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Overview of metazoan
Diversity
LEARNING OUTCOMES
• Identify three features that characterize all animals
and four that characterize only some types of
animals.
• Understand how the metazoans are organized and
how this organization is different from that of
plants, fungi, protists, and prokaryotes.
• Know the five key innovations in body plans.
• Compare and contrast Parazoa and Eumetazoa in
terms of evolution, complexity, symmetry, and
organization of embryonic cell layers.
• Compare and contrast asymmetry, radial
symmetry, and bilateral symmetry.
• Differentiate among acoelomate,
pseudocoelomate, and coelomate organisms;
indicate how they are evolutionarily related and
give examples of each.
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• Differentiate between protostomes and
deuterostomes.
• Understand the advantages of
segmentation; give at least one example of
segmentation in each of the coelomate
phyla.
• Compare the tradition methods using
morphology in classification of metazoans to
the new molecular systematics using DNA
and RNA analysis to classify related
metazoan groups. What is the problems of
classification regarding homology and
analogy?
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General Features of metazoans
Are “metazoans” monophyletic?
• Animals are so diverse that few criteria fit
them all. But some, such as metazoans being
eaters, or consumers, apply to all.
• ALL:
– Are heterotrophs
– Are multicellular (It was such a great move,
it evolved at least 16 different times.
Animals, land plants, fungi and algae all
joined in)
– Have cells without cell walls
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General Features of Animals
• MOST:
–
–
–
–
Most are able to move
Are very diverse in form and habitat
Most reproduce sexually
Have a characteristic pattern of embryonic
development
– Cells of all metazoans (except sponges)
are organized into tissues
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Traditional Classification of Metazoans
Five key innovations can be noted in animal evolution:
1. The evolution of symmetry
2. The evolution of tissues, allowing specialized structures
and functions
3. The evolution of a body cavity
4. The evolution of various patterns of embryonic
development
5. The evolution of segmentation, or repeated body units
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• Traditional methods of classification:
–
–
–
–
Morphology
Embryology
Symmetry
Germ layers
Problem: these comparisons can be
analogous or homologous.
– What’s the difference?
– What is the difference in
classification and phylogenetics?
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Classification of Animals
5 Key Transitions
(1) Tissues
(2) Body Symmetry
(3) Body Cavity
(4) Development
(5) Segmentation
Figure 31.3
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Chapter 31
Phylogeny
Metazoans are divided into two main branches:
• Parazoa = Lack symmetry and tissues
– These “simplest” metazoans lack defined tissues
and organs
– Have the ability to disaggregate and aggregate
their cells
• Eumetazoa = Have symmetry and tissues
– Diploblastic = Have two germ layers
– Triploblastic = Have three germ layers
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Evolution of the Animal Body
Plan
1. Evolution of tissues—Parazoa/Eumetazoa
split
• Have irreversible differentiation for most cell
types
• The evolution of tissues allowed for
specialized structures and functions
• Eumetazoa (all other metazoans) have
distinct and well-defined tissues
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Evolution of the Animal Body
Plan
2. Evolution of symmetry
• Radiata/Bilateria split.
• Sponges lack any definite symmetry
• Eumetazoa have a symmetry defined along
an imaginary axis drawn through the
metazoan’s body
• There are two main types of symmetry
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Evolution of the Animal Body
Plan
-Radial symmetry (The Radiata)
-Body parts arranged around central axis
-Can be bisected into two equal halves in
any 2-D plane
-Bilateral symmetry (The Bilateria)
-Body has right and left halves that are
mirror images
-Only the sagittal plane bisects the
animal into two equal halves
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Top
Back
Front
Bottom
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Evolution of the Animal Body
Plan
Bilaterally symmetrical metazoans have
two main advantages over radially
symmetrical ones
1. Cephalization
-Evolution of a definite brain area
2. Greater mobility
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Evolution of the Animal Body
Plan
3. Evolution of a body cavity
• Eumetazoa produce two or three germ layers
• Body cavity = Space surrounded by mesoderm
tissue that is formed during development
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Evolution of the Animal Body
Plan
3. Evolution of a body cavity
Three basic kinds of body plans
• Acoelomates = No body cavity
• Pseudocoelomates = Body cavity between
mesoderm and endoderm
– Called the pseudocoel
• Coelomates = Body cavity entirely within the
mesoderm
– Called the coelom
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Diploblastic vs. Triploblastic – Cell
Layers
Diploblastic – two cell layers
– Ectoderm – outer layer
– Endoderm – inner layer
– The Radiata
Triploblastic – three cell layers
– Ectoderm, endoderm
– Mesoderm – layer between ectoderm
and endoderm
– The Bilateria
Ectoderm – outer covering of the body; nervous system
Endoderm – digestive organs and intestines
Mesoderm – skeleton and muscles
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Evolution of the Animal Body
Plan
The body cavity made possible the
development of advanced organs systems
• Pseudocoelomates use pseudocoel for circ.
