1.
Apply Concepts Design a “new” invertebrate.
Create and illustration on which you point out its
body plan features. Then show its place on the
cladogram of invertebrates and write a caption
explaining how its features helped you decide
where it belongs.
CH 26 ANIMAL EVOLUTION AND DIVERSITY
26.1 Invertebrate Evolution and Diversity
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Fossil evidence indicates that the first animals
began evolving long before the Cambrian
Explosion.
Origins of the Invertebrates
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Roughly 3 billion years after the first prokaryotic
cells evolved, all prokaryotes and eukaryotes were
single-celled
Animals evolved from ancestors they shared with
organisms called choanoflagellates
 Single-celled
eukaryotes that sometimes grow in
colonies
 Similar to sponges.
Traces of Early Animals
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Oldest evidence of multicellular life comes from
microscopic fossils that are roughly 600 million
years old
First animals were tiny and soft-bodied, so few
fossilized bodies exist.
Ediacaran Fauna
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Before the Cambrian Period
From the Ediacara Hills of Australia
Fossils 565 to about 544 million years old
Flat and lived on the bottom of shallow seas
Little evidence of cell, tissue, or organ
specialization, and no organization into a front and
back end.
Cambrian Explosion
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Cambrian Period began about 542 million years
ago
Burgess Shale of Canada
Fossils show that over a period of 10–15 million
years, animals evolved complex body plans,
including specialized cells, tissues, and organs.
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By the end of the Cambrian Period, all the basic
body plans of modern phyla had been established
Later evolutionary changes, which produced the
more familiar body structures of modern animals,
involved variations on these basic body plans.
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Invertebrates are the most abundant animals on
Earth.
Sponges
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Phylum: Porifera
Most ancient members of the kingdom Animalia
Multicellular, heterotrophic, lack cell walls, and
contain a few specialized cells.
Cnidarians
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Phylum: Cnidaria
Includes jellyfishes, sea fans, sea anemones,
hydras, and corals
Aquatic, soft-bodied, carnivorous, radially
symmetrical animals with stinging tentacles
arranged in circles around their mouths.
Arthropods
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Phylum: Arthropoda
Includes spiders, centipedes, insects, and
crustaceans
Bodies divided into segments, a tough external
skeleton called an exoskeleton, cephalization, and
jointed appendages (legs and antennae)
Appeared in the sea about 600 million years ago.
Nematodes (Roundworms)
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Phylum: Nematoda
Unsegmented worm with pseudocoeloms,
specialized tissues and organ systems, and
digestive tracts with two openings
More closely related to the arthropods than
flatworms.
Flatworms
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Phylum: Platyhelminthes
Soft, unsegmented, flattened worms that have
tissues and internal organ systems
Simplest animals to have three embryonic germ
layers, bilateral symmetry, and cephalization
No coelms.
Annelids
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Phylum: Annelida
Includes earthworms, some marine worms, and
leeches
Worms with segmented bodies and a true coelom.
Mollusks
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Phylum: Mollusca
Includes snails, slugs, clams, squids, and octopi
Soft-bodied animals that have an internal or
external shell
Have true coeloms and complex organ systems
May have a trochophore
 Immature
free-swimming larva stage.
Echinoderms
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Phylum: Echinodermata
Includes sea stars, sea urchins, and sand dollars
Have spiny skin and an internal skeleton
Have a water vascular system that include suctioncuplike tube feet
Deuterostomes.