Chapter 17 - The Evolution of Animals

What Is an Animal?• Are eukaryotic, multicellular, heterotrophic

organisms that obtain nutrients by ingestion– Digest their food within their bodies..

Animal Phylogeny• To reconstruct the evolutionary history of

animal phyla, researchers must depend on clues from comparative anatomy and embryology

• Four key evolutionary branch points have been hypothesized

1. Presence or absence of true tissues

2. Radial or bilateral symmetry

3. Presence or absence of a body cavity (coelom)

4. Mechanism of coelom formation..

(a) Radial symmetry

(b) Bilateral symmetry

(a) No body cavity (e.g., flatworm)

Body covering (from ectoderm)

Digestive tract (from endoderm)

Tissue-filled region (from mesoderm)

(b) Pseudocoelom (e.g., roundworm)

Pseudocoelom

Digestive tract (from endoderm)

Body covering (from ectoderm)

Muscle layer (from mesoderm)

(c) True coelom (e.g., annelid)

Coelom

Digestive tract (from endoderm)

Body covering (from ectoderm)

Tissue layer lining coelom and suspending internal organs (from mesoderm)

Mesentery

Multicellularity

True tissues

Radial symmetry Bilateral

symmetry

No body cavity

Body cavities

PseudocoelomTrue coelom

Coelom from cell masses

Coelom from digestive tube

1

2

3

4

Sponges Cnidarians Flatworms Roundworms Mollusks Annelids Arthropods Echinoderms Chordates

• Phylum Porifera– Includes

sessile animals once believed to be plants

– Lack true tissues

– Resembles a sac perforated with holes (pores)

– Draws water into a central cavity, where food is collected..

Pores

Water flow

Skeleton fiber

Central cavity

Choanocyte

Amoebocyte

Choanocyte in contact with an amoebocyte

Flagella

• Phylum Cnidaria– Characterized by organisms with radial symmetry

and tentacles with stinging cells (cnidocytes)– Sac body plan with a gastrovascular cavity– Do not have true organs– Life cycle includes two body forms:

• Sessile polyp• Floating medusa..

Mouth/anus

Tentacle

Gastrovascular cavity

Tentacle

Mouth/anus

MedusaPolyp

• Cnidarians are carnivores that use tentacles armed with cnidocytes, or “stinging cells,” to capture prey..

TentacleCoiled thread Capsule

“Trigger”

Discharge of thread

Cnidocyte

Prey

• Phylum Platyhelminthes– Is represented by the simplest bilateral animals– Have true organs but no body cavity –

acoelomate– Digestive tract is two directional – only one

opening = incomplete digestive tract

• Includes free-living forms such as planarian• Includes many parasitic forms such as

tapeworms and flukes..

Digestive tract (gastrovascular cavity) Nerve cords

Mouth

Eyespots

Nervous tissue clusters

Reproductive structures

Head

Hooks

Sucker

• Phylum Nematoda– exhibit an important evolutionary adaptation; a

digestive tube with two openings, a mouth and an anus

• A complete digestive tract can process food and absorb nutrients efficiently

– Only phylum with a pseudocoelom..

• Phylum Mollusca– Is represented by soft-bodied animals, but most

are protected by a hard shell– All have a similar body plan:

• Muscular foot; maybe modified into tentacles – for locomotion and defense and food gathering

• Mantle – secretes shell and aids in respiration• Visceral mass containing internal organs

– First group with a true coelom – eucoelomates..

Mantle

Mantle cavity

Shell

Anus

Gill

FootNerve cords

Visceral mass

Kidney

Coelom

Heart

Reproductive organs

Digestive tract

RadulaRadula

Mouth

Mouth

• Phylum Annelida– Includes worms with body segmentation –

division of the body along into length into a series of repeated segments; look like rings..

• Phylum Arthropoda– All have exoskeleton of chitin– Appendages are jointed– Appendages are highly specialized for the

environment– Undergo metamorphosis – change in body form

during development

• This is the largest group of animals– Includes insects – most abundant animals..

Metamorphosis = egg larva pupa adult..

• Phylum Echinodermata– Is named for the spiny surfaces of the organisms– Have endoskeleton– Have water-vascular system – hydraulic pumps

that circulate water for gas-exchange, waste disposal, circulation of nutrients, and locomotion with tube feet

– All aquatic..

Muscle segments

Post-anal tail

Notochord

Anus

Dorsal, hollow nerve cord

Pharyngeal slits

Brain

Mouth

• Phylum Chordata – all possess these characters at some point during development– Have notochord = supporting rod of tissue;

becomes vertebrae in one subphylum– Have dorsal, hollow nerve cord– Have post-anal tail– Have pharyngeal pouches..

Era

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Me

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Pal

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Pre

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Ter

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De

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Silu

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Tu

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Lan

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such

as

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Ch

on

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(sh

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Ost

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(bo

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Am

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(fr

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Rep

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a (r

epti

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Ave

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Mam

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(ma

mm

als)

Ancestral chordate

Vertebrae

Jaws

Lungs or lung derivatives

Legs

Amniotic egg

Hair

Feathers

Chordates

Vertebrates

Tetrapods

Amniotes

Pe

rio

ds

Chordate Evolution

• The subphyla include the invertebrate chordates – lancelets (cephalochordata) and tunicates (urochordata), which also share four key chordate characteristics..

• This phylum includes the subphylum Vertebrata – The notochord develops into the vertebral

column..

• The first three major classes of the phylum are fishes– Agnatha – jawless fishes, eg. lampreys– Chondrichthyes – cartilage fishes– Osteichthyes – bony fishes..

• Cartilaginous fishes have a flexible skeleton made of cartilage; the only bone is found in the jaw– Includes sharks, rays, and skates..

• Bony Fish– Have a skeleton reinforced by hard calcium

salts– Have a lateral line system, a keen sense of

smell, and excellent eyesight• Most bony fishes are ray-finned fishes• A second evolutionary branch includes lungfishes

and lobe-finned fishes..

Terrestrial vertebrates are collectively called

tetrapods, which means “four legs”

Class Amphibia• Exhibit a mixture of aquatic

and terrestrial adaptations• Usually need water to

reproduce..

Class Reptilia - can live totally on land

• Have scales to prevent dehydration

• Have claws for digging and protection

• Have lungs for breathing

• First group to develop the amniotic egg..

• Class Aves– Evolved the ability to fly and only animals with

feathers• Bird anatomy and physiology are modified for flight

– Bones are honeycombed, which makes them lighter

– Some specific organs are absent, which reduces weight

– A warm, constant body temperature is maintained through endothermy..

Airfoil

• Class Mammalia• Includes mostly terrestrial organisms

– Have hair– Have mammary glands that produce milk and

nourish the young

There are three major groups of mammals– Monotremes – lay eggs– Marsupials – do not have a placenta– Eutherians (placental mammals) – have

placenta to nourish young inside mother’s body..