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 enormous 2

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 tissues 3

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. b. a. c. a: Royalty-Free/Corbis; b: Edward S. Ross; c: Cabisco/Phototake Some characteristics shared by animals include mobility, diversity of shapes and sizes, and development that involves complex cleavage patterns 4

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

Evolution of the Animal Body Plan 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

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Radial Symmetry Comparison of radial and bilateral symmetry a. Bilateral Symmetry Top Dorsal Sagittal plane Back Posterior Frontal plane Anterior Front Ventral Bottom Transverse plane 8 b.

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 9

EVOLUTION OF THE ANIMAL BODY PLAN 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 welldefined tissues Have irreversible differentiation for most cell types 10

EVOLUTION OF THE ANIMAL BODY PLAN 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

EVOLUTION OF THE ANIMAL BODY PLAN 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

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Acoelomate Ectodermally derived tissue Endodermally derived tissue Digestive cavity Flatworm Mesodermally derived tissue Three body plans for bilaterally symmetrical animals Pseudocoelomate Endodermally derived tissue Ectodermally derived tissue Digestive cavity Roundworm Pseudocoelom Mesodermally derived tissue Coelomate Ectodermally derived tissue Endodermally derived tissue Digestive cavity 13 Annelid Coelom Mesodermally derived tissue

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 - Blastula indents to form a two-layerthick ball with: - Blastopore = Opening to outside - Archenteron = Primitive body cavity 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 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 17