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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.

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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.
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?
general features of metazoans
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
general features of animals
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
traditional classification of metazoans
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


5. The evolution of segmentation, or repeated body units

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?
classification of animals 5 key transitions
Classification of Animals 5 Key Transitions
  • Tissues
  • Body Symmetry
  • Body Cavity
  • Development
  • Segmentation

Figure 31.3

Chapter 31


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
evolution of the animal body plan
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
evolution of the animal body plan11
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
evolution of the animal body plan12
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






evolution of the animal body plan14
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

evolution of the animal body plan15
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
evolution of the animal body plan16
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
diploblastic vs triploblastic cell layers
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

evolution of the animal body plan19
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?
evolution of the animal body plan21
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

evolution of the animal body plan22
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








Mesoderm splits

Mouth forms

from blastopore








Archenteron outpockets

to form coelom

Anus forms

from blastopore



Embryonic development in protostomes and deuterostomes



evolution of the animal body plan24
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

evolution of the animal body plan25
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

5 segmentation
(5) - Segmentation


Body 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.
a new look at metazoans
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

a new look at metazoans29
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

evolutionary developmental biology
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

evolutionary developmental biology32
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