• Coelomates developed a circulatory system
to flow nutrients and remove wastes
-Open circulatory system: blood
passes from vessels into sinuses, mixes
with body fluids and reenters the vessels
-Closed circulatory system: blood
moves continuously through vessels that
are separated from body fluids
• Why do you think closed is more advanced?19
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Evolution of the Animal Body
Plan
4. Evolution of different patterns of development
The basic Bilaterian pattern of development:
• Mitotic cell divisions of the egg form a hollow
ball of cells, called the blastula
• Blastula indents to form a two-layer-thick ball
with:
-Blastopore = Opening to outside
-Archenteron = Primitive body cavity
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Evolution of the Animal Body
Plan
Bilaterians can be divided into two groups:
-Protostomes develop the mouth first from or
near the blastopore
-Anus (if present) develops either from
blastopore or another region of embryo
-Deuterostomes develop the anus first from the
blastopore
-Mouth develops later from another region
of the embryo
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Embryonic development in protostomes and
deuterostomes
Protostomes
Mesoderm
Archenteron
Coelom
Mesoderm splits
Blastula
Blastopore
Archenteron
Blastula
Blastopore
Coelom
Archenteron outpockets
to form coelom
Anus
Coelom
Mouth forms Mouth
from blastopore
Mouth
Coelom
Anus
Anus forms
from blastopore
Deuterostomes
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Evolution of the Animal Body
Plan
Deuterostomes differ from protostomes in three
other fundamental embryological features:
-1. Cleaveage pattern of embryonic cells
-Protostomes = Spiral cleavage
-Deuterostomes = Radial cleavage
-2. Developmental fate of cells
-Protostomes = Determinate development
-Deuterostomes = Indeterminate
development
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Evolution of the Animal Body
Plan
-3. Origination of coelom
•
Protostomes = Forms simply and
directly from the mesoderm
•
Deuterostomes = Forms indirectly from
the archenteron
Deuterostomes evolved from protostomes
more than 500 MYA
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(5) - Segmentation
SegmentationBody is assembled from succession
of similar segments
• Each segment may develop
into complete set of adult
organs
• Damage to one segment is less
fatal
• Locomotion is easier when
segments can move
independently
• Earthworms, Arthropods, and
Chordates
• Originated multiple times in
metazoans.
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A New Look At Metazoans
The traditional metazoan phylogeny is being
reevaluated using molecular data.
(Remember the homology/analogy
problem.)
Therefore, key morphological characters used
in traditional classification are not necessarily
conservative
Molecular systematics uses unique sequences
within certain genes to identify clusters of
related groups
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A New Look At Metazoans
Molecular data has helped to clarify the
relationship of different groups with
the animals (metazoans) for example
annelids and arthropods
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A New Look At Metazoans
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Evolutionary Developmental
Biology
Most taxonomists agree that the
metazoan kingdom is monophyletic
Three prominent hypotheses have been
proposed for the origin of metazoans
from single-celled protists
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Evolutionary Developmental
Biology
1. The multinucleate hypothesis
2. The colonial flagellate hypothesis
3. The polyphyletic origin hypothesis
Molecular systematics using rRNA
sequences settles this argument in favor
of the colonial flagellate hypothesis
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