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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.
2.
3.
4.
Presence or absence of true tissues
Radial or bilateral symmetry
Presence or absence of a body cavity (coelom)
Mechanism of coelom formation..
Body covering
(from ectoderm)
Tissue-filled
region (from
mesoderm)
Digestive
tract (from
endoderm)
(a) No body cavity (e.g., flatworm)
(a) Radial symmetry
Pseudocoelom
Body
covering
(from
ectoderm)
Digestive
tract (from
endoderm)
Muscle layer
(from
mesoderm)
(b) Pseudocoelom (e.g., roundworm)
(b) Bilateral symmetry
Coelom
Digestive
tract (from
endoderm)
Body covering (from
ectoderm)
Mesentery
(c) True coelom (e.g., annelid)
Tissue layer
lining coelom
and
suspending
internal organs
(from
mesoderm)
Sponges
Cnidarians
Flatworms Roundworms
Mollusks
Annelids
Arthropods
Coelom from
cell masses
4
Pseudocoelom
True coelom
No body
cavity
3
Radial
symmetry
Bilateral
2 symmetry
True tissues
1
Multicellularity
Body cavities
Echinoderms
Coelom from
digestive tube
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
Choanocyte
in contact
with an
amoebocyte
Water flow
Skeleton fiber
Central cavity
Choanocyte
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
Polyp
Medusa
• Cnidarians are carnivores that use tentacles
armed with cnidocytes, or “stinging cells,” to
capture prey..
Coiled
thread
Tentacle
Capsule
“Trigger”
Cnidocyte
Discharge of
thread
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
Head
Reproductive
structures
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..
Visceral mass
Coelom
Mantle
Kidney
Reproductive
organs
Heart
Digestive
tract
Mantle
cavity
Radula
Shell
Radula
Anus
Gill
Foot
Mouth
Nerve cords
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..
• 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
Dorsal,
hollow
– Have post-anal tail
nerve cord
Notochord
– Have pharyngeal pouches..
Brain
Muscle segments
Mouth
Anus
Post-anal
tail
Pharyngeal
slits
Precambrian
Cambrian
Jurassic
Tertiary
Cenozoic
Lancelets
Tunicates
Cretaceous
Mesozoic
Vertebrae
Ancestral chordate
Jaws
Amniotic egg
Legs
Lungs or lung derivatives
Feathers
Hair
Mammalia
(mammals)
Aves
(birds)
Reptilia (reptiles)
Eras
Periods
Chordate Evolution
Amphibia (frogs and
salamanders)
Osteichthyes (bony fishes)
Chondrichthyes (sharks and rays)
Agnatha (jawless vertebrates,
such as lampreys)
Ordovician Silurian Devonian Carboniferous Permian Triassic
Paleozoic
Chordates
Vertebrates
Tetrapods
Amniotes
• 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..
• 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..