ANIMAL DIVERSITY AND THE EVOLUTION OF BODY...

ANIMAL DIVERSITY AND THE EVOLUTION OF BODY PLANS

GENERAL FEATURES OF ANIMALS

• Heterotrophy - obtain energy and organic

molecules by ingesting other organisms

• Multicellularity - Many have complex bodies

• No cell walls - They lack rigid cell walls and are

usually flexible

• Active movement - Move more rapidly and in more

complex ways

• Diversity of form - Vary greatly in form, ranging in

size from organisms too small to see with the

unaided eye to enormous2

GENERAL FEATURES OF ANIMALS

• Diversity of habitat - Grouped into 35–40 phyla,

most that occur only in the sea but many occur in

fresh water and on land

• Sexual reproduction - Most animals reproduce

sexually. Animal eggs, which are nonmobile.

• Embryonic development - Zygote first undergoes a

series of mitotic divisions that produces a ball of

cells

• Tissues - Cells of most animals are organized into

structural and functional units called tissues3

4

Some characteristics shared by animals include

mobility, diversity of shapes and sizes, and

development that involves complex cleavage

patterns

a. c.

b.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

a: © Royalty-Free/Corbis; b: © Edward S. Ross; c: © Cabisco/Phototake

EVOLUTION OF THE ANIMAL BODY PLAN

• Five key transitions can be noted in animal evolution

1. Symmetry

2. Tissues

3. Body cavity

4. Patterns of Development

5. Segmentation

5

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 6

Evolution of the Animal Body Plan

Radial symmetry (i.e. jellyfish, anemones, corals)

Body parts arranged around central axis

Can be bisected into two equal halves in any

2-D plane

Bilateral symmetry (i.e. worms, octopus, spiders)

Body has right and left halves that are mirror images

Only the sagittal plane bisects the animal into two

equal halves

7

Evolution of the Animal Body Plan


a.

Radial Symmetry

b.

Bilateral Symmetry

Ventral

Dorsal

Frontal plane

Sagittal plane

Anterior

Posterior

Transverse

plane8

Top

Bottom

Back

Front

Comparison of

radial and

bilateral

symmetry


Bilaterally symmetrical animals have two main

advantages over radially symmetrical ones

1. Cephalization

Evolution of a definite brain area

2. Greater mobility

9


2. Evolution of tissues

Parazoa (Sponges – the simplest animals) lack

defined tissues and organs

• Have the ability to disaggregate and aggregate

their cell

Eumetazoa (all other animals) have distinct and well-

defined tissues

Have irreversible differentiation for most cell

types10


3. Evolution of a body cavity

Eumetazoa produce three germ layers

Outer ectoderm (body coverings and nervous system)

Middle mesoderm (skeleton and muscles)

Inner endoderm (digestive organs and intestines)

Body cavity = Space surrounded by mesoderm tissue that is

formed during development

11


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

13

Three body plans

for bilaterally

symmetrical

animals


Flatworm

Pseudocoelomate

RoundwormPseudocoelom

Coelomate

AnnelidCoelom

Ectodermally

derived tissue

Endodermally

derived tissue

Mesodermally

derived tissue

Digestive

cavity

Ectodermally

derived tissue

Endodermally

derived tissue

Mesodermally

derived tissue

Acoelomate

Endodermally

derived tissue

Ectodermally

derived tissue

Digestive

cavity

Mesodermally

derived tissue

Digestive

cavity


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


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

15


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


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

animals17

ANIMAL DIVERSITY AND THE EVOLUTION OF BODY...

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