity among the Mollusks (Part 5)