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1
Q

Evolution of the Animal Body Plan

A
Five key transitions can be noted in animal evolution
Tissues
Symmetry
Body cavity
Development
Segmentation
2
Q

Evolution of the Animal Body Plan

A

. Evolution of tissues
-Parazoa (Sponges – the simplest animals) lack defined tissues and organs
Have the ability to disaggregate and aggregate their cells
-Eumetazoa (all other animals) have distinct and well-defined tissues
-Have irreversible differentiation for most cell types

3
Q

Evolution of the Animal Body Plan

A
  1. Evolution of symmetry
    - Sponges also lack any definite symmetry
  • Eumetazoa have a symmetry defined along an imaginary axis drawn through the animal’s body
  • There are two main types of symmetry
4
Q

Evolution of the Animal Body Plan

A

Radial symmetry

  • Body parts arranged around central axis-Can be bisected into two equal halves in any 2-D plane
  • Bilateral symmetry (Bilateria)
  • Body has right and left halves that are mirror images
  • Only the sagittal plane bisects the animal into two equal halves
5
Q

Evolution of the Animal Body Plan

A

Bilaterally symmetrical animals have two main advantages over radially symmetrical ones

  1. Cephalization
    - Evolution of a definite brain area
  2. Greater mobility
6
Q

Evolution of the Animal Body Plan

A
  1. Evolution of a body cavity
    - Three basic kinds of body plans
    - Acoelomates = No body cavity
    - Pseudocoelomates = Body cavity between mesoderm and endoderm
    - Called the pseudocoelom
    - Coelomates = Body cavity entirely within the mesoderm-Called the coelom
7
Q

Evolution of the Animal Body Plan

A

The body cavity made possible the development of advanced organ systems

  • 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—more efficient
8
Q

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
A

. 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 gut
9
Q
Deuterostomes differ from protostomes in three other fundamental embryological features: 
-1. Cleavage pattern of embryonic cells 
-Protostomes = Spiral cleavage
-Deuterostomes = Radial cleavage 
-2. Developmental fate of cells 
-Protostomes = Determinate development
fate determined early
-Deuterostomes = Indeterminate 	development—fate determined later
A

-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

10
Q

Evolution of segmentation
-Segmentation provides two advantages
-1. Allows redundant organ systems in adults such as occurs in the annelids
-2. Allows for more efficient and flexible movement because each segment can move independently
Segmentation appeared several times in the evolution of animals

A

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11
Q

Traditional Classification of Animals

A

Multicellular animals, or metazoans, are traditionally divided into 36 or so distinct phyla based on shared anatomy and embryology
Metazoans are divided into two main branches:
-Parazoa = Lack symmetry and tissues
-Eumetazoa = Have symmetry and tissues
-Diploblastic = Have two germ layers
-Triploblastic = Have three germ layers

12
Q

A New Look At Metazoans

A

The traditional animal phylogeny is being reevaluated using molecular data
Myzostomids are marine animals that are parasites of echinoderms
-Have no body cavity and only incomplete segmentation
-And so until recently have been allied with annelids

13
Q

Recent analysis of the translation machinery revealed that myzostomids have no close link to the annelids at all

A

Instead, they are more closely allied with the flatworms (planaria and tapeworms)

14
Q

Therefore, key morphological characters used in traditional classification do not necessarily reflect evolutionary relationships.

A

Molecular systematics uses unique sequences within certain genes to identify clusters of related groups

15
Q

Most new phylogenies agree on two revolutionary features:

  1. Separation of annelids and arthropods into different clades
  2. Division of the protostome group into Ecdysozoa and Spiralia
    - The latter is then broken down into Lophotrochozoa and Platyzoa
A
--lophophore--feeding structure
horseshoe-shaped crown of 
ciliated tentacles around the 
mouth—used in filter-feeding
Bryozoa and Brachiopoda
--Free-living larva called
a trochophore
Mollusca and Annelida
16
Q

Evolutionary Developmental Biology

A

Most taxonomists agree that the animal kingdom is monophyletic
-Three prominent hypotheses have been proposed for the origin of metazoans from single-celled protists

17
Q
  1. The multinucleate hypothesis—evolved from a multinucleated protistan
  2. The colonial flagellate hypothesis—
    evolved from a choanoflagellate protistan
  3. The polyphyletic origin hypothesis—
    evolved more than once
    Molecular systematics using rRNA sequences settles this argument in favor of the colonial flagellate hypothesis
A

Molecular analysis may also explain the Cambrian explosion

  • The enormous expansion of animal diversity in the Cambrian period (543–525 MYA)
  • The homeobox (Hox) developmental gene complex evolved
  • Provided a tool that can produce rapid changes in body plan
18
Q

Metazoa—multicellular
Parazoa—”near animals”
Platyzoa—acoelomate protostomes
Lophotrochozoa—coelomate protostomes

A

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