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Biology
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2. 29–1 Invertebrate Evolution
Photo Credit: ©Brandon Cole/Visuals Unlimited
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3. Origin of Invertebrates
Invertebrate fossils, dating between 575 and 543 million years ago, were discovered in the Ediacara Hills of Australia and in Chengjiang, China.
The Ediacaran fossils include some of the earliest and most primitive animals known.
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4. Origin of Invertebrates
The fossils:
were flat and plate shaped
were segmented
had bilateral symmetry
lived on the bottom of shallow seas
were made of soft tissues
absorbed nutrients from the surrounding water
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5. Origin of Invertebrates
Some of these animals may have had photosynthetic algae living within their bodies.
Some may have been related to soft-bodied invertebrates.
They were probably simple and had little internal specialization.
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6. Origin of Invertebrates
Beginnings of Invertebrate Diversity
By the Cambrian Period, 544 million years ago, some animals had evolved shells, skeletons, and other hard body parts.
One of the best-known sites of Cambrian fossils is the Burgess Shale of Canada.
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7. Origin of Invertebrates
By the Cambrian period, animals had acquired specialized cells, tissues, and organs.
During that time, the ancestors of most modern animal phyla first appeared in the fossil record.
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8. Origin of Invertebrates
The animals of the Burgess Shale had many of the characteristics of modern day invertebrates including:
body symmetry
segmentation
a skeleton
a front and a back end
appendages adapted for many functions
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9. Invertebrate Phylogeny
Many features of modern invertebrates evolved during the Cambrian period such as:
tissues and organs
patterns of early development
body symmetry
cephalization
segmentation
formation of three germ layers and a coelom
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10. Invertebrate Phylogeny
Invertebrate Evolutionary Relationships
This diagram illustrates one recent theory about the phylogenetic relationships among groups of living animals.
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11. Invertebrate Phylogeny
Roundworms
Flatworms
Cnidarians
This diagram illustrates one recent theory about the phylogenetic relationships among groups of living animals. Labels indicate the evolution of major features such as radial symmetry.
Sponges
Unicellular ancestor
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12. Invertebrate Phylogeny
This diagram illustrates one recent theory about the phylogenetic relationships among groups of living animals.
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13. Invertebrate Phylogeny
This diagram illustrates one recent theory about the phylogenetic relationships among groups of living animals.
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14. Invertebrate Phylogeny
What are the major trends in invertebrate evolution?
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Evolutionary Trends
Evolutionary Trends
The appearance of each phylum in the fossil record represents the evolution of a successful and unique body plan.
Features of this body plan typically change over time, leading to the formation of many new traits.
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Evolutionary Trends
This table shows the major characteristics of the main groups of invertebrates. Germ layers, body symmetry, cephalization, and development of a coelom are more common in complex invertebrates than in simple ones. Mollusks, for example, have all of these features, but sponges have none of them.
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Evolutionary Trends
This table shows the major characteristics of the main groups of invertebrates. Germ layers, body symmetry, cephalization, and development of a coelom are more common in complex invertebrates than in simple ones. Mollusks, for example, have all of these features, but sponges have none of them.
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Evolutionary Trends
Specialized Cells, Tissues, and Organs
As larger and more complex animals evolved, specialized cells joined together to form tissues, organs, and organ systems that work together to carry out complex functions.
Flatworms have simple organs for digestion, excretion, response, and reproduction.
More complex animals, such as mollusks and arthropods, have organ systems.
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Evolutionary Trends
Body Symmetry
All invertebrates, except sponges, exhibit some type of body symmetry.
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Evolutionary Trends
Cnidarians and echinoderms exhibit radial symmetry where parts extend from the center of the body.
Radial symmetry
Planes of symmetry
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Evolutionary Trends
Worms, mollusks, and arthropods exhibit bilateral symmetry, or have mirror-image left and right sides.
Bilateral symmetry
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Evolutionary Trends
Cephalization
Cephalization is the concentration of sense organs and nerve cells in the front of the body.
Invertebrates with cephalization can respond to the environment in more sophisticated ways than can simpler invertebrates.
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Evolutionary Trends
In most worms and arthropods, nerve cells are arranged in structures called ganglia.
In more complex invertebrates, nerve cells form an organ called a brain.
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Evolutionary Trends
Segmentation
Over the course of evolution, different segments in invertebrates have often become specialized for specific functions.
Segmentation allows an animal to increase its size with minimal new genetic material.
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Evolutionary Trends
Coelom Formation 
Flatworms are acoelomates. This means they have no coelom, or body cavity, that forms between the germ layers.
Ectoderm
Mesoderm
Endoderm
Acoelomates do not have a coelom, or body cavity, between their body wall and digestive cavity.
Digestive cavity
Acoelomate
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Evolutionary Trends
Pseudocoelomates have a body cavity lined partially with mesoderm.
Pseudocoelom
Pseudocoelomates have body cavities that are partially lined with tissues from mesoderm.
Digestive tract
Pseudocoelomate
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Evolutionary Trends
Most complex animal phyla have a true coelom that is lined completely with tissue derived from mesoderm.
Coelom
Most complex animal phyla are coelomates, meaning that they have a true coelom that is lined completely with tissues from mesoderm.
Digestive tract
Coelomate
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Evolutionary Trends
Embryological Development  
In most invertebrates, the zygote divides to form a blastula—a hollow ball of cells.
In protostomes, the blastopore, or the opening of the blastula, develops into a mouth.
In deuterostomes, the blastopore forms an anus.
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29–1
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29–1
According to the most recent studies of animal fossils, which of the following is correct?
Annelids with a true coelom appeared before cnidarians with two germ layers.
Radial symmetry appears in cnidarians and adult echinoderms.
Protostome development appears after deuterostome development.
Bilateral symmetry appears before tissues.
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29–1
Acoelomates lack
tissues.
a coelom.
radial symmetry.
specialized cells.
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29–1
Worms, mollusks, and arthropods exhibit
bilateral symmetry.
radial symmetry.
no internal specialization.
similar larval forms.
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29–1
In most invertebrates, the zygote divides repeatedly to form a
coelom.
digestive tract.
blastula.
mesoderm.
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29–1
The animal group that has no germ layers, body symmetry, cephalization, or coelom is the
flatworms.
annelids.
sponges.
cnidarians.
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35. END OF SECTION