CHAPTER 32

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General Features of Animals

2

3

4

Evolution of the Animal Body Plan • Five key transitions can be noted in animal evolution 1. Tissues 2. Symmetry 3. Body cavity 4. Development 5. Segmentation 5

Evolution of the Animal Body Plan

1. 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 6

Evolution of the Animal Body Plan

2. 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 7

Evolution of the Animal Body Plan -

Radial symmetry

-Body parts arranged around central axis -Can be bisected into two equal halves in any 2-D plane -

Bilateral symmetry

-Body has right and left halves that are mirror images -Only the sagittal plane bisects the animal into two equal halves 8

Top Front Bottom Back

9

Evolution of the Animal Body Plan Bilaterally symmetrical animals have two main advantages over radially symmetrical ones 1.

Cephalization

-Evolution of a definite brain area 2. Greater mobility 10

Evolution of the Animal Body Plan

3. Evolution of a body cavity

-Eumetazoa produce three germ layers -Outer

ectoderm

nervous system) (body coverings and -Middle

mesoderm

(skeleton and muscles) -Inner

endoderm

(digestive organs and intestines) Body cavity = Space surrounded by mesoderm tissue that is formed during development 11

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

pseudocoelom

-

Coelomates

= Body cavity entirely within the mesoderm -Called the

coelom

12

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Evolution of the Animal Body Plan -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 14

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

15

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 16

Evolution of the Animal Body Plan 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 -Deuterostomes = Indeterminate development 17

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 18

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Evolution of the Animal Body Plan

5. 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 20

Traditional Classification of Animals 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 21

A New Look At Metazoans Recent analysis of the translation machinery revealed that myzostomids have no close link to the annelids at all -Instead, they are more closely allied with the flatworms (planaria and tapeworms) 22

A New Look At Metazoans 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 23

A New Look At Metazoans 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

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A New Look At Metazoans 25

Evolutionary Developmental Biology Molecular analysis may also explain the

Cambrian explosion

-The enormous expansion of animal diversity in the Cambrian period (543 –525 MYA) -The

homeobox

complex evolved (

Hox

) developmental gene -Provided a tool that can produce rapid changes in body plan 26

Evolutionary Developmental Biology 